which includes the #dff original        
illustrations4       ,see #ehhaj-;h4htm 
or #ehhaj-;h4zip3                       
7https3_/_/www4gutenberg4org_/cache_/epu
b_/#ehhaj_/pg#ehhaj-;images4html7       
or                                      
7https3_/_/www4gutenberg4org_/files_/#eh
haj_/ehhaj-;h4zip7         ,images of   
the original pages are available through
       ,internet ,archive4 ,see         
https3_/_/archive4org_/details_/heavensa
bovepopu#jj;gillrich    ,transcriber's  
note3        ,text enclosed by          
underscores is in italics 7italics74    
,superscripts1 such as ,p to the second 
power1 are shown by       the caret     
character  before the superscript1 such 
as ,p#b4        ,subscripts are         
similarly shown by an underscore before 
the       subscript which is wrapped in 
curly braces1 such as ,m#b4             
,illustration3 ,,spectra ,,of ,,various 
,,sources ,,of ,,light4             ,the
 ,heavens ,above3 ,a ,popular ,handbook 
of ,astronomy   ,,the ,,heavens ,,above3

  ,a ,popular ,handbook of ,astronomy4  
by  ,j4 ,a4 ,,gillet1  ,professor of    
,physics in the ,normal ,college of the 
,city of ,new ,york1  and  ,w4 ,j4      
,,rolfe1  ,formerly ,head ,master of the
 ,high ,school1 ,cambridge1 ,mass4      
,with ,six ,lithographic ,plates and    
,four ,hundred and ,sixty ,wood         
,engravings4       ,potter1 ,ainsworth1 
,co41 ,new ,york and ,chicago4 #ahhb4   
,copyright by ,j4 ,a4 ,gillet and ,w4   
,j4 ,rolfe1 #ahhb4  ,franklin ,press3   
,rand1 ,avery1 and ,company1 ,boston4   
,,preface4   ,it has been the aim of the
 authors to give in this little book a  
brief1 simple1 and accurate account of  
the heavens as they are known to        
astronomers of the present day4 ,it is  
believed that there is nothing in the   
book beyond the comprehension of readers
 of ordinary intelligence1 and that it  
contains all the information on the     
subject of astronomy that is needful to 
a person of ordinary culture4 ,the      
authors have carefully avoided dry and  

abstruse mathematical calculations1 yet 
they have sought to make clear the      
methods by which astronomers have gained
 their knowledge of the heavens4 ,the   
various kinds of telescopes and         
spectroscopes have been described1 and  
their use in the study of the heavens   
has been fully explained4  ,the cuts    
with which the book is illustrated have 
been drawn from all available sources2  
and it is believed that they excel in   
number1 freshness1 beauty1 and accuracy 
those to be found in any similar work4  
,the lithographic plates are1 with a    
single exception1 reductions of the     
plates prepared at the ,observatory at  
,cambridge1 ,mass4 ,the remaining       
lithographic plate is a reduced copy of 
,professor ,langley's celebrated        
sun-spot engraving4 ,many of the views  
of the moon are from drawings made from 
the photographs in ,carpenter and       
,nasmyth's work on the moon4 ,the       
majority of the cuts illustrating the   
solar system are copied from the ,french

 edition of ,guillemin's ,heavens4 ,most
 of the remainder are from ,lockyer's   
,solar ,physics1 ,young's ,sun1 and     
other recent authorities4 ,the cuts     
illustrating comets1 meteors1 and nebul1
 are nearly all taken from the ,french  
editions of ,guillemin's ,comets and    
,guillemin's ,heavens4                  
,,contents4   ,i4 ,,the ,,celestial     
,,sphere #c  ,,ii4 ,,the ,,solar        
,,system #da      ,i4 ,,theory ,,of     
,,the ,,solar ,,system #da          ,the
 ,ptolemaic ,system #da          ,the   
,copernican ,system #dd          ,tycho 
,brahe's ,system #dd          ,kepler's 
,system #dd          ,the ,newtonian    
,system #dh      ,,ii4 ,,the ,,sun ,,and
 ,,planets #ec          ,i4 ,the ,earth 
#ec              ,form and ,size #ec    
,day and ,night #eg              ,the   
,seasons #fd              ,tides #fh    
,the ,day and ,time #gd                 
,the ,year #gh              ,weight of  
the ,earth and ,precession #hc          
,,ii4 ,the ,moon #hf                    

,distance1 ,size1 and ,motions #hf      
,the ,atmosphere of the ,moon #aji      
,the ,surface of the ,moon #aad         
,,iii4 ,inferior and ,superior ,planets 
#acj          ,inferior ,planets #acj   
,superior ,planets #acd      ,,iv4 ,the 
,sun #adj          ,i4 ,magnitude and   
,distance of the ,sun #adj              
,,ii4 ,physical and ,chemical ,condition
 of the ,sun #adi              ,physical
 ,condition of the ,sun #adi            
,the ,spectroscope #aeb                 
,spectra #aeh              ,chemical    
,constitution of the ,sun #afd          
,motion at the ,surface of the ,sun #afh
          ,,iii4 ,the ,photosphere and  
,sun-,spots #age              ,the      
,photosphere #age                       
,sun-,spots #agi          ,,iv4 ,the    
,chromosphere and ,prominences #aif     
,v4 ,the ,corona #bjd      ,v4 ,eclipses
 #baj      ,,vi4 ,the ,three ,groups of 
,planets #bba          ,i4 ,general     
,characteristics of the ,groups #bba    
,,ii4 ,the ,inner ,group of ,planets    

#bbe              ,mercury #bbe         
,venus #bcj              ,mars #bce     
,,iii4 ,the ,asteroids #bda             
,,iv4 ,outer ,group of ,planets #bdd    
,jupiter #bdd                  ,the     
,satellites of ,jupiter #bej            
,saturn #bee                  ,the      
,planet and his ,moons #bee             
,the ,rings of ,saturn #bfa             
,uranus #bfi              ,neptune #bga 
,,vii4 ,comets and ,meteors #bgd        
,i4 ,comets #bgd              ,general  
,phenomena of ,comets #bgd              
,motion and ,origin of ,comets #bha     
,remarkable ,comets #bij                
,connection between ,meteors and        
,comets1 #cjj              ,physical and
 ,chemical ,constitution of ,comets #cad
          ,,ii4 ,the ,zodiacal ,light   
#cah  ,,iii4 ,,the ,,stellar ,,universe 
#cbb      ,i4 ,general ,aspect of the   
,heavens #cbb      ,,ii4 ,the ,stars    
#ccj          ,the ,constellations #ccj 
,clusters #cej          ,double and     
,multiple ,stars #cee          ,new and 

,variable ,stars #ceh          ,distance
 of the ,stars #cfd          ,proper    
,motion of the ,stars #cfe              
,chemical and ,physical ,constitution of
 the ,stars #cga      ,,iii4 ,nebul #cgc
          ,classification of ,nebul #cgc
          ,irregular ,nebul #cgf        
,spiral ,nebul #chd          ,the       
,nebular ,hypothesis #cia      ,,iv4    
,the ,structure of the ,stellar         
,universe #cif                          
,i4                          ,,the      
,,celestial ,,sphere4   ,i4 ,the        
,sphere4--,a sphere is a solid figure   
bounded by a surface which curves       
equally in all directions at every      
point4 ,the rate at which the surface   
curves is called the curvature of the   
sphere4 ,the smaller the sphere1 the    
greater is its curvature4 ,every point  
on the surface of a sphere is equally   
distant from a point within1 called the 
centre of the sphere4 ,the circumference
 of a sphere is the distance around its 
centre4 ,the diameter of a sphere is the

 distance through its centre4 ,the      
radius of a sphere is the distance from 
the surface to the centre4 ,the surfaces
 of two spheres are to each other as the
 squares of their radii or diameters2   
and the volumes of two spheres are to   
each other as the cubes of their radii  
or diameters4  ,distances on the surface
 of a sphere are usually denoted in     
degrees4 ,a degree is #a_/cfj of the    
circumference of the sphere4 ,the larger
 a sphere1 the longer are the degrees on
 it4  ,a curve described about any point
 on the surface of a sphere1 with a     
radius of uniform length1 will be a     
circle4 ,as the radius of a circle      
described on a sphere is a curved line1 
its length is usually denoted in        
degrees4 ,the circle described on the   
surface of a sphere increases with the  
length of the radius1 until the radius  
becomes #ij1 in which case the circle is
 the largest that can possibly be       
described on the sphere4 ,the largest   
circles that can be described on the    

surface of a sphere are called great    
circles1 and all other circles small    
circles4      ,any number of great      
circles may be described on the surface 
of a     sphere1 since any point on the 
sphere may be used for the centre of    
the circle4 ,the plane of every great   
circle passes through the     centre of 
the sphere1 while the planes of all the 
small circles pass     through the      
sphere away from the centre4 ,all great 
circles on the     same sphere are of   
the same size1 while the small circles  
differ in     size according to the     
distance of their planes from the centre
 of     the sphere4 ,the farther the    
plane of a circle is from the centre of 
the sphere1 the smaller is the circle4  
,by a section of a sphere we usually    
mean the figure of the surface          
formed by the cutting2 by a plane       
section we mean one whose     surface is
 plane4 ,every plane section of a sphere
 is a circle4 ,when     the section     
passes through the centre of the sphere1

 it is a great     circle2 in every     
other case the section is a small       
circle4 ,thus1     ,,an and ,,sb 7,fig4 
#a7 are small circles1 and ,,mm' and    
,,sn are     large circles4             
,illustration3 ,fig4 #a4      ,in a     
diagram representing a sphere in        
section1 all the circles whose          
planes cut the section are represented  
by straight lines4 ,thus1 in     ,fig4  
#b1 we have a diagram representing in   
section the sphere of ,fig4     #a4 ,the
 straight lines ,,an1 ,,sb1 ,,mm'1 and  
,,sn1 represent the     corresponding   
circles of ,fig4 #a4  ,the axis of a    
sphere is the diameter on which it      
rotates4 ,the poles of a sphere are the 
ends of its axis4 ,thus1 supposing the  
spheres of ,figs4 #a and #b to rotate on
 the diameter ,,pp'1 this line would be 
called the axis of the sphere1 and the  
points ,p and ,p' the poles of the      
sphere4 ,a great circle1 ,,mm'1 situated
 half way between the poles of a sphere1
 is called the equator of the sphere4   

,every great circle of a sphere has two 
poles4 ,these are the two points on the 
surface of the sphere which lie #ij away
 from the circle4 ,the poles of a sphere
 are the poles of its equator4          
,illustration3 ,fig4 #b4  #b4 ,the      
,celestial ,sphere4--,the heavens appear
 to have the form of a sphere1 whose    
centre is at the eye of the observer2   
and all the stars seem to lie on the    
surface of this sphere4 ,this form of   
the heavens is a mere matter of         
perspective4 ,the stars are really at   
very unequal distances from us2 but they
 are all seen projected upon the        
celestial sphere in the direction in    
which they happen to lie4 ,thus1 suppose
 an observer situated at ,c 7,fig4 #c71 
stars situated at a1 b1 d1 e1 f1 and g1 
would be projected upon the sphere at   
,a1 ,b1 ,d1 ,e1 ,f1 and ,g1 and would   
appear to lie on the surface of the     
heavens4  ,illustration3 ,fig4 #c4  #c4 
,the ,horizon4--,only half of the       
celestial sphere is visible at a time4  

,the plane that separates the visible   
from the invisible portion is called the
 horizon4 ,this plane is tangent to the 
earth at the point of observation1 and  
extends indefinitely into space in every
 direction4 ,in ,fig4 #d1 ,e represents 
the earth1 ,o the point of observation1 
and ,,sn the horizon4 ,the points on the
 celestial sphere directly above and    
below the observer are the poles of the 
horizon4 ,they are called respectively  
the zenith and the nadir4 ,no two       
observers in different parts of the     
earth have the same horizon2 and as a   
person moves over the earth he carries  
his horizon with him4  ,illustration3   
,fig4 #d4  ,the dome of the heavens     
appears to rest on the earth1 as shown  
in ,fig4 #e4 ,this is because distant   
objects on the earth appear projected   
against the heavens in the direction of 
the horizon4  ,illustration3 ,fig4 #e4  
,the sensible horizon is a plane tangent
 to the earth at the point of           
observation4 ,the rational horizon is a 

plane parallel with the sensible        
horizon1 and passing through the centre 
of the earth4 ,as it cuts the celestial 
sphere through the centre1 it forms a   
great circle4 ,,sn 7,fig4 #f7 represents
 the sensible horizon1 and ,s',n' the   
rational horizon4 ,although these two   
horizons are really four thousand miles 
apart1 they appear to meet at the       
distance of the celestial sphere2 a line
 four thousand miles long at the        
distance of the celestial sphere        
becoming a mere point1 far too small to 
be detected with the most powerful      
telescope4  ,illustration3 ,fig4 #f4    
,illustration3 ,fig4 #g4  #d4 ,rotation 
of the ,celestial ,sphere4--,it is well 
known that the sun and the majority of  
the stars rise in the east1 and set in  
the west4 ,in our latitude there are    
certain stars in the north which never  
disappear below the horizon4 ,these     
stars are called the circumpolar stars4 
,a close watch1 however1 reveals the    
fact that these all appear to revolve   

around one of their number called the   
pole star1 in the direction indicated by
 the arrows in ,fig4 #g4 ,in a word1 the
 whole heavens appear to rotate once a  
day1 from east to west1 about an axis1  
which is the prolongation of the axis of
 the earth4 ,the ends of this axis are  
called the poles of the heavens2 and the
 great circle of the heavens1 midway    
between these poles1 is called the      
celestial equator1 or the equinoctial4  
,this rotation of the heavens is        
apparent only1 being due to the rotation
 of the earth from west to east4  #e4   
,diurnal ,circles4--,in this rotation of
 the heavens1 the stars appear to       
describe circles which are perpendicular
 to the celestial axis1 and parallel    
with the celestial equator4 ,these      
circles are called diurnal circles4 ,the
 position of the poles in the heavens   
and the direction of the diurnal circles
 with reference to the horizon1 change  
with the position of the observer on the
 earth4 ,this is owing to the fact that 

the horizon changes with the position of
 the observer4  ,illustration3 ,fig4 #h4
  ,when the observer is north of the    
equator1 the north pole of the heavens  
is elevated above the horizon1 and the  
south pole is depressed below it1 and   
the diurnal circles are oblique to the  
horizon1 leaning to the south4 ,this    
case is represented in ,fig4 #h1 in     
which ,,pp' represents the celestial    
axis1 ,,eq the celestial equator1 ,,sn  
the horizon1 and ab1 c,n1 de1 fg1 ,sh1  
kl1 diurnal circles4 ,o is the point of 
observation1 ,z the zenith1 and ,z' the 
nadir4  ,illustration3 ,fig4 #i4  ,when 
the observer is south of the equator1 as
 at ,o in ,fig4 #i1 the south pole is   
elevated1 the north pole depressed1 and 
the diurnal circles are oblique to the  
horizon1 leaning to the north4 ,when the
 diurnal circles are oblique to the     
horizon1 as in ,figs4 #h and #i1 the    
celestial sphere is called an oblique   
sphere4  ,when the observer is at the   
equator1 as in ,fig4 #aj1 the poles of  

the heavens are on the horizon1 and the 
diurnal circles are perpendicular to the
 horizon4  ,when the observer is at one 
of the poles1 as in ,fig4 #aa1 the poles
 of the heavens are in the zenith and   
the nadir1 and the diurnal circles are  
parallel with the horizon4              
,illustration3 ,fig4 #aj4               
,illustration3 ,fig4 #aa4  #f4          
,elevation of the ,pole and of the      
,equinoctial4--,at the equator the poles
 of the heavens lie on the horizon1 and 
the celestial equator passes through the
 zenith4 ,as a person moves north from  
the equator1 his zenith moves north from
 the celestial equator1 and his horizon 
moves down from the north pole1 and up  
from the south pole4 ,the distance of   
the zenith from the equinoctial1 and of 
the horizon from the celestial poles1   
will always be equal to the distance of 
the observer from the equator4 ,in other
 words1 the elevation of the pole is    
equal to the latitude of the place4 ,in 
,fig4 #ab1 ,o is the point of           

observation1 ,z the zenith1 and ,,sn the
 horizon4 ,,np1 the elevation of the    
pole1 is equal to ,,ze1 the distance of 
the zenith from the equinoctial1 and to 
the distance of ,o from the equator1 or 
the latitude of the place4  ,two angles1
 or two arcs1 which together equal #ij1 
are said to be complements of each      
other4 ,,ze and ,,es in ,fig4 #ab are   
together equal to #ij3 hence they are   
complements of each other4 ,,ze is equal
 to the latitude of the place1 and ,,es 
is the elevation of the equinoctial     
above the horizon3 hence the elevation  
of the equinoctial is equal to the      
complement of the latitude of the place4
  ,illustration3 ,fig4 #ab4  ,were the  
observer south of the equator1 the      
zenith would be south of the            
equinoctial1 and the south pole of the  
heavens would be the elevated pole4     
,illustration3 ,fig4 #ac4  #g4 ,four    
,sets of ,stars4--,at most points of    
observation there are four sets of      
stars4 ,these four sets are shown in    

,fig4 #ac4  7#a7 ,the stars in the      
neighborhood of the elevated pole never 
set4 ,it will be seen from ,fig4 #ac1   
that if the distance of a star from the 
elevated pole does not exceed the       
elevation of the pole1 or the latitude  
of the place1 its diurnal circle will be
 wholly above the horizon4 ,as the      
observer approaches the equator1 the    
elevation of the pole becomes less and  
less1 and the belt of circumpolar stars 
becomes narrower and narrower3 at the   
equator it disappears entirely4 ,as the 
observer approaches the pole1 the       
elevation of the pole increases1 and the
 belt of circumpolar stars becomes      
broader and broader1 until at the pole  
it includes half of the heavens4 ,at the
 poles1 no stars rise or set1 and only  
half of the stars are ever seen at all4 
7#b7 ,the stars in the neighborhood of  
the depressed pole never rise4 ,the     
breadth of this belt also increases as  
the observer approaches the pole1 and   
decreases as he approaches the equator1 

to vanish entirely when he reaches the  
equator4 ,the distance from the         
depressed pole to the margin of this    
belt is always equal to the latitude of 
the place4  7#c7 ,the stars in the      
neighborhood of the equinoctial1 on the 
side of the elevated pole1 set1 but are 
above the horizon longer than they are  
below it4 ,this belt of stars extends   
from the equinoctial to a point whose   
distance from the elevated pole is equal
 to the latitude of the place3 in other 
words1 the breadth of this third belt of
 stars is equal to the complement of the
 latitude of the place4 ,hence this belt
 of stars becomes broader and broader as
 the observer approaches the equator1   
and narrower and narrower as he         
approaches the pole4 ,however1 as the   
observer approaches the equator1 the    
horizon comes nearer and nearer the     
celestial axis1 and the time a star is  
below the horizon becomes more nearly   
equal to the time it is above it4 ,as   
the observer approaches the pole1 the   

horizon moves farther and farther from  
the axis1 and the time any star of this 
belt is below the horizon becomes more  
and more unequal to the time it is above
 it4 ,the farther any star of this belt 
is from the equinoctial1 the longer the 
time it is above the horizon1 and the   
shorter the time it is below it4  7#d7  
,the stars which are in the neighborhood
 of the equinoctial1 on the side of the 
depressed pole1 rise1 but are below the 
horizon longer than they are above it4  
,the width of this belt is also equal to
 the complement of the latitude of the  
place4 ,the farther any star of this    
belt is from the equinoctial1 the longer
 time it is below the horizon1 and the  
shorter time it is above it2 and1 the   
farther the place from the equator1 the 
longer every star of this belt is below 
the horizon1 and the shorter the time it
 is above it4  ,at the equator every    
star is above the horizon just half of  
the time2 and any star on the           
equinoctial is above the horizon just   

half of the time in every part of the   
earth1 since the equinoctial and        
horizon1 being great circles1 bisect    
each other4  #h4 ,vertical              
,circles4--,great circles perpendicular 
to the horizon are called vertical      
circles4 ,all vertical circles pass     
through the zenith and nadir4 ,a number 
of these circles are shown in ,fig4 #ad1
 in which ,,senw represents the horizon1
 and ,z the zenith4  ,illustration3     
,fig4 #ad4  ,the vertical circle which  
passes through the north and south      
points of the horizon is called the     
meridian2 and the one which passes      
through the east and west points1 the   
prime vertical4 ,these two circles are  
shown in ,fig4 #ae2 ,,szn being the     
meridian1 and ,,ezw the prime vertical4 
,these two circles are at right angles  
to each other1 or #ij apart2 and        
consequently they divide the horizon    
into four quadrants4  ,illustration3    
,fig4 #ae4  #i4 ,altitude and ,zenith   
,distance4--,the altitude of a heavenly 

body is its distance above the horizon1 
and its zenith distance is its distance 
from the zenith4 ,both the altitude and 
the zenith distance of a body are       
measured on the vertical circle which   
passes through the body4 ,the altitude  
and zenith distance of a heavenly body  
are complements of each other4  #aj4    
,azimuth and ,amplitude4--,azimuth is   
distance measured east or west from the 
meridian4 ,when a heavenly body lies    
north of the prime vertical1 its azimuth
 is measured from the meridian on the   
north2 and1 when it lies south of the   
prime vertical1 its azimuth is measured 
from the meridian on the south4 ,the    
azimuth of a body can1 therefore1 never 
exceed #ij4 ,the azimuth of a body is   
the angle which the plane of the        
vertical circle passing through it makes
 with that of the meridian4  ,the       
amplitude of a body is its distance     
measured north or south from the prime  
vertical4 ,the amplitude and azimuth of 
a body are complements of each other4   

#aa4 ,alt-azimuth ,instrument4--,an     
instrument for measuring the altitude   
and azimuth of a heavenly body is called
 an alt-azimuth instrument4 ,one form of
 this instrument is shown in ,fig4 #af4 
,it consists essentially of a telescope 
mounted on a vertical circle1 and       
capable of turning on a horizontal axis1
 which1 in turn1 is mounted on the      
vertical axis of a horizontal circle4   
,both the horizontal and the vertical   
circles are graduated1 and the          
horizontal circle is placed exactly     
parallel with the plane of the horizon4 
,when the instrument is properly        
adjusted1 the axis of the telescope will
 describe a vertical circle when the    
telescope is turned on the horizontal   
axis1 no matter to what part of the     
heavens it has been pointed4  ,the      
horizontal and vertical axes carry each 
a pointer4 ,these pointers move over the
 graduated circles1 and mark how far    
each axis turns4  ,to find the azimuth  
of a star1 the instrument is turned on  

its vertical axis till its vertical     
circle is brought into the plane of the 
meridian1 and the reading of the        
horizontal circle noted4 ,the telescope 
is then directed to the star by turning 
it on both its vertical and horizontal  
axes4 ,the reading of the horizontal    
circle is again noted4 ,the difference  
between these two readings of the       
horizontal circle will be the azimuth of
 the star4  ,illustration3 ,fig4 #af4   
,to find the altitude of a star1 the    
reading of the vertical circle is first 
ascertained when the telescope is       
pointed horizontally1 and again when the
 telescope is pointed at the star4 ,the 
difference between these two readings of
 the vertical circle will be the        
altitude of the star4  #ab4 ,the        
,vernier4--,to enable the observer to   
read the fractions of the divisions on  
the circles1 a device called a vernier  
is often employed4 ,it consists of a    
short1 graduated arc1 attached to the   
end of an arm c 7,fig4 #ag71 which is   

carried by the axis1 and turns with the 
telescope4 ,this arc is of the length of
 nine divisions on the circle1 and it is
 divided into ten equal parts4 ,if #j of
 the vernier coincides with any         
division1 say #f1 of the circle1 #a of  
the vernier will be #a_/aj of a division
 to the left of #g1 #b will be #b_/aj of
 a division to the left of #h1 #c will  
be #c_/aj1 of a division to the left of 
#i1 etc4 ,hence1 when #a coincides with 
#g1 #j will be at #f-a_/aj2 when #b     
coincides with #h1 #j will be at        
#f-b_/aj2 when #c coincides with #i1 #j 
will be at #f-c_/aj1 etc4               
,illustration3 ,fig4 #ag4  ,to ascertain
 the reading of the circle by means of  
the vernier1 we first notice the zero   
line4 ,if it exactly coincides with any 
division of the circle1 the number of   
that division will be the reading of the
 circle4 ,if there is not an exact      
coincidence of the zero line with any   
division of the circle1 we run the eye  
along the vernier1 and note which of its

 divisions does coincide with a division
 of the circle4 ,the reading of the     
circle will then be the number of the   
first division on the circle behind the 
#j of the vernier1 and a number of      
tenths equal to the number of the       
division of the vernier1 which coincides
 with a division of the circle4 ,for    
instance1 suppose #j of the vernier     
beyond #f of the circle1 and #g of the  
vernier to coincide with #ac of the     
circle4 ,the reading of the circle will 
then be #f-g_/aj4  #ac4 ,hour           
,circles4--,great circles perpendicular 
to the celestial equator are called hour
 circles4 ,these circles all pass       
through the poles of the heavens1 as    
shown in ,fig4 #ah4 ,,eq is the         
celestial equator1 and ,p and ,p' are   
the poles of the heavens4  ,the point ,a
 on the equinoctial 7,fig4 #ai7 is      
called the vernal equinox1 or the first 
point of ,aries4 ,the hour circle1      
,,app'1 which passes through it1 is     
called the equinoctial colure4          

,illustration3 ,fig4 #ah4  #ad4         
,declination and ,right                 
,ascension4--,the declination of a      
heavenly body is its distance north or  
south of the celestial equator4 ,the    
polar distance of a heavenly body is its
 distance from the nearer pole4         
,declination and polar distance are     
measured on hour circles1 and for the   
same heavenly body they are complements 
of each other4  ,illustration3 ,fig4    
#ai4  ,the right ascension of a heavenly
 body is its distance eastward from the 
first point of ,aries1 measured from the
 equinoctial colure4 ,it is equal to the
 arc of the celestial equator included  
between the first point of ,aries and   
the hour circle which passes through the
 heavenly body4 ,as right ascension is  
measured eastward entirely around the   
celestial sphere1 it may have any value 
from #j up to #cfj4 ,right ascension    
corresponds to longitude on the earth1  
and declination to latitude4  #ae4 ,the 
,meridian ,circle4--,the right ascension

 and declination of a heavenly body are 
ascertained by means of an instrument   
called the meridian circle1 or transit  
instrument4 ,a side-view of this        
instrument is shown in ,fig4 #bj4       
,illustration3 ,fig4 #bj4  ,it consists 
essentially of a telescope mounted      
between two piers1 so as to turn in the 
plane of the meridian1 and carrying a   
graduated circle4 ,the readings of this 
circle are ascertained by means of fixed
 microscopes1 under which it turns4 ,a  
heavenly body can be observed with this 
instrument1 only when it is crossing the
 meridian4 ,for this reason it is often 
called the transit circle4  ,to find the
 declination of a star with this        
instrument1 we first ascertain the      
reading of the circle when the telescope
 is pointed to the pole1 and then the   
reading of the circle when pointed to   
the star on its passage across the      
meridian4 ,the difference between these 
two readings will be the polar distance 
of the star1 and the complement of them 

the declination of the star4  ,to       
ascertain the reading of the circle when
 the telescope is pointed to the pole1  
we must select one of the circumpolar   
stars near the pole1 and then point the 
telescope to it when it crosses the     
meridian1 both above and below the pole1
 and note the reading of the circle in  
each case4 ,the mean of these two       
readings will be the reading of the     
circle when the telescope is pointed to 
the pole4  #af4 ,astronomical           
,clock4--,an astronomical clock1 or     
sidereal clock as it is often called1 is
 a clock arranged so as to mark hours   
from #a to #bd1 instead of from #a to   
#ab1 as in the case of an ordinary      
clock1 and so adjusted as to mark #j    
when the vernal equinox1 or first point 
of ,aries1 is on the meridian4  ,as the 
first point of ,aries makes a complete  
circuit of the heavens in twenty-four   
hours1 it must move at the rate of #ae  
an hour1 or of #a in four minutes3      
hence1 when the astronomical clock marks

 #a1 the first point of ,aries must be  
#ae west of the meridian1 and when it   
marks #b1 #cj west of the meridian1 etc4
 ,that is to say1 by observing an       
accurate astronomical clock1 one can    
always tell how far the meridian at any 
time is from the first point of ,aries4 
#ag4 ,how to find ,right ,ascension with
 the ,meridian ,circle4--,to find the   
right ascension of a heavenly body1 we  
have merely to ascertain the exact time1
 by the astronomical clock1 at which the
 body crosses the meridian4 ,if a star  
crosses the meridian at #a hour #bj     
minutes by the astronomical clock1 its  
right ascension must be #ai2 if at #bj  
hours1 its right ascension must be #cjj4
  ,to enable the observer to ascertain  
with great exactness the time at which a
 star crosses the meridian1 a number of 
equidistant and parallel spider-lines   
are stretched across the focus of the   
telescope1 as shown in ,fig4 #ba4 ,the  
observer notes the time when the star   
crosses each spider-line2 and the mean  

of all of these times will be the time  
when the star crosses the meridian4 ,the
 mean of several observations is likely 
to be more nearly exact than any single 
observation4  ,illustration3 ,fig4 #ba4 
,illustration3 ,fig4 #bb4  #ah4 ,the    
,equatorial ,telescope4--,the equatorial
 telescope is mounted on two axes1--one 
parallel with the axis of the earth1 and
 the other at right angles to this1 and 
therefore parallel with the plane of the
 earth's equator4 ,the former is called 
the polar axis1 and the latter the      
declination axis4 ,each axis carries a  
graduated circle4 ,these circles are    
called respectively the hour circle and 
the declination circle4 ,the telescope  
is attached directly to the declination 
axis4 ,when the telescope is fixed in   
any declination1 and then turned on its 
polar axis1 the line of sight will      
describe a diurnal circle2 so that1 when
 the tube is once directed to a star1 it
 can be made to follow the star by      
simply turning the telescope on its     

polar axis4  ,in the case of large      
instruments of this class1 the polar    
axis is usually turned by clock-work at 
the rate at which the heavens rotate2 so
 that1 when the telescope has once been 
pointed to a planet or other heavenly   
body1 it will continue to follow the    
body and keep it steadily in the field  
of view without further trouble on the  
part of the observer4  ,the great       
,washington ,equatorial is shown in     
,fig4 #bb4 ,its object-glass is #bf     
inches in diameter1 and its focal length
 is #cb-a_/b feet4 ,it was constructed  
by ,alvan ,clark  ,sons of ,cambridge1  
,mass4 ,it is one of the three largest  
refracting telescopes at present in use4
 ,the ,newall refractor at ,gateshead1  
,eng41 has an objective #be inches in   
diameter1 and a focal length of #bi     
feet4 ,the great refractor at ,vienna   
has an objective #bg inches in diameter4
 ,there are several large refractors now
 in process of construction4            
,illustration3 ,fig4 #bc4  #ai4 ,the    

,wire ,micrometer4--,large arcs in the  
heavens are measured by means of the    
graduated circles attached to the axes  
of the telescopes2 but small arcs within
 the field of view of the telescope are 
measured by means of instruments called 
micrometers1 mounted in the focus of the
 telescope4 ,one of the most convenient 
of these micrometers is that known as   
the wire micrometer1 and shown in ,fig4 
#bc4  ,the frame ,,aa covers two slides1
 ,c and ,d4 ,these slides are moved by  
the screws ,f and ,g4 ,the wires ,e and 
,b are stretched across the ends of the 
slides so as to be parallel to each     
other4 ,on turning the screws ,f and ,g 
one way1 these wires are carried apart2 
and on turning them the other way they  
are brought together again4 ,sometimes  
two parallel wires1 x and y1 shown in   
the diagram at the right1 are stretched 
across the frame at right angles to the 
wires ,e1 ,b4 ,we may call the wires x  
and y the longitudinal wires of the     
micrometer1 and ,e and ,b the transverse

 wires4 ,many instruments have only one 
longitudinal wire1 which is stretched   
across the middle of the focus4 ,the    
longitudinal wires are just in front of 
the transverse wires1 but do not touch  
them4  ,to find the distance between any
 two points in the field of view with a 
micrometer1 with a single longitudinal  
wire1 turn the frame till the           
longitudinal wire passes through the two
 points2 then set the wires ,e and ,b   
one on each point1 turn one of the      
screws1 known as the micrometer screw1  
till the two wires are brought together1
 and note the number of times the screw 
is turned4 ,having previously           
ascertained over what arc one turn of   
the screw will move the wire1 the number
 of turns will enable us to find the    
length of the arc between the two       
points4  ,the threads of the micrometer 
screw are cut with great accuracy2 and  
the screw is provided with a large head1
 which is divided into a hundred or more
 equal parts4  ,these divisions1 by     

means of a fixed pointer1 enable us to  
ascertain what fraction of a turn the   
screw has made over and above its       
complete revolutions4  #bj4 ,reflecting 
,telescopes4--,it is possible to        
construct mirrors of much larger size   
than lenses3 hence reflecting telescopes
 have an advantage over refracting      
telescopes as regards size of aperture  
and of light-gathering power4 ,they are1
 however1 inferior as regards           
definition2 and1 in order to prevent    
flexure1 it is necessary to give the    
speculum1 or mirror1 a massiveness which
 makes the telescope unwieldy4 ,it is   
also necessary frequently to repolish   
the speculum2 and this is an operation  
of great delicacy1 as the slightest     
change in the form of the surface       
impairs the definition of the image4    
,these defects have been remedied1 to a 
certain extent1 by the introduction of  
silver-on-glass mirrors2 that is1 glass 
mirrors covered in front with a thin    
coating of silver4 ,glass is only       

one-third as heavy as speculum-metal1   
and silver is much superior to that     
metal in reflecting power2 and when the 
silver becomes tarnished1 it can be     
removed and renewed without danger of   
changing the form of the glass4  ,the   
,herschelian ,reflector4--,in this form 
of telescope the mirror is slightly     
tipped1 so that the image1 instead of   
being formed in the centre of the tube1 
is formed near one side of it1 as in    
,fig4 #bd4 ,the observer can then view  
it without putting his head inside the  
tube1 and therefore without cutting off 
any material portion of the light4 ,in  
observation1 he must stand at the upper 
or outer end of the tube1 and look into 
it1 his back being turned towards the   
object4 ,from his looking directly into 
the mirror1 it is also sometimes called 
the front-view telescope4 ,the great    
disadvantage of this arrangement is1    
that the rays cannot be brought to an   
exact focus when they are thrown so far 
to one side of the axis1 and the injury 

to the definition is so great that the  
front-view plan is now entirely         
abandoned4  ,illustration3 ,fig4 #bd4   
,the ,newtonian ,reflector4--,the plan  
proposed by ,sir ,isaac ,newton was to  
place a small plane mirror just inside  
the focus1 inclined to the telescope at 
an angle of #de1 so as to throw the rays
 to the side of the tube1 where they    
come to a focus1 and form the image4 ,an
 opening is made in the side of the     
tube1 just below where the image is     
formed2 and in this opening the         
eye-piece is inserted4 ,the small mirror
 cuts off some of the light1 but not    
enough to be a serious defect4 ,an      
improvement which lessens this defect   
has been made by ,professor ,henry      
,draper4 ,the inclined mirror is        
replaced by a small rectangular prism   
7,fig4 #be71 by reflection from which   
the image is formed very near the prism4
 ,a pair of lenses are then inserted in 
the course of the rays1 by which a      
second image is formed at the opening in

 the side of the tube2 and this second  
image is viewed by an ordinary          
eye-piece4  ,illustration3 ,fig4 #be4   
,the ,gregorian ,reflector4--,this is a 
form proposed by ,james ,gregory1 who   
probably preceded ,newton as an inventor
 of the reflecting telescope4 ,behind   
the focus1 ,f 7,fig4 #bf71 a small      
concave mirror1 ,r1 is placed1 by which 
the light is reflected back again down  
the tube4 ,the larger mirror1 ,m1 has an
 opening through its centre2 and the    
small mirror1 ,r1 is so adjusted as to  
form a second image of the object in    
this opening4 ,this image is then viewed
 by an eye-piece which is screwed into  
the opening4  ,illustration3 ,fig4 #bf4 
,the ,cassegrainian ,reflector4--,in    
principle this is the same with the     
,gregorian2 but the small mirror1 ,r1 is
 convex1 and is placed inside the focus1
 ,f1 so that the rays are reflected from
 it before reaching the focus1 and no   
image is formed until they reach the    
opening in the large mirror4 ,this form 

has an advantage over the ,gregorian1 in
 that the telescope may be made shorter1
 and the small mirror can be more easily
 shaped to the required figure4 ,it has1
 therefore1 entirely superseded the     
original ,gregorian form4               
,illustration3 ,fig4 #bg4  ,optically   
these forms of telescope are inferior to
 the ,newtonian2 but the latter is      
subject to the inconvenience1 that the  
observer must be stationed at the upper 
end of the telescope1 where he looks    
into an eye-piece screwed into the side 
of the tube4  ,on the other hand1 the   
,cassegrainian ,telescope is pointed    
directly at the object to be viewed1    
like a refractor2 and the observer      
stands at the lower end1 and looks in at
 the opening through the large mirror4  
,this is1 therefore1 the most convenient
 form of all in management4             
,illustration3 ,fig4 #bh4  ,the largest 
reflecting telescope yet constructed is 
that of ,lord ,rosse1 at ,parsonstown1  
,ireland4 ,its speculum is #f feet in   

diameter1 and its focal length #ee feet4
 ,it is commonly used as a ,newtonian4  
,this telescope is shown in ,fig4 #bg4  
,the great telescope of the ,melbourne  
,observatory1 ,australia1 is a          
,cassegranian reflector4 ,its speculum  
is #d feet in diameter1 and its focal   
length is #cb feet4 ,it is shown in     
,fig4 #bh4  ,illustration3 ,fig4 #bi4   
,the great reflector of the ,paris      
,observatory is a ,newtonian reflector4 
,its mirror of silvered glass is #d feet
 in diameter1 and its focal length is   
#bc feet4 ,this telescope is shown in   
,fig4 #bi4  #ba4 ,the ,sun's ,motion    
among the ,stars4--,if we notice the    
stars at the same hour night after      
night1 we shall find that the           
constellations are steadily advancing   
towards the west4 ,new constellations   
are continually appearing in the east1  
and old ones disappearing in the west4  
,this continual advancing of the heavens
 towards the west is due to the fact    
that the sun's place among the stars is 

continually moving towards the east4    
,the sun completes the circuit of the   
heavens in a year1 and is therefore     
moving eastward at the rate of about a  
degree a day4  ,illustration3 ,fig4 #cj4
  ,this motion of the sun's place among 
the stars is due to the revolution of   
the earth around the sun1 and not to any
 real motion of the sun4 ,in ,fig4 #cj  
suppose the inner circle to represent   
the orbit of the earth around the sun1  
and the outer circle to represent the   
celestial sphere4 ,when the earth is at 
,e1 the sun's place on the celestial    
sphere is at ,s'4 ,as the earth moves in
 the direction ,,ef1 the sun's place on 
the celestial sphere must move in the   
direction ,s',t3 hence the revolution of
 the earth around the sun would cause   
the sun's place among the stars to move 
around the heavens in the same direction
 that the earth is moving around the    
sun4  #bb4 ,the ,ecliptic4--,the circle 
described by the sun in its apparent    
motion around the heavens is called the 

ecliptic4 ,the plane of this circle     
passes through the centre of the earth1 
and therefore through the centre of the 
celestial sphere2 the earth being so    
small1 compared with the celestial      
sphere1 that it practically makes no    
difference whether we consider a point  
on its surface1 or one at its centre1 as
 the centre of the celestial sphere4    
,the ecliptic is1 therefore1 a great    
circle4  ,the earth's orbit lies in the 
plane of the ecliptic2 but it extends   
only an inappreciable distance from the 
sun towards the celestial sphere4       
,illustration3 ,fig4 #ca4  #bc4 ,the    
,obliquity of the ,ecliptic4--,the      
ecliptic is inclined to the celestial   
equator by an angle of about #bc-a_/b4  
,this inclination is called the         
obliquity of the ecliptic4 ,the         
obliquity of the ecliptic is due to the 
deviation of the earth's axis from a    
perpendicular to the plane of its orbit4
 ,the axis of a rotating body tends to  
maintain the same direction2 and1 as the

 earth revolves around the sun1 its axis
 points all the time in nearly the same 
direction4 ,the earth's axis deviates   
about #bc-a_/b from the perpendicular to
 its orbit2 and1 as the earth's equator 
is at right angles to its axis1 it will 
deviate about #bc-a_/b from the plane of
 the ecliptic4 ,the celestial equator   
has the same direction as the           
terrestrial equator1 since the axis of  
the heavens has the same direction as   
the axis of the earth4  ,illustration3  
,fig4 #cb4  ,suppose the globe at the   
centre of the tub 7,fig4 #ca7 to        
represent the sun1 and the smaller      
globes to represent the earth in various
 positions in its orbit4 ,the surface of
 the water will then represent the plane
 of the ecliptic1 and the rod projecting
 from the top of the earth will         
represent the earth's axis1 which is    
seen to point all the time in the same  
direction1 or to lean the same way4 ,the
 leaning of the axis from the           
perpendicular to the surface of the     

water would cause the earth's equator to
 be inclined the same amount to the     
surface of the water1 half of the       
equator being above1 and half of it     
below1 the surface4 ,were the axis of   
the earth perpendicular to the surface  
of the water1 the earth's equator would 
coincide with the surface1 as is evident
 from ,fig4 #cb4  ,illustration3 ,fig4  
#cc4  #bd4 ,the ,equinoxes and          
,solstices4--,the ecliptic and celestial
 equator1 being great circles1 bisect   
each other4 ,half of the ecliptic is    
north1 and half of it is south1 of the  
equator4 ,the points at which the two   
circles cross are called the equinoxes4 
,the one at which the sun crosses the   
equator from south to north is called   
the vernal equinox1 and the one at which
 it crosses from north to south the     
autumnal equinox4 ,the points on the    
ecliptic midway between the equinoxes   
are called the solstices4 ,the one north
 of the equator is called the summer    
solstice1 and the one south of the      

equator the winter solstice4 ,in ,fig4  
#cc1 ,,eq is the celestial equator1     
,ec,e'c' the ecliptic1 ,v the vernal    
equinox1 ,a the autumnal equinox1 ,ec   
the winter solstice1 and ,e'c' the      
summer solstice4  ,illustration3 ,fig4  
#cd4  #be4 ,the ,inclination of the     
,ecliptic to the ,horizon4--,since the  
celestial equator is perpendicular to   
the axis of the heavens1 it makes the   
same angle with it on every side3 hence1
 at any place1 the equator makes always 
the same angle with the horizon1        
whatever part of it is above the        
horizon4 ,but1 as the ecliptic is       
oblique to the equator1 it makes        
different angles with the celestial axis
 on different sides2 and hence1 at any  
place1 the angle which the ecliptic     
makes with the horizon varies according 
to the part which is above the horizon4 
,the two extreme angles for a place more
 than #bc-a_/b north of the equator are 
shown in ,figs4 #cd and #ce4  ,the least
 angle is formed when the vernal equinox

 is on the eastern horizon1 the autumnal
 on the western horizon1 and the winter 
solstice on the meridian1 as in ,fig4   
#cd4 ,the angle which the ecliptic then 
makes with the horizon is equal to the  
elevation of the equinoctial minus      
#bc-a_/b4 ,in the latitude of ,new ,york
 this angle  #di - #bc-a_/b  #be-a_/b4  
,illustration3 ,fig4 #ce4  ,the greatest
 angle is formed when the autumnal      
equinox is on the eastern horizon1 the  
vernal on the western horizon1 and the  
summer solstice is on the meridian      
7,fig4 #ce74 ,the angle between the     
ecliptic and the horizon is then equal  
to the elevation of the equinoctial plus
 #bc-a_/b4 ,in the latitude of ,new     
,york this angle  #di  #bc-a_/b         
#gb-a_/b4  ,of course the equinoxes1 the
 solstices1 and all other points on the 
ecliptic1 describe diurnal circles1 like
 every other point in the heavens3      
hence1 in our latitude1 these points    
rise and set every day4  #bf4 ,celestial
 ,latitude and ,longitude4--,celestial  

latitude is distance measured north or  
south from the ecliptic2 and celestial  
longitude is distance measured on the   
ecliptic eastward from the vernal       
equinox1 or the first point of ,aries4  
,great circles perpendicular to the     
ecliptic are called celestial meridians4
 ,these circles all pass through the    
poles of the ecliptic1 which are some   
#bc-a_/b from the poles of the          
equinoctial4 ,the latitude of a heavenly
 body is measured by the arc of a       
celestial meridian included between the 
body and the ecliptic4 ,the longitude of
 a heavenly body is measured by the arc 
of the ecliptic included between the    
first point of ,aries and the meridian  
which passes through the body4 ,there   
are1 of course1 always two arcs included
 between the first point of ,aries and  
the meridian1--one on the east1 and the 
other on the west1 of the first point of
 ,aries4 ,the one on the east is always 
taken as the measure of the longitude4  
#bg4 ,the ,precession of the            

,equinoxes4--,the equinoctial points    
have a slow westward motion along the   
ecliptic4 ,this motion is at the rate of
 about #ej'' a year1 and would cause the
 equinoxes to make a complete circuit of
 the heavens in a period of about       
twenty-six thousand years4 ,it is called
 the precession of the equinoxes4 ,this 
westward motion of the equinoxes is due 
to the fact that the axis of the earth  
has a slow gyratory motion1 like the    
handle of a spinning-top which has begun
 to wabble a little4 ,this gyratory     
motion causes the axis of the heavens to
 describe a cone in about twenty-six    
thousand years1 and the pole of the     
heavens to describe a circle about the  
pole of the ecliptic in the same time4  
,the radius of this circle is #bc-a_/b4 
,illustration3 ,fig4 #cf4      #bh4     
,illustration of ,precession4--,the     
precession of the equinoxes     may be  
illustrated by means of the apparatus   
shown in ,fig4 #cf4 ,the     horizontal 
and stationary ring ,,ec represents the 

ecliptic2 the     oblique ring ,e',q    
represents the equator2 ,v and ,a       
represent the     equinoctial point1 and
 ,e and ,c the solstitial points2 ,b    
represents the pole of the ecliptic1 ,p 
the pole of the equator1     and ,,po   
the celestial axis4 ,the ring ,e',q is  
supported on a pivot     at ,o2 and the 
rod ,,bp1 which connects ,b and ,p1 is  
jointed at     each end so as to admit  
of the movement of ,p and ,b4      ,on  
carrying ,p around ,b1 we shall see that
 ,e',q will always     preserve the same
 obliquity to ,,ec1 and that the points 
,v and ,a     will move around the      
circle ,,ec4 ,the same will also be true
 of the     points ,e and ,c4  #bi4     
,effects of ,precession4--,one effect of
 precession1 as has already been stated1
 is the revolution of the pole of the   
heavens around the pole of the ecliptic 
in a period of about twenty-six thousand
 years4 ,the circle described by the    
pole of the heavens1 and the position of
 the pole at various dates1 are shown in

 ,fig4 #cg1 where o indicates the       
position of the pole at the birth of    
,christ4 ,the numbers round the circle  
to the left of o are dates ,a4,d41 and  
those to the right of o are dates ,b4,c4
 ,it will be seen that the star at the  
end of the ,little ,bear's tail1 which  
is now near the north pole1 will be     
exactly at the pole about the year      
#bjjj4 ,it will then recede farther and 
farther from the pole till the year     
#aejjj ,a4,d41 when it will be about    
forty-seven degrees away from the pole4 
,it will be noticed that one of the     
stars of the ,dragon was the pole star  
about #bhjj years ,b4,c4 ,there are     
reasons to suppose that this was about  
the time of the building of the ,great  
,pyramid4  ,a second effect of          
precession is the shifting of the signs 
along the zodiac4 ,the zodiac is a belt 
of the heavens along the ecliptic1      
extending eight degrees from it on each 
side4 ,this belt is occupied by twelve  
constellations1 known as the zodiacal   

constellations4 ,they are ,aries1       
,taurus1 ,gemini1 ,cancer1 ,leo1 ,virgo1
 ,libra1 ,scorpio1 ,sagittarius1        
,capricornus1 ,aquarius1 and ,pisces4   
,the zodiac is also divided into twelve 
equal parts of thirty degrees each1     
called signs4 ,these signs have the same
 names as the twelve zodiacal           
constellations1 and when they were first
 named1 each sign occupied the same part
 of the zodiac as the corresponding     
constellation2 that is to say1 the sign 
,aries was in the constellation ,aries1 
and the sign ,taurus in the             
constellation ,taurus1 etc4 ,now the    
signs are always reckoned as beginning  
at the vernal equinox1 which is         
continually shifting along the ecliptic2
 so that the signs are continually      
moving along the zodiac1 while the      
constellations remain stationary3 hence 
it has come about that the first point  
of ,aries 7the sign7 is no longer in the
 constellation ,aries1 but in ,pisces4  
,illustration3 ,fig4 #cg4  ,fig4 #ch    

shows the position of the vernal equinox
 #bagj ,b4,c4 ,it was then in ,taurus1  
just south of the ,pleiades4 ,it has    
since moved from ,taurus1 through       
,aries1 and into ,pisces1 as shown in   
,fig4 #ci4  ,illustration3 ,fig4 #ch4   
,illustration3 ,fig4 #ci4  ,since       
celestial longitude and right ascension 
are both measured from the first point  
of ,aries1 the longitude and right      
ascension of the stars are slowly       
changing from year to year4 ,it will be 
seen1 from ,figs4 #ch and #ci1 that the 
declination is also slowly changing4    
#cj4 ,nutation4--,the gyratory motion of
 the earth's axis is not perfectly      
regular and uniform4 ,the earth's axis  
has a slight tremulous motion1          
oscillating to and fro through a short  
distance once in about nineteen years4  
,this tremulous motion of the axis      
causes the pole of the heavens to       
describe an undulating curve1 as shown  
in ,fig4 #dj1 and gives a slight        
unevenness to the motion of the         

equinoxes along the ecliptic4 ,this     
nodding motion of the axis is called    
nutation4  ,illustration3 ,fig4 #dj4    
#ca4 ,refraction4--,when a ray of light 
from one of the heavenly bodies enters  
the earth's atmosphere obliquely1 it    
will be bent towards a perpendicular to 
the surface of the atmosphere1 since it 
will be entering a denser medium4 ,as   
the ray traverses the atmosphere1 it    
will be continually passing into denser 
and denser layers1 and will therefore be
 bent more and more towards the         
perpendicular4 ,this bending of the ray 
is shown in ,fig4 #da4 ,a ray which     
started from ,a would enter the eye at  
,c1 as if it came from ,i3 hence a star 
at ,a would appear to be at ,i4         
,illustration3 ,fig4 #da4  ,atmospheric 
refraction displaces all the heavenly   
bodies from the horizon towards the     
zenith4 ,this is evident from ,fig4 #db4
 ,,od is the horizon1 and ,z the zenith1
 of an observer at ,o4 ,refraction would
 make a star at ,q appear at ,p3 in     

other words1 it would displace it       
towards the zenith4 ,a star in the      
zenith is not displaced by refraction1  
since the rays which reach the eye from 
it traverse the atmosphere vertically4  
,the farther a star is from the zenith1 
the more it is displaced by refraction1 
since the greater is the obliquity with 
which the rays from it enter the        
atmosphere4  ,illustration3 ,fig4 #db4  
,at the horizon the displacement by     
refraction is about half a degree2 but  
it varies considerably with the state of
 the atmosphere4 ,refraction causes a   
heavenly body to appear above the       
horizon longer than it really is above  
it1 since it makes it appear to be on   
the horizon when it is really half a    
degree below it4  ,the increase of      
refraction towards the horizon often    
makes the sun1 when near the horizon1   
appear distorted1 the lower limb of the 
sun being raised more than the upper    
limb4 ,this distortion is shown in ,fig4
 #dc4 ,the vertical diameter of the sun 

appears to be considerably less than the
 horizontal diameter4  ,illustration3   
,fig4 #dc4  #cb4 ,parallax4--,parallax  
is the displacement of an object caused 
by a change in the point of view from   
which it is seen4 ,thus in ,fig4 #dd1   
the top of the tower ,s would be seen   
projected against the sky at a by an    
observer at ,a1 and at b by an observer 
at ,b4 ,in passing from ,a to ,b1 the   
top of the tower is displaced from a to 
b1 or by the angle a,sb4 ,this angle is 
called the parallax of ,s1 as seen from 
,b instead of ,a4  ,illustration3 ,fig4 
#dd4  ,the geocentric parallax of a     
heavenly body is its displacement caused
 by its being seen from the surface of  
the earth1 instead of from the centre of
 the earth4 ,in ,fig4 #de1 ,r is the    
centre of the earth1 and ,o the point of
 observation on the surface of the      
earth4 ,stars at ,s1 ,s'1 and ,s''1     
would1 from the centre of the earth1    
appear at ,q1 ,q'1 and ,q''2 while from 
the point ,o on the surface of the      

earth1 these same stars would appear at 
,p1 ,p' and ,p''1 being displaced from  
their position1 as seen from the centre 
of the earth1 by the angles ,,qsp1      
,q',s',p'1 and ,q'',s'',p''4 ,it will be
 seen that parallax displaces a body    
from the zenith towards the horizon1 and
 that the parallax of a body is greatest
 when it is on the horizon4 ,the        
parallax of a heavenly body when on the 
horizon is called its horizontal        
parallax4 ,a body in the zenith is not  
displaced by parallax1 since it would be
 seen in the same direction from both   
the centre and the surface of the earth4
  ,illustration3 ,fig4 #de4  ,the       
parallax of a body at ,s''' is          
,q''',s''',p1 which is seen to be       
greater than ,,qsp2 that is to say1 the 
parallax of a heavenly body increases   
with its nearness to the earth4 ,the    
distance and parallax of a body are so  
related1 that1 either being known1 the  
other may be computed4      #cc4        
,aberration4--,aberration is a slight   

displacement of a star1     owing to an 
apparent change in the direction of the 
rays of light     which proceed from it1
 caused by the motion of the earth in   
its     orbit4 ,if we walk rapidly in   
any direction in the rain1 when the     
drops are falling vertically1 they will 
appear to come into our     faces from  
the direction in which we are walking4  
,our own motion has     apparently      
changed the direction in which the drops
 are falling4      ,illustration3 ,fig4 
#df4      ,in ,fig4 #df let ,a be a gun 
of a battery1 from which a shot is fired
     at a ship1 ,,de1 that is passing4  
,let ,,abc be the course of the         
shot4 ,the shot enters the ship's side  
at ,b1 and passes out at the     other  
side at ,c2 but in the mean time the    
ship has moved from ,e     to e1 and the
 part ,b1 where the shot entered1 has   
been carried     to b4 ,if a person on  
board the ship could see the ball as it 
crossed the ship1 he would see it cross 
in the diagonal line b,c2     and he    

would at once say that the cannon was in
 the direction of     ,cb4 ,if the ship 
were moving in the opposite direction1  
he would     say that the cannon was    
just as far the other side of its true  
position4      ,now1 we see a star in   
the direction in which the light coming 
from     the star appears to be moving4 
,when we examine a star with a          
telescope1 we are in the same condition 
as the person who on     shipboard saw  
the cannon-ball cross the ship4 ,the    
telescope is     carried along by the   
earth at the rate of eighteen miles a   
second3     hence the light will appear 
to pass through the tube in a slightly  
different direction from that in which  
it is really moving3 just as     the    
cannon-ball appears to pass through the 
ship in a different     direction from  
that in which it is really moving4 ,thus
 in ,fig4 #dg1 a     ray of light coming
 in the direction ,,sot would appear to 
traverse     the tube ,,ot of a         
telescope1 moving in the direction of   

the arrow1     as if it were coming in  
the direction ,s',o4  ,illustration3    
,fig4 #dg4      ,as light moves with    
enormous velocity1 it passes through the
 tube so     quickly1 that it is        
apparently changed from its true        
direction only     by a very slight     
angle3 but it is sufficient to displace 
the star4     ,this apparent change in  
the direction of light caused by the    
motion     of the earth is called       
aberration of light4  #cd4 ,the         
,planets4--,on watching the stars       
attentively night after night1 it will  
be found1 that while the majority of    
them appear fixed on the surface of the 
celestial sphere1 so as to maintain     
their relative positions1 there are a   
few that wander about among the stars1  
alternately advancing towards the east1 
halting1 and retrograding towards the   
west4 ,these wandering stars are called 
planets4  ,their motions appear quite   
irregular2 but1 on the whole1 their     
eastward motion is in excess of their   

westward1 and in a longer or shorter    
time they all complete the circuit of   
the heavens4 ,in almost every instance1 
their paths are found to lie wholly in  
the belt of the zodiac4  ,illustration3 
,fig4 #dh4  ,fig4 #dh shows a portion of
 the apparent path of one of the        
planets4                                
,,ii4                            ,,the  
,,solar ,,system4                       
,i4 ,,theory ,,of ,,the ,,solar         
,,system4   #ce4 ,members of the ,solar 
,system4--,the solar system is composed 
of the sun1 planets1 moons1 comets1 and 
meteors4 ,five planets1 besides the     
earth1 are readily distinguished by the 
naked eye1 and were known to the        
ancients3 these are ,mercury1 ,venus1   
,mars1 ,jupiter1 and ,saturn4 ,these1   
with the sun and moon1 made up the seven
 planets of the ancients1 from which the
 seven days of the week were named4     
,the ,ptolemaic ,system4   #cf4 ,the    
,crystalline ,spheres4--,we have seen   
that all the heavenly bodies appear to  

be situated on the surface of the       
celestial sphere4 ,the ancients assumed 
that the stars were really fixed on the 
surface of a crystalline sphere1 and    
that they were carried around the earth 
daily by the rotation of this sphere4   
,they had1 however1 learned to          
distinguish the planets from the stars1 
and they had come to the conclusion that
 some of the planets were nearer the    
earth than others1 and that all of them 
were nearer the earth than the stars    
are4 ,this led them to imagine that the 
heavens were composed of a number of    
crystalline spheres1 one above another1 
each carrying one of the planets1 and   
all revolving around the earth from east
 to west1 but at different rates4 ,this 
structure of the heavens is shown in    
section in ,fig4 #di4  ,illustration3   
,fig4 #di4  #cg4 ,cycles and            
,epicycles4--,the ancients had also     
noticed that1 while all the planets move
 around the heavens from west to east1  
their motion is not one of uniform      

advancement4 ,mercury and ,venus appear 
to oscillate to and fro across the sun1 
while ,jupiter and ,saturn appear to    
oscillate to and fro across a centre    
which is moving around the earth1 so as 
to describe a series of loops1 as shown 
in ,fig4 #ej4  ,illustration3 ,fig4 #ej4
  ,the ancients assumed that the planets
 moved in exact circles1 and1 in fact1  
that all motion in the heavens was      
circular1 the circle being the simplest 
and most perfect curve4 ,to account for 
the loops described by the planets1 they
 imagined that each planet revolved in a
 circle around a centre1 which1 in turn1
 revolved in a circle around the earth4 
,the circle described by this centre    
around the earth they called the cycle1 
and the circle described by the planet  
around this centre they called the      
epicycle4  #ch4 ,the ,eccentric4--,the  
ancients assumed that the planets moved 
at a uniform rate in describing the     
epicycle1 and also the centre in        
describing the cycle4 ,they had1        

however1 discovered that the planets    
advance eastward more rapidly in some   
parts of their orbits than in others4   
,to account for this they assumed that  
the cycles described by the centre1     
around which the planets revolved1 were 
eccentric2 that is to say1 that the     
earth was not at the centre of the      
cycle1 but some distance away from it1  
as shown in ,fig4 #ea4 ,e is the        
position of the earth1 and ,c is the    
centre of the cycle4 ,the lines from ,e 
are drawn so as to intercept equal arcs 
of the cycle4 ,it will be seen at once  
that the angle between any pair of lines
 is greatest at ,p1 and least at ,a2 so 
that1 were a planet moving at the same  
rate at ,p and ,a1 it would seem to be  
moving much faster at ,p4 ,the point ,p 
of the planet's cycle was called its    
perigee1 and the point ,a its apogee4   
,illustration3 ,fig4 #ea4  ,as the      
apparent motion of the planets became   
more accurately known1 it was found     
necessary to make the system of cycles1 

epicycles1 and eccentrics exceedingly   
complicated to represent that motion4   
,the ,copernican ,system4   #ci4        
,copernicus4--,copernicus simplified the
 ,ptolemaic system greatly by assuming  
that the earth and all the planets      
revolved about the sun as a centre4 ,he1
 however1 still maintained that all     
motion in the heavens was circular1 and 
hence he could not rid his system       
entirely of cycles and epicycles4       
,tycho ,brahe's ,system4   #dj4 ,tycho  
,brahe4--,tycho ,brahe was the greatest 
of the early astronomical observers4    
,he1 however1 rejected the system of    
,copernicus1 and adopted one of his own1
 which was much more complicated4 ,he   
held that all the planets but the earth 
revolved around the sun1 while the sun  
and moon revolved around the earth4     
,this system is shown in ,fig4 #eb4     
,illustration3 ,fig4 #eb4               
,kepler's ,system4   #da4               
,kepler4--,while ,tycho ,brahe devoted  
his life to the observation of the      

planets4 ,kepler gave his to the study  
of ,tycho's observations1 for the       
purpose of discovering the true laws of 
planetary motion4 ,he banished the      
complicated system of cycles1 epicycles1
 and eccentrics forever from the        
heavens1 and discovered the three laws  
of planetary motion which have rendered 
his name immortal4  #db4 ,the           
,ellipse4--,an ellipse is a closed curve
 which has two points within it1 the sum
 of whose distances from every point on 
the curve is the same4 ,these two points
 are called the foci of the ellipse4    
,illustration3 ,fig4 #ec4  ,one method  
of describing an ellipse is shown in    
,fig4 #ec4 ,two tacks1 ,f and ,f'1 are  
stuck into a piece of paper1 and to     
these are fastened the two ends of a    
string which is longer than the distance
 between the tacks4 ,a pencil is then   
placed against the string1 and carried  
around1 as shown in the figure4 ,the    
curve described by the pencil is an     
ellipse4 ,the two points ,f and ,f' are 

the foci of the ellipse3 the sum of the 
distances of these two points from every
 point on the curve is equal to the     
length of the string4 ,when half of the 
ellipse has been described1 the pencil  
must be held against the other side of  
the string in the same way1 and carried 
around as before4  ,the point ,o1 half  
way between ,f and ,f'1 is called the   
centre of the ellipse2 ,,aa' is the     
major axis of the ellipse1 and ,,cd is  
the minor axis4  #dc4 ,the ,eccentricity
 of the ,ellipse4--,the ratio of the    
distance between the two foci to the    
major axis of the ellipse is called the 
eccentricity of the ellipse4 ,the       
greater the distance between the two    
foci1 compared with the major axis of   
the ellipse1 the greater is the         
eccentricity of the ellipse2 and the    
less the distance between the foci1     
compared with the length of the major   
axis1 the less the eccentricity of the  
ellipse4 ,the ellipse of ,fig4 #ed has  
an eccentricity of #a_/h4 ,this ellipse 

scarcely differs in appearance from a   
circle4 ,the ellipse of ,fig4 #ee has an
 eccentricity of #a_/b1 and that of     
,fig4 #ef an eccentricity of #g_/h4     
,illustration3 ,fig4 #ed4               
,illustration3 ,fig4 #ee4               
,illustration3 ,fig4 #ef4  #dd4         
,kepler's ,first ,law4--,kepler first   
discovered that all the planets move    
from west to east in ellipses which have
 the sun as a common focus4 ,this law of
 planetary motion is known as ,kepler's 
,first ,law4 ,the planets appear to     
describe loops1 because we view them    
from a moving point4  ,the ellipses     
described by the planets differ in      
eccentricity2 and1 though they all have 
one focus at the sun1 their major axes  
have different directions4 ,the         
eccentricity of the planetary orbits is 
comparatively small4 ,the ellipse of    
,fig4 #ed has seven times the           
eccentricity of the earth's orbit1 and  
twice that of the orbit of any of the   
larger planets except ,mercury2 and its 

eccentricity is more than half of that  
of the orbit of ,mercury4 ,owing to     
their small eccentricity1 the orbits of 
the planets are usually represented by  
circles in astronomical diagrams4       
,illustration3 ,fig4 #eg4  #de4         
,kepler's ,second ,law4--,kepler next   
discovered that a planet's rate of      
motion in the various parts of its orbit
 is such that a line drawn from the     
planet to the sun would always sweep    
over equal areas in equal times4 ,thus1 
in ,fig4 #eg1 suppose the planet would  
move from ,p to ,p#a in the same time   
that it would move from ,p#b to ,p#c1 or
 from ,p#d to ,p#e2 then the dark       
spaces1 which would be swept over by a  
line joining the sun and the planet1 in 
these equal times1 would all be equal4  
,a line drawn from the sun to a planet  
is called the radius vector of the      
planet4 ,the radius vector of a planet  
is shortest when the planet is nearest  
the sun1 or at perihelion1 and longest  
when the planet is farthest from the    

sun1 or at aphelion3 hence1 in order to 
have the areas equal1 it follows that a 
planet must move fastest when at        
perihelion1 and slowest at aphelion4    
,kepler's ,second ,law of planetary     
motion is usually stated as follows3    
,the radius vector of a planet describes
 equal areas in equal times in every    
part of the planet's orbit4  #df4       
,kepler's ,third ,law4--,kepler finally 
discovered that the periodic times of   
the planets bear the following relation 
to the distances of the planets from the
 sun3 ,the squares of the periodic times
 of the planets are to each other as the
 cubes of their mean distances from the 
sun4 ,this is known as ,kepler's ,third 
,law of planetary motion4 ,by periodic  
time is meant the time it takes a planet
 to revolve around the sun4  ,these     
three laws of ,kepler's are the         
foundation of modern physical astronomy4
                            ,the        
,newtonian ,system4   #dg4 ,newton's    
,discovery4--,newton followed ,kepler1  

and by means of his three laws of       
planetary motion made his own immortal  
discovery of the law of gravitation4    
,this law is as follows3 ,every portion 
of matter in the universe attracts every
 other portion with a force varying     
directly as the product of the masses   
acted upon1 and inversely as the square 
of the distances between them4  #dh4    
,the ,conic ,sections4--,the conic      
sections are the figures formed by the  
various plane sections of a right cone4 
,there are four classes of figures      
formed by these sections1 according to  
the angle which the plane of the section
 makes with the axis of the cone4       
,,opq1 ,fig4 #eh1 is a right cone1 and  
,,on is its axis4 ,any section1 ,,ab1 of
 this cone1 whose plane is perpendicular
 to the axis of the cone1 is a circle4  
,illustration3 ,fig4 #eh4  ,any section1
 ,,cd1 of this cone1 whose plane is     
oblique to the axis1 but forms with it  
an angle greater than ,,nop1 is an      
ellipse4 ,the less the angle which the  

plane of the section makes with the     
axis1 the more elongated is the ellipse4
  ,any section1 ,,ef1 of this cone1     
whose plane makes with the axis an angle
 equal to ,,nop1 is a parabola4 ,it will
 be seen1 that1 by changing the         
obliquity of the plane ,,cd to the axis 
,,no1 we may pass uninterruptedly from  
the circle through ellipses of greater  
and greater elongation to the parabola4 
,any section1 ,,gh1 of this cone1 whose 
plane makes with the axis ,,on an angle 
less than ,,nop1 is a hyperbola4        
,illustration3 ,fig4 #ei4  ,it will be  
seen from ,fig4 #ei1 in which           
comparative views of the four conic     
sections are given1 that the circle and 
the ellipse are closed curves1 or curves
 which return into themselves4 ,the     
parabola and the hyperbola are1 on the  
contrary1 open curves1 or curves which  
do not return into themselves4      #di4
 ,a ,revolving ,body is continually     
,falling towards its ,centre of         
,revolution4--,in ,fig4 #fj let ,m      

represent the moon1 and ,e the     earth
 around which the moon is revolving in  
the direction ,,mn4 ,it     will be seen
 that the moon1 in moving from ,m to ,n1
 falls towards the     earth a distance 
equal to m,n4 ,it is kept from falling  
into the     earth by its orbital       
motion4  ,illustration3 ,fig4 #fj4      
,the fact that a body might be projected
 forward fast enough to keep     it from
 falling into the earth is evident from 
,fig4 #fa4 ,,ab     represents the level
 surface of the ocean1 ,c a mountain    
from the     summit of which a          
cannon-ball is supposed to be fired in  
the     direction ,,ce4 ,,ad is a line  
parallel with ,,ce2 ,,db is a line      
equal to the distance between the two   
parallel lines ,,ad and ,,ce4     ,this 
distance is equal to that over which    
gravity would pull a ball     towards   
the centre of the earth in a minute4 ,no
 matter1 then1 with     what velocity   
the ball ,c is fired1 at the end of a   
minute it will     be somewhere on the  

line ,,ad4 ,suppose it were fired fast  
enough to     reach the point ,d in a   
minute3 it would be on the line ,,ad at 
the     end of the minute1 but still    
just as far from the surface of the     
water as when it started4 ,it will be   
seen1 that1 although it has all     the 
while been falling towards the earth1 it
 has all the while kept     at exactly  
the same distance from the surface4 ,the
 same thing would     of course be true 
during each succeeding minute1 till the 
ball came     round to ,c again1 and the
 ball would continue to revolve in a    
circle around the earth4  ,illustration3
 ,fig4 #fa4  #ej4 ,the ,form of a       
,body's ,orbit depends upon the ,rate of
 its ,forward ,motion4--,if the ball ,c 
were fired fast enough to reach the line
 ,,ad beyond the point ,d1 it would be  
farther from the surface at the end of  
the second than when it started4 ,its   
orbit would no longer be circular1 but  
elliptical4 ,if the speed of projection 
were gradually augmented1 the orbit     

would become a more and more elongated  
ellipse4 ,at a certain rate of          
projection1 the orbit would become a    
parabola2 at a still greater rate1 a    
hyperbola4  #ea4 ,the ,moon held in her 
,orbit by ,gravity4--,newton compared   
the distance m,n that the moon is drawn 
to the earth in a given time1 with the  
distance a body near the surface of the 
earth would be pulled toward the earth  
in the same time2 and he found that     
these distances are to each other       
inversely as the squares of the         
distances of the two bodies from the    
centre of the earth4 ,he therefore      
concluded that the moon is drawn to the 
earth by gravity1 and that the intensity
 of gravity decreases as the square of  
the distance increases4  ,illustration3 
,fig4 #fb4  #eb4 ,any ,body whose ,orbit
 is a ,conic ,section1 and which moves  
according to ,kepler's ,second ,law1 is 
acted upon by a ,force varying inversely
 as the ,square of the                  
,distance4--,newton compared the        

distance which any body1 moving in an   
ellipse1 according to ,kepler's ,second 
,law1 is drawn towards the sun in the   
same time in different parts of its     
orbit4 ,he found these distances in all 
cases to vary inversely as the square of
 the distance of the planet from the    
sun4 ,thus1 in ,fig4 #fb1 suppose a     
planet would move from ,k to ,b in the  
same time that it would move from k to b
 in another part of its orbit4 ,in the  
first instance the planet would be drawn
 towards the sun the distance ,,ab1 and 
in the second instance the distance ab4 
,newton found that ,,ab 3 ab  7,,sk7#b 3
 7,sk7#b4 ,he also found that the same  
would be true when the body moved in a  
parabola or a hyperbola3 hence he       
concluded that every body that moves    
around the sun in an ellipse1 a         
parabola1 or a hyperbola1 is moving     
under the influence of gravity4         
,transcriber's ,note3 ,in ,newton's     
equation above1 7,,sk7#b means to group 
,s and ,k together and square their     

product4 ,in the original book1 instead 
of using parentheses1 there was a       
vinculum1 a horizontal bar1 drawn over  
the ,s and the ,k to express the same   
grouping4  ,illustration3 ,fig4 #fc4    
#ec4 ,the ,force that draws the         
,different ,planets to the ,sun ,varies 
inversely as the ,squares of the        
,distances of the ,planets from the     
,sun4--,newton compared the distances   
j,k and e,f1 over which two planets are 
drawn towards the sun in the same time1 
and found these distances to vary       
inversely as the squares of the         
distances of the planets from the sun3  
hence he concluded that all the planets 
are held in their orbits by gravity4 ,he
 also showed that this would be true of 
any two bodies that were revolving      
around the sun's centre1 according to   
,kepler's ,third ,law4  #ed4 ,the       
,copernican ,system4--,the theory of the
 solar system which originated with     
,copernicus1 and which was developed and
 completed by ,kepler and ,newton1 is   

commonly known as the ,copernican       
,system4 ,this system is shown in ,fig4 
#fd4  ,illustration3 ,fig4 #fd4         
,,ii4 ,,the ,,sun ,,and ,,planets4      
,i4 ,,the ,,earth4                      
,form and ,size4   #ee4 ,form of the    
,earth4--,in ordinary language the term 
horizon denotes the line that bounds the
 portion of the earth's surface that is 
visible at any point4  7#a7 ,it is well 
known that the horizon of a plain       
presents the form of a circle           
surrounding the observer4 ,if the latter
 moves1 the circle moves also2 but its  
form remains the same1 and is modified  
only when mountains or other obstacles  
limit the view4 ,out at sea1 the        
circular form of the horizon is still   
more decided1 and changes only near the 
coasts1 the outline of which breaks the 
regularity4  ,here1 then1 we obtain a   
first notion of the rotundity of the    
earth1 since a sphere is the only body  
which is presented always to us under   
the form of a circle1 from whatever     

point on its surface it is viewed4  7#b7
 ,moreover1 it cannot be maintained that
 the horizon is the vanishing point of  
distinct vision1 and that it is this    
which causes the appearance of a        
circular boundary1 because the horizon  
is enlarged when we mount above the     
surface of the plain4 ,this will be     
evident from ,fig4 #fe1 in which a      
mountain is depicted in the middle of a 
plain1 whose uniform curvature is that  
of a sphere4 ,from the foot of the      
mountain the spectator will have but a  
very limited horizon4 ,let him ascend   
half way1 his visual radius extends1 is 
inclined below the first horizon1 and   
reveals a more extended circular area4  
,at the summit of the mountain the      
horizon still increases2 and1 if the    
atmosphere is pure1 the spectator will  
see numerous objects where from the     
lower stations the sky alone was        
visible4  ,illustration3 ,fig4 #fe4     
,this extension of the horizon would be 
inexplicable if the earth had the form  

of an extended plane4  7#c7 ,the        
curvature of the surface of the sea     
manifests itself in a still more        
striking manner4 ,if we are on the coast
 at the summit of a hill1 and a vessel  
appears on the horizon 7,fig4 #ff71 we  
see only the tops of the masts and the  
highest sails2 the lower sails and the  
hull are invisible4 ,as the vessel      
approaches1 its lower part comes into   
view above the horizon1 and soon it     
appears entire4  ,illustration3 ,fig4   
#ff4  ,in the same manner the sailors   
from the ship see the different parts of
 objects on the land appear             
successively1 beginning with the        
highest4 ,the reason of this will be    
evident from ,fig4 #fg1 where the course
 of a vessel1 seen in profile1 is       
figured on the convex surface of the    
sea4  ,illustration3 ,fig4 #fg4  ,as the
 curvature of the ocean is the same in  
every direction1 it follows that the    
surface of the ocean is spherical4 ,the 
same is true of the surface of the land1

 allowance being made for the various   
inequalities of the surface4 ,from these
 and various other indications1 we      
conclude that the earth is a sphere4    
#ef4 ,size of the ,earth4--,the size of 
the earth is ascertained by measuring   
the length of a degree of a meridian1   
and multiplying this by three hundred   
and sixty4 ,this gives the circumference
 of the earth as about twenty-five      
thousand miles1 and its diameter as     
about eight thousand miles4 ,we know    
that the two stations between which we  
measure are one degree apart when the   
elevation of the pole at one station is 
one degree greater than at the other4   
#eg4 ,the ,earth ,flattened at the      
,poles4--,degrees on the meridian have  
been measured in various parts of the   
earth1 and it has been found that they  
invariably increase in length as we     
proceed from the equator towards the    
pole3 hence the earth must curve less   
and less rapidly as we approach the     
poles2 for the less the curvature of a  

circle1 the larger the degrees on it4   
,illustration3 ,fig4 #fh4  #eh4 ,the    
,earth in ,space4--,in ,fig4 #fh we have
 a view of the earth suspended in space4
 ,the side of the earth turned towards  
the sun is illumined1 and the other side
 is in darkness4 ,as the planet rotates 
on its axis1 successive portions of it  
will be turned towards the sun4 ,as     
viewed from a point in space between it 
and the sun1 it will present light and  
dark portions1 which will assume        
different forms according to the portion
 which is illumined4 ,these different   
appearances are shown in ,fig4 #fi4     
,illustration3 ,fig4 #fi4               
,day and ,night4   #ei4 ,day and        
,night4--,the succession of day and     
night is due to the rotation of the     
earth on its axis1 by which a place on  
the surface of the earth is carried     
alternately into the sunshine and out of
 it4 ,as the sun moves around the       
heavens on the ecliptic1 it will be on  
the celestial equator when at the       

equinoxes1 and #bc-a_/b north of the    
equator when at the summer solstice1 and
 #bc-a_/b south of the equator when at  
the winter solstice4  #fj4 ,day and     
,night when the ,sun is at the          
,equinoxes4--,when the sun is at either 
equinox1 the diurnal circle described by
 the sun will coincide with the         
celestial equator2 and therefore half of
 this diurnal circle will be above the  
horizon at every point on the surface of
 the globe4 ,at these times day and     
night will be equal in every part of the
 earth4  ,illustration3 ,fig4 #gj4      
,illustration3 ,fig4 #ga4      ,the     
equality of days and nights when the sun
 is on the celestial     equator is also
 evident from the following             
considerations3 one-half     of the     
earth is in sunshine all of the time2   
when the sun is on the     celestial    
equator1 it is directly over the equator
 of the earth1 and     the illumination 
extends from pole to pole1 as is evident
 from ,figs4     #gj and #ga1 in the    

former of which the sun is represented  
as on the     eastern horizon at a place
 along the central line of the figure1  
and     in the latter as on the meridian
 along the same line4 ,in each          
diagram it is seen that the illumination
 extends from pole to pole3     hence1  
as the earth rotates on its axis1 every 
place on the surface     will be in the 
sunshine and out of it just half of the 
time4  #fa4 ,day and ,night when the    
,sun is at the ,summer ,solstice4--,when
 the sun is at the summer solstice1 it  
will be #bc-a_/b north of the celestial 
equator4 ,the diurnal circle described  
by the sun will then be #bc-a_/b north  
of the celestial equator2 and more than 
half of this diurnal circle will be     
above the horizon at all places north of
 the equator1 and less than half of it  
at places south of the equator3 hence   
the days will be longer than the nights 
at places north of the equator1 and     
shorter than the nights at places south 
of the equator4 ,at places within       

#bc-a_/b of the north pole1 the entire  
diurnal circle described by the sun will
 be above the horizon1 so that the sun  
will not set4 ,at places within #bc-a_/b
 of the south pole of the earth1 the    
entire diurnal circle will be below the 
horizon1 so that the sun will not rise4 
,illustration3 ,fig4 #gb4               
,illustration3 ,fig4 #gc4      ,the     
illumination of the earth at this time  
is shown in ,figs4 #gb and     #gc4 ,in 
,fig4 #gb the sun is represented as on  
the western horizon     along the middle
 line of the figure1 and in ,fig4 #gc as
 on the     meridian4 ,it is seen at    
once that the illumination extends      
#bc-a_/b     beyond the north pole1 and 
falls #bc-a_/b short of the south pole4 
,as     the earth rotates on its axis1  
places near the north pole will be in   
the sunshine all the time1 while places 
near the south pole will be     out of  
the sunshine all the time4 ,all places  
north of the equator     will be in the 
sunshine longer than they are out of it1

 while all     places south of the      
equator will be out of the sunshine     
longer than     they are in it4  #fb4   
,day and ,night when the ,sun is at the 
,winter ,solstice4--,when the sun is at 
the winter solstice1 it is #bc-a_/b     
south of the celestial equator4 ,the    
diurnal circle described by the sun is  
then #bc-a_/b south of the celestial    
equator4 ,more than half of this diurnal
 circle will therefore be above the     
horizon at all places south of the      
equator1 and less than half of it at all
 places north of the equator3 hence the 
days will be longer than the nights     
south of the equator1 and shorter than  
the nights at places north of the       
equator4 ,at places within #bc-a_/b of  
the south pole1 the diurnal circle      
described by the sun will be entirely   
above the horizon1 and the sun will     
therefore not set4 ,at places within    
#bc-a_/b of the north pole1 the diurnal 
circle described by the sun will be     
wholly below the horizon1 and therefore 

the sun will not rise4      ,the        
illumination of the earth at this time  
is shown in ,figs4 #gd and     #ge1 and 
is seen to be the reverse of that shown 
in ,figs4 #gb and #gc4  ,illustration3  
,fig4 #gd4  ,illustration3 ,fig4 #ge4   
#fc4 ,variation in the ,length of ,day  
and ,night4--,as long as the sun is     
north of the equinoctial1 the nights    
will be longer than the days south of   
the equator1 and shorter than the days  
north of the equator4 ,it is just the   
reverse when the sun is south of the    
equator4  ,the farther the sun is from  
the equator1 the greater is the         
inequality of the days and nights4  ,the
 farther the place is from the equator1 
the greater the inequality of its days  
and nights4  ,when the distance of a    
place from the north pole is less than  
the distance of the sun north of the    
equinoctial1 it will have continuous day
 without night1 since the whole of the  
sun's diurnal circle will be above the  
horizon4 ,a place within the same       

distance of the south pole will have    
continuous night4  ,when the distance of
 a place from the north pole is less    
than the distance of the sun south of   
the equinoctial1 it will have continuous
 night1 since the whole of the sun's    
diurnal circle will then be below the   
horizon4 ,a place within the same       
distance of the south pole will then    
have continuous day4  ,at the equator   
the days and nights are always equal2   
since1 no matter where the sun is in the
 heavens1 half of all the diurnal       
circles described by it will be above   
the horizon1 and half of them below it4 
#fd4 ,the ,zones4--,it will be seen1    
from what has been stated above1 that   
the sun will at some time during the    
year be directly overhead at every place
 within #bc-a_/b of the equator on      
either side4 ,this belt of the earth is 
called the torrid zone4 ,the torrid zone
 is bounded by circles called the       
tropics2 that of ,cancer on the north1  
and that of ,capricorn on the south4    

,it will also be seen1 that1 at every   
place within #bc-a_/b of either pole1   
there will be1 some time during the     
year1 a day during which the sun will   
not rise1 or on which it will not set4  
,these two belts of the earth's surface 
are called the frigid zones4 ,these     
zones are bounded by the arctic circles4
 ,the nearer a place is to the poles1   
the greater the number of days on which 
the sun does not rise or set4  ,between 
the frigid zones and the torrid zones1  
there are two belts on the earth which  
are called the temperate zones4 ,the sun
 is never overhead at any place in these
 two zones1 but it rises and sets every 
day at every place within their limits4 
#fe4 ,the ,width of the ,zones4--,the   
distance the frigid zones extend from   
the poles1 and the torrid zones from the
 equator1 is exactly equal to the       
obliquity of the ecliptic1 or the       
deviation of the axis of the earth from 
the perpendicular to the plane of its   
orbit4 ,were this deviation forty-five  

degrees1 the obliquity of the ecliptic  
would be forty-five degrees1 the torrid 
zone would extend forty-five degrees    
from the equator1 and the frigid zones  
forty-five degrees from the poles4 ,in  
this case there would be no temperate   
zones4 ,were this deviation fifty       
degrees1 the torrid and frigid zones    
would overlap ten degrees1 and there    
would be two belts of ten degrees on the
 earth1 which would experience          
alternately during the year a torrid and
 a frigid climate4  ,were the axis of   
the earth perpendicular to the plane of 
the earth's orbit1 there would be no    
zones on the earth1 and no variation in 
the length of day and night4  #ff4      
,twilight4--,were it not for the        
atmosphere1 the darkness of midnight    
would begin the moment the sun sank     
below the horizon1 and would continue   
till he rose again above the horizon in 
the east1 when the darkness of the night
 would be suddenly succeeded by the full
 light of day4 ,the gradual transition  

from the light of day to the darkness of
 the night1 and from the darkness of the
 night to the light of day1 is called   
twilight1 and is due to the diffusion of
 light from the upper layers of the     
atmosphere after the sun has ceased to  
shine on the lower layers at night1 or  
before it has begun to shine on them in 
the morning4  ,illustration3 ,fig4 #gf4 
,let ,,abcd 7,fig4 #gf7 represent a     
portion of the earth1 ,a a point on its 
surface where the sun ,s is setting2 and
 let ,,sah be a ray of light just       
grazing the earth at ,a1 and leaving the
 atmosphere at the point ,h4 ,the point 
,a is illuminated by the whole          
reflective atmosphere ,,hgfe4 ,the point
 ,b1 to which the sun has set1 receives 
no direct solar light1 nor any reflected
 from that part of the atmosphere which 
is below ,,alh2 but it receives a       
twilight from the portion ,,hlf1 which  
lies above the visible horizon ,,bf4    
,the point ,c receives a twilight only  
from the small portion of the           

atmosphere2 while at ,d the twilight has
 ceased altogether4  #fg4 ,duration of  
,twilight4--,the astronomical limit of  
twilight is generally understood to be  
the instant when stars of the sixth     
magnitude begin to be visible in the    
zenith at evening1 or disappear in the  
morning4      ,twilight is usually      
reckoned to last until the sun's        
depression     below the horizon amounts
 to eighteen degrees3 this1 however1    
varies2 in the tropics a depression of  
sixteen or seventeen degrees     being  
sufficient to put an end to the         
phenomenon1 while in ,england a         
depression of seventeen to twenty-one   
degrees is required4 ,the     duration  
of twilight differs in different        
latitudes2 it varies also     in the    
same latitude at different seasons of   
the year1 and depends1     in some      
measure1 on the meteorological condition
 of the atmosphere4     ,when the sky is
 of a pale color1 indicating the        
presence of an     unusual amount of    

condensed vapor1 twilight is of longer  
duration4     ,this happens habitually  
in the polar regions4 ,on the contrary1 
within the tropics1 where the air is    
pure and dry1 twilight     sometimes    
lasts only fifteen minutes4 ,strictly   
speaking1 in the     latitude of        
,greenwich there is no true night from  
,may #bb to ,july #ba1     but constant 
twilight from sunset to sunrise4        
,twilight reaches its     minimum three 
weeks before the vernal equinox1 and    
three weeks after     the autumnal      
equinox1 when its duration is an hour   
and fifty     minutes4 ,at midwinter it 
is longer by about seventeen minutes2   
but     the augmentation is frequently  
not perceptible1 owing to the greater   
prevalence of clouds and haze at that   
season of the year1 which     intercept 
the light1 and hinder it from reaching  
the earth4 ,the     duration is least at
 the equator 7an hour and twelve        
minutes71 and     increases as we       
approach the poles2 for at the former   

there are two     twilights every       
twenty-four hours1 but at the latter    
only two in a     year1 each lasting    
about fifty days4 ,at the north pole the
 sun is     below the horizon for six   
months1 but from ,jan4 #bi to the vernal
     equinox1 and from the autumnal     
equinox to ,nov4 #ab1 the sun is less   
than eighteen degrees below the horizon2
 so that there is twilight     during   
the whole of these intervals1 and thus  
the length of the     actual night is   
reduced to two months and a half4 ,the  
length of the     day in these regions  
is about six months1 during the whole of
 which     time the sun is constantly   
above the horizon4 ,the general rule is1
     that to the inhabitants of an      
oblique sphere the twilight is longer   
in proportion as the place is nearer the
 elevated pole1 and the sun     is      
farther from the equator on the side of 
the elevated pole4                      
,the ,seasons4   #fh4 ,the              
,seasons4--,while the sun is north of   

the celestial equator1 places north of  
the equator are receiving heat from the 
sun by day longer than they are losing  
it by radiation at night1 while places  
south of the equator are losing heat by 
radiation at night longer than they are 
receiving it from the sun by day4 ,when1
 therefore1 the sun passes north of the 
equator1 the temperature begins to rise 
at places north of the equator1 and to  
fall at places south of it4 ,the rise of
 temperature is most rapid north of the 
equator when the sun is at the summer   
solstice2 but1 for some time after this1
 the earth continues to receive more    
heat by day than it loses by night1 and 
therefore the temperature continues to  
rise4 ,for this reason1 the heat is more
 excessive after the sun passes the     
summer solstice than before it reaches  
it4  #fi4 ,the ,duration of the         
,seasons4--,summer is counted as        
beginning in ,june1 when the sun is at  
the summer solstice1 and as continuing  
until the sun reaches the autumnal      

equinox1 in ,september4 ,autumn then    
begins1 and continues until the sun is  
at the winter solstice1 in ,december4   
,winter follows1 continuing until the   
sun comes to the vernal equinox1 in     
,march1 when spring begins1 and         
continues to the summer solstice4 ,in   
popular reckoning the seasons begin with
 the first day of ,june1 ,september1    
,december1 and ,march4  ,the reason why 
winter is counted as occurring after the
 winter solstice is similar to the      
reason why the summer is placed after   
the summer solstice4 ,the earth north of
 the equator is losing heat most rapidly
 at the time of the winter solstice2 but
 for some time after this it loses more 
heat by night than it receives by day3  
hence for some time the temperature     
continues to fall1 and the cold is more 
intense after the winter solstice than  
before it4  ,illustration3 ,fig4 #gg4   
,of course1 when it is summer in the    
northern hemisphere1 it is winter in the
 southern hemisphere1 and the reverse4  

,fig4 #gg shows the portion of the      
earth's orbit included in each season4  
,it will be seen that the earth is at   
perihelion in the winter season for     
places north of the equator1 and at     
aphelion in the summer season4 ,this    
tends to mitigate somewhat the extreme  
temperatures of our winters and summers4
  ,illustration3 ,fig4 #gh4  #gj4 ,the  
,illumination of the ,earth at the      
different ,seasons4--,fig4 #gh shows the
 earth as it would appear to an observer
 at the sun during each of the four     
seasons2 that is to say1 the portion of 
the earth that is receiving the sun's   
rays4 ,figs4 #gi1 #hj1 #ha1 and #hb are 
enlarged views of the earth1 as seen    
from the sun at the time of the summer  
solstice1 of the autumnal equinox1 of   
the winter solstice1 and of the vernal  
equinox4  ,illustration3 ,fig4 #gi4     
,illustration3 ,fig4 #hj4               
,illustration3 ,fig4 #ha4               
,illustration3 ,fig4 #hb4               
,illustration3 ,fig4 #hc4  ,fig4 #hc is1

 so to speak1 a side view of the earth1 
showing the limit of sunshine on the    
earth when the sun is at the summer     
solstice2 and ,fig4 #hd1 showing the    
limit of sunshine when the sun is at the
 autumnal equinox4  ,illustration3 ,fig4
 #hd4  #ga4 ,cause of the ,change of    
,seasons4--,variety in the length of day
 and night1 and diversity in the        
seasons1 depend upon the obliquity of   
the ecliptic4 ,were there no obliquity  
of the ecliptic1 there would be no      
inequality in the length of day and     
night1 and but slight diversity of      
seasons4 ,the greater the obliquity of  
the ecliptic1 the greater would be the  
variation in the length of the days and 
nights1 and the more extreme the changes
 of the seasons4                        
,tides4   #gb4 ,tides4--,the alternate  
rise and fall of the surface of the sea 
twice in the course of a lunar day1 or  
of twenty-four hours and fifty-one      
minutes1 is known as the tides4 ,when   
the water is rising1 it is said to be   

flood tide2 and when it is falling1 ebb 
tide4 ,when the water is at its greatest
 height1 it is said to be high water2   
and when at its least height1 low water4
      #gc4 ,cause of the ,tides4--,it   
has been known to seafaring nations     
from a remote antiquity that there is a 
singular connection between     the ebb 
and flow of the tides and the diurnal   
motion of the moon4  ,illustration3     
,fig4 #he4      ,this tidal movement in 
seeming obedience to the moon was a     
mystery     until the study of the law  
of gravitation showed it to be due to   
the attraction of the moon on the waters
 of the ocean4 ,the reason     why there
 are two tides a day will appear from   
,fig4 #he4 ,let ,m be     the moon1 ,e  
the earth1 and ,,em the line joining    
their centres4     ,now1 strictly       
speaking1 the moon does not revolve     
around the earth     any more than the  
earth around the moon2 but the centre of
 each body     moves around the common  
centre of gravity of the two bodies4    

,the     earth being eighty times as    
heavy as the moon1 this centre is       
situated within the former1 about       
three-quarters of the way from its      
centre to its surface1 at the point ,g4 
,the body of the earth     itself being 
solid1 every part of it1 in consequence 
of the moon's     attraction1 may be    
considered as describing a circle once  
in a     month1 with a radius equal to  
,,eg4 ,the centrifugal force caused by  
this rotation is just balanced by the   
mean attraction of the moon     upon the
 earth4 ,if this attraction were the    
same on every part of     the earth1    
there would be everywhere an exact      
balance between it and     the          
centrifugal force4 ,but as we pass from 
,e to ,d the attraction     of the moon 
diminishes1 owing to the increased      
distance3 hence at     ,d the           
centrifugal force predominates1 and the 
water therefore     tends to move away  
from the centre ,e4 ,as we pass from ,e 
towards     ,c1 the attraction of the   

moon increases1 and therefore exceeds   
the     centrifugal force3 consequently 
at ,c there is a tendency to draw       
the water towards the moon1 but still   
away from the centre ,e4 ,at     ,a and 
,b the attraction of the moon increases 
the gravity of the     water1 owing to  
the convergence of the lines ,,bm and   
,,am1 along     which it acts3 hence the
 action of the moon tends to make the   
waters     rise at ,d and ,c1 and to    
fall at ,a and ,b1 causing two tides    
to each apparent diurnal revolution of  
the moon4      #gd4 ,the ,lagging of the
 ,tides4--,if the waters everywhere     
yielded     immediately to the          
attractive force of the moon1 it would  
always be     high water when the moon  
was on the meridian1 low water when she 
was     rising or setting1 and high     
water again when she was on the meridian
     below the horizon4 ,but1 owing to  
the inertia of the water1 some time     
is necessary for so slight a force to   
set it in motion2 and1 once in          

motion1 it continues so after the force 
has ceased1 and until it has     acted  
some time in the opposite direction4    
,therefore1 if the motion     of the    
water were unimpeded1 it would not be   
high water until some     hours after   
the moon had passed the meridian4 ,the  
free motion of the     water is also    
impeded by the islands and continents4  
,these deflect     the tidal wave from  
its course in such a way that it may1 in
 some     cases1 be many hours1 or even 
a whole day1 behind its time4           
,sometimes two waves meet each other1   
and raise a very high tide4 ,in     some
 places the tides run up a long bay1    
where the motion of a large     mass of 
water will cause an enormous tide to be 
raised4 ,in the ,bay     of ,fundy both 
of these causes are combined4 ,a tidal  
wave coming up     the ,atlantic coast  
meets the ocean wave from the east1 and1
 entering     the bay with their        
combined force1 they raise the water at 
the head     of it to the height of     

sixty or seventy feet4  #ge4            
,spring-,tides and ,neap-,tides4--,the  
sun produces a tide as well as the moon2
 but the tide-producing force of the sun
 is only about four-tenths of that of   
the moon4 ,at new and full moon the two 
bodies unite their forces1 the ebb and  
flow become greater than the average1   
and we have the spring-tides4 ,when the 
moon is in her first or third quarter1  
the two forces act against each other2  
the tide-producing force is the         
difference of the two2 the ebb and flow 
are less than the average2 and we have  
the neap-tides4  ,illustration3 ,fig4   
#hf4  ,illustration3 ,fig4 #hg4         
,illustration3 ,fig4 #hh4  ,fig4 #hf    
shows the tide that would be produced by
 the moon alone2 ,fig4 #hg1 the tide    
produced by the combined action of the  
sun and moon2 and ,fig4 #hh1 by the sun 
and moon acting at right angles to each 
other4  ,the tide is affected by the    
distance of the moon from the earth1    
being highest near the time when the    

moon is in perigee1 and lowest near the 
time when she is in apogee4 ,when the   
moon is in perigee1 at or near the time 
of a new or full moon1 unusually high   
tides occur4      #gf4 ,diurnal         
,inequality of ,tides4--,the height of  
the tide at a     given place is        
influenced by the declination of the    
moon4 ,when the     moon has no         
declination1 the highest tides should   
occur along the     equator1 and the    
heights should diminish from thence     
toward the     north and south2 but the 
two daily tides at any place should have
     the same height4 ,when the moon has
 north declination1 as shown in         
,fig4 #hi1 the highest tides on the side
 of the earth next the moon     will be 
at places having a corresponding north  
latitude1 as at ,b1     and on the      
opposite side at those which have an    
equal south     latitude4 ,of the two   
daily tides at any place1 that which    
occurs     when the moon is nearest the 
zenith should be the greatest3 hence1   

when the moon's declination is north1   
the height of the tide at a     place in
 north latitude should be greater when  
the moon is above the     horizon than  
when she is below it4 ,at the same time1
 places south of     the equator have   
the highest tides when the moon is below
 the     horizon1 and the least when she
 is above it4 ,this is called the       
diurnal inequality1 because its cycle is
 one day2 but it varies     greatly in  
amount at different places4             
,illustration3 ,fig4 #hi4  #gg4 ,height 
of ,tides4--,at small islands in        
mid-ocean the tides never rise to a     
great height1 sometimes even less than  
one foot2 and the average height of the 
tides for the islands of the ,atlantic  
and ,pacific ,oceans is only three feet 
and a half4 ,upon approaching an        
extensive coast where the water is      
shallow1 the height of the tide is      
increased2 so that1 while in mid-ocean  
the average height does not exceed three
 feet and a half1 the average in the    

neighborhood of continents is not less  
than four or five feet4                 
,the ,day and ,time4   #gh4 ,the        
,day4--,by the term day we sometimes    
denote the period of sunshine as        
contrasted with that of the absence of  
sunshine1 which we call night1 and      
sometimes the period of the earth's     
rotation on its axis4 ,it is with the   
latter signification that the term is   
used in this section4 ,as the earth     
rotates on its axis1 it carries the     
meridian of a place with it2 so that1   
during each complete rotation of the    
earth1 the portion of the meridian which
 passes overhead from pole to pole      
sweeps past every star in the heavens   
from west to east4 ,the interval between
 two successive passages of this portion
 of the meridian across the same star is
 the exact period of the complete       
rotation of the earth4 ,this period is  
called a sidereal day4 ,the sidereal day
 may also be defined as the interval    
between two successive passages of the  

same star across the meridian2 the      
passage of the meridian across the star1
 and the passage or transit of the star 
across the meridian1 being the same     
thing looked at from a different point  
of view4 ,the interval between two      
successive passages of the meridian     
across the sun1 or of the sun across the
 meridian1 is called a solar day4  #gi4 
,length of the ,solar ,day4--,the solar 
day is a little longer than the sidereal
 day4 ,this is owing to the sun's       
eastward motion among the stars4 ,we    
have already seen that the sun's        
apparent position among the stars is    
continually shifting towards the east at
 a rate which causes it to make a       
complete circuit of the heavens in a    
year1 or three hundred and sixty-five   
days4 ,this is at the rate of about one 
degree a day3 hence1 were the sun and a 
star on the meridian together to-day1   
when the meridian again came around to  
the star1 the sun would appear about one
 degree to the eastward3 hence the      

meridian must be carried about one      
degree farther in order to come up to   
the sun4 ,the solar day must therefore  
be about four minutes longer than the   
sidereal day4  ,illustration3 ,fig4 #ij4
  ,illustration3 ,fig4 #ia4  ,the fact  
that the earth must make more than a    
complete rotation is also evident from  
,figs4 #ij and #ia4 ,in ,fig4 #ij1 ba   
represents the plane of the meridian1   
and the small arrows indicate the       
direction the earth is rotating on its  
axis1 and revolving in its orbit4 ,when 
the earth is at #a1 the sun is on the   
meridian at a4 ,when the earth has moved
 to #b1 it has made a complete rotation1
 as is shown by the fact that the plane 
of the meridian is parallel with its    
position at #a2 but it is evident that  
the meridian has not yet come up with   
the sun4 ,in ,fig4 #ia1 ,,oa represents 
the plane of the meridian1 and ,,os the 
direction of the sun4 ,the small arrows 
indicate the direction of the rotation  
and revolution of the earth4 ,in passing

 from the first position to the second  
the earth makes a complete rotation1 but
 the meridian is not brought up to the  
sun4  #hj4 ,inequality in the ,length of
 ,solar ,days4--,the sidereal days are  
all of the same length2 but the solar   
days differ somewhat in length4 ,this   
difference is due to the fact that the  
sun's apparent position moves eastward1 
or away from the meridian1 at a variable
 rate4      ,there are three reasons why
 this rate is variable3--      7#a7 ,the
 sun's eastward motion is due to the    
revolution of the earth     in its      
orbit4 ,now1 the earth's orbital motion 
is not uniform1     being fastest when  
the earth is at perihelion1 and slowest 
when the     earth is at aphelion3      
hence1 other things being equal1 solar  
days     will be longest when the earth 
is at perihelion1 and shortest when     
the earth is at aphelion4               
,illustration3 ,fig4 #ib4               
,illustration3 ,fig4 #ic4      7#b7 ,the
 sun's eastward motion is along the     

ecliptic4 ,now1 from ,figs4     #ib and 
#ic1 it will be seen1 that1 when the sun
 is at one of the     equinoxes1 it will
 be moving away from the meridian       
obliquely2     and1 from ,figs4 #id and 
#ie1 that1 when the sun is at one of the
     solstices1 it will be moving away  
from the meridian     perpendicularly3  
hence1 other things being equal1 the sun
 would     move away from the meridian  
fastest1 and the days be longest1       
when the sun is at the solstices2 while 
it would move away from     the meridian
 slowest1 and the days be shortest1 when
 the sun is     at the equinoxes4 ,that 
a body moving along the ecliptic must be
     moving at a variable angle to the  
meridian becomes very evident on        
turning a celestial globe so as to bring
 each portion of the     ecliptic under 
the meridian in turn4  ,illustration3   
,fig4 #id4  ,illustration3 ,fig4 #ie4   
7#c7 ,the sun1 moving along the         
ecliptic1 always moves in a great       
circle1 while the point of the meridian 

which is to overtake the     sun moves  
in a diurnal circle1 which is sometimes 
a great circle     and sometimes a small
 circle4 ,when the sun is at the        
equinoxes1     the point of the meridian
 which is to overtake it moves in a     
great     circle4 ,as the sun passes    
from the equinoxes to the solstices1 the
     point of the meridian which is to  
overtake it moves on a smaller and      
smaller circle3 hence1 as we pass away  
from the celestial equator1     the     
points of the meridian move slower and  
slower4 ,therefore1 other     things    
being equal1 the meridian will gain upon
 the sun most     rapidly1 and the days 
be shortest1 when the sun is at the     
equinoxes2 while it will gain on the sun
 least rapidly1 and the     days will be
 longest1 when the sun is at the        
solstices4  ,the ordinary or civil day  
is the mean of all the solar days in a  
year4  #ha4 ,sun ,time and ,clock       
,time4--,it is noon by the sun when the 
sun is on the meridian1 and by the clock

 at the middle of the civil day4 ,now1  
as the civil days are all of the same   
length1 while solar days are of variable
 length1 it seldom happens that the     
middles of these two days coincide1 or  
that sun time and clock time agree4 ,the
 difference between sun time and clock  
time1 or what is often called apparent  
solar time and mean solar time1 is      
called the equation of time4 ,the sun is
 said to be slow when it crosses the    
meridian after noon by the clock1 and   
fast when it crosses the meridian before
 noon by the clock4 ,sun time and clock 
time coincide four times a year2 during 
two intermediate seasons the clock time 
is ahead1 and during two it is behind4  
99       99       99       99       99  
,the following are the dates of         
coincidence and of maximum deviation1   
which vary but slightly from year to    
year3--      ,february #aj      ,true   
sun fifteen minutes slow4     ,april #ae
         ,true sun correct4     ,may #ad
           ,true sun four minutes fast4 

,june #ad          ,true sun correct4   
,july #be          ,true sun six minutes
 slow4     ,august #ca        ,true sun 
correct4     ,november #b       ,true   
sun sixteen minutes fast4     ,december 
#bd      ,true sun correct4  ,one of the
 effects of the equation of time which  
is frequently misunderstood is1 that the
 interval from sunrise until noon1 as   
given in the almanacs1 is not the same  
as that between noon and sunset4 ,the   
forenoon could not be longer or shorter 
than the afternoon1 if by noon we meant 
the passage of the sun across the       
meridian2 but the noon of our clocks    
being sometimes fifteen minutes before  
or after noon by the sun1 the former may
 be half an hour nearer to sunrise than 
to sunset1 or vice versa4               
,the ,year4   #hb4 ,the ,year4--,the    
year is the time it takes the earth to  
revolve around the sun1 or1 what amounts
 to the same thing1 the time it takes   
the sun to pass around the ecliptic4    
7#a7 ,the time it takes the sun to pass 

from a star around to the same star     
again is called a sidereal year4 ,this  
is1 of course1 the exact time it takes  
the earth to make a complete revolution 
around the sun4  ,illustration3 ,fig4   
#if4  7#b7 ,the time it takes the sun to
 pass around from the vernal equinox1 or
 the first point of ,aries1 to the      
vernal equinox again1 is called the     
tropical year4 ,this is a little shorter
 than the sidereal year1 owing to the   
precession of the equinoxes4 ,this will 
be evident from ,fig4 #if4 ,the circle  
represents the ecliptic1 ,s the sun1 and
 ,e the vernal equinox4 ,the sun moves  
around the ecliptic eastward1 as        
indicated by the long arrow1 while the  
equinox moves slowly westward1 as       
indicated by the short arrow4 ,the sun  
will therefore meet the equinox before  
it has quite completed the circuit of   
the heavens4 ,the exact lengths of these
 respective years are3--      ,sidereal 
year   #cfe4befcfcfe days  #f hours   #i
 min   #i sec     ,tropical year        

#cfe4bdbbjcfe days  #e hours  #dh min   
#df sec  ,since the recurrence of the   
seasons depends on the tropical year1   
the latter is the one to be used in     
forming the calendar and for the        
purposes of civil life generally4 ,its  
true length is eleven minutes and       
fourteen seconds less than three hundred
 and sixty-five days and a fourth4  ,it 
will be seen that the tropical year is  
about twenty minutes shorter than the   
sidereal year4      7#c7 ,the time it   
takes the earth to pass from its        
perihelion point     around to the      
perihelion point again is called the    
anomalistic     year4 ,this year is     
about four minutes longer than the      
sidereal     year4 ,this is owing to the
 fact that the major axis of the earth's
     orbit is slowly moving around to   
the east at the rate of about ten       
seconds a year4 ,this causes the        
perihelion point ,p 7,fig4 #ig7 to      
move eastward at that rate1 as indicated
 by the short arrow4 ,the     earth ,e  

is also moving eastward1 as indicated by
 the long arrow4     ,hence the earth1  
on starting at the perihelion1 has to   
make a little     more than a complete  
circuit to reach the perihelion point   
again4  ,illustration3 ,fig4 #ig4       
#hc4 ,the ,calendar4--,the solar year1  
or the interval between two             
successive passages of the same equinox 
by the sun1 is #cfe days1 #e     hours1 
#dh minutes1 #df seconds4 ,if1 then1 we 
reckon only #cfe days to     a common or
 civil year1 the sun will come to the   
equinox #e hours1     #dh minutes1 #df  
seconds1 or nearly a quarter of a day1  
later each     year2 so that1 if the sun
 entered ,aries on the #bjth of ,march  
one     year1 he would enter it on the  
#bast four years after1 on the #bb;d    
eight years after1 and so on4 ,thus in a
 comparatively short time the     spring
 months would come in the winter1 and   
the summer months in the     spring4    
,among different ancient nations        
different methods of computing the      

year were in use4 ,some reckoned it by  
the revolution of the moon1     some by 
that of the sun2 but none1 so far as we 
know1 made proper     allowances for    
deficiencies and excesses4 ,twelve moons
 fell short of     the true year1       
thirteen exceeded it2 #cfe days were not
 enough1 #cff     were too many4 ,to    
prevent the confusion resulting from    
these errors1     ,julius ,csar reformed
 the calendar by making the year consist
 of #cfe     days1 #f hours 7which is   
hence called a ,julian year71 and made  
every fourth year consist of #cff days4 
,this method of reckoning is     called 
,old ,style4      ,but as this made the 
year somewhat too long1 and the error in
 #aehb     amounted to ten days1 ,pope  
,gregory ,,xiii41 in order to bring the 
vernal equinox back to the #bast of     
,march again1 ordered ten days to     be
 struck out of that year1 calling the   
next day after the #dth of     ,october 
the #aeth2 and1 to prevent similar      
confusion in the future1     he decreed 

that three leap-years should be omitted 
in the course of     every four hundred 
years4 ,this way of reckoning time is   
called ,new     ,style4 ,it was         
immediately adopted by most of the      
,european nations1     but was not      
accepted by the ,english until the year 
#ageb4 ,the error     then amounted to  
eleven days1 which were taken from the  
month of     ,september by calling the  
#c;d of that month the #adth4 ,the ,old 
,style is     still retained by ,russia4
      ,according to the ,gregorian      
calendar1 every year whose number is    
divisible by four is a leap-year1       
except1 that1 in the case of     the    
years whose numbers are exact hundreds1 
those only are     leap-years which are 
divisible by four after cutting off the 
last     two figures4 ,thus the years   
#afjj1 #bjjj1 #bdjj1 etc41 are          
leap-years2     #agjj1 #ahjj1 #aijj1    
#bajj1 #bbjj1 etc41 are not4 ,the error 
will not     amount to a day in over    
three thousand years4      #hd4 ,the    

,dominical ,letter4--,the dominical     
letter for any year is     that which we
 often see placed against ,sunday in the
 almanacs1 and     is always one of the 
first seven in the alphabet4 ,since a   
common     year consists of #cfe days1  
if this number is divided by seven 7the 
number of days in a week71 there will be
 a remainder of one3 hence a     year   
commonly begins one day later in the    
week than the preceding     one did4 ,if
 a year of #cfe days begins on ,sunday1 
the next will begin     on ,monday2 if  
it begins on ,thursday1 the next will   
begin on ,friday2     and so on4 ,if    
,sunday falls on the #ast of ,january1  
the first letter     of the alphabet1 or
 ,a1 is the dominical letter4 ,if       
,sunday falls     on the #gth of        
,january 7as it will the next year1     
unless the first is     leap-year71 the 
seventh letter1 ,g1 is the dominical    
letter4 ,if     ,sunday falls on the    
#fth of ,january 7as it will the third  
year1     unless the first or second is 

leap-year71 the sixth letter1 ,f1       
will be the dominical letter4 ,thus1 if 
there were no leap-years1 the           
dominical letters would regularly follow
 a retrograde order1 ,g1     ,f1 ,e1 ,d1
 ,c1 ,b1 ,a4      ,but leap-years have  
#cff days1 which1 divided by seven1     
leaves two     remainder3 hence the     
years following leap-years will begin   
two days     later in the week than the 
leap-years did4 ,to prevent the         
interruption which would hence occur in 
the order of the dominical     letters1 
leap-years have two dominical letters1  
one indicating     ,sunday till the     
#bith of ,february1 and the other for   
the rest of the     year4  ,by ,table   
,i4 below1 the dominical letter for any 
year 7,new ,style7 for four thousand    
years from the beginning of the         
,christian ,era may be found2 and it    
will be readily seen how the ,table     
could be extended indefinitely by       
continuing the centuries at the top in  
the same order4  ,to find the dominical 

letter by this table1 look for the      
hundreds of years at the top1 and for   
the years below a hundred1 at the left  
hand4  ,thus the letter for #ahhb will  
be opposite the number #hb1 and in the  
column having #ahjj at the top2 that is1
 it will be ,a4 ,in the same way1 the   
letters for #ahhd1 which is a leap-year1
 will be found to be ,,fe4  ,having the 
dominical letter of any year1 ,table    
,,ii4 shows what days of every month of 
the year will be ,sundays4  ,to find the
 ,sundays of any month in the year by   
this table1 look in the column1 under   
the dominical letter1 opposite the name 
of the month given at the left4  ,from  
the ,sundays the date of any other day  
of the week can be readily found4       
,thus1 if we wish to know on what day of
 the week ,christmas falls in #ahhi1 we 
look opposite ,december1 under the      
letter ,f 7which we have found to be the
 dominical letter for the year71 and    
find that the #bb;d of the month is a   
,sunday2 the #beth1 or ,christmas1 will 

then be ,wednesday4  ,in the same way we
 may find the day of the week           
corresponding to any date 7,new ,style7 
in history4 ,for instance1 the #agth of 
,june1 #agge1 the day of the fight at   
,bunker ,hill1 is found to have been a  
,saturday4  ,these two tables then serve
 as a perpetual almanac4  ,,table ,i4   
#ajj  #bjj  #cjj  #djj                  
#ejj  #fjj  #gjj  #hjj                  
#ijj #ajjj #aajj #abjj                  
#acjj #adjj #aejj #afjj                 
#agjj #ahjj #aijj #bjjj                 
#bajj #bbjj #bcjj #bdjj                 
---  ---  --- ----                      
,c    ,e    ,g   ,,ba                   
#a #bi #eg #he     ,b    ,d    ,f    ,g 
#b #cj #eh #hf     ,a    ,c    ,e    ,f 
#c #ca #ei #hg     ,g    ,b    ,d    ,e 
#d #cb #fj #hh    ,,fe   ,,ag   ,,cb    
,,dc                      #e #cc #fa #hi
     ,d    ,f    ,a    ,b               
#f #cd #fb #ij     ,c    ,e    ,g    ,a 
#g #ce #fc #ia     ,b    ,d    ,f    ,g 
#h #cf #fd #ib    ,,ag   ,,cb   ,,ed    

,,fe                      #i #cg #fe #ic
     ,f    ,a    ,c    ,d               
#aj #ch #ff #id     ,e    ,g    ,b    ,c
                     #aa #ci #fg #ie    
,d    ,f    ,a    ,b                    
#ab #dj #fh #if    ,,cb   ,,ed   ,,gf   
,,ag                     #ac #da #fi #ig
     ,a    ,c    ,e    ,f               
#ad #db #gj #ih     ,g    ,b    ,d    ,e
                     #ae #dc #ga #ii    
,f    ,a    ,c    ,d                    
#af #dd #gb 44    ,,ed   ,,gf   ,,ba    
,,cb                     #ag #de #gc 44 
,c    ,e    ,g    ,a                    
#ah #df #gd 44     ,b    ,d    ,f    ,g 
#ai #dg #ge 44     ,a    ,c    ,e    ,f 
#bj #dh #gf 44    ,,gf   ,,ba   ,,dc    
,,ed                     #ba #di #gg 44 
,e    ,g    ,b    ,c                    
#bb #ej #gh 44     ,d    ,f    ,a    ,b 
#bc #ea #gi 44     ,c    ,e    ,g    ,a 
#bd #eb #hj 44    ,,ba   ,,dc   ,,fe    
,,gf                     #be #ec #ha 44 
,g    ,b    ,d    ,e                    
#bf #ed #hb 44     ,f    ,a    ,c    ,d 

#bg #ee #hc 44     ,e    ,g    ,b    ,c 
#bh #ef #hd 44    ,,dc   ,,fe   ,,ag    
,,ba  ,,table ,,ii4                     
,a  ,b  ,c  ,d  ,e  ,f  ,g              
#a  #b  #c  #d  #e  #f  #g              
,jan4 #ca4      #h  #i #aj #aa #ab #ac  
#ad                                 #ae 
#af #ag #ah #ai #bj #ba                 
,oct4 #ca4     #bb #bc #bd #be #bf #bg  
#bh                                 #bi 
#cj #ca 44 44 44 44                     
,feb4 #bh-bi4  44 44 44  #a  #b  #c  #d 
#e  #f  #g  #h  #i #aj #aa              
,march #ca4    #ab #ac #ad #ae #af #ag  
#ah                                 #ai 
#bj #ba #bb #bc #bd #be                 
,nov4 #cj4     #bf #bg #bh #bi #cj #ca  
44                                  44  
44 44 44 44 44  #a                      
,april #cj4     #b  #c  #d  #e  #f  #g  
#h                                  #i  
#aj #aa #ab #ac #ad #ae                 
,july #ca      #af #ag #ah #ai #bj #ba  
#bb                                 #bc 
#bd #be #bf #bg #bh #bi                 

#cj #ca 44 44 44 44 44                  
44 44  #a  #b  #c  #d  #e               
#f  #g  #h  #i #aj #aa #ab              
,aug4 #ca4     #ac #ad #ae #af #ag #ah  
#ai                                 #bj 
#ba #bb #bc #bd #be #bf                 
#bg #bh #bi #cj #ca 44 44               
44 44 44 44 44  #a  #b                  
,sept4 #cj4     #c  #d  #e  #f  #g  #h  
#i                                 #aj  
#aa #ab #ac #ad #ae #af                 
#ag #ah #ai #bj #ba #bb #bc             
,dec4 #ca4     #bd #be #bf #bg #bh #bi  
#cj                                 #ca 
44 44 44 44 44 44                       
44  #a  #b  #c  #d  #e  #f              
#g  #h  #i #aj #aa #ab #ac              
,may4 #ca4     #ad #ae #af #ag #ah #ai  
#bj                                 #ba 
#bb #bc #bd #be #bf #bg                 
#bh #bi #cj #ca 44 44 44                
44 44 44 44  #a  #b  #c                 
#d  #e  #f  #g  #h  #i #aj              
,june #cj4     #aa #ab #ac #ad #ae #af  
#ag                                 #ah 

#ai #bj #ba #bb #bc #bd                 
#be #bf #bg #bh #bi #cj 44              
,weight of the ,earth and ,precession4  
#he4 ,the ,weight of the ,earth4--,there
 are several methods of ascertaining the
 weight and mass of the earth4 ,the     
simplest1 and perhaps the most          
trustworthy method is to compare the    
pull of the earth upon a ball of lead   
with that of a known mass of lead upon  
it4 ,the pull of a known mass of lead   
upon the ball may be measured by means  
of a torsion balance4 ,one form of the  
balance employed for this purpose is    
shown in ,figs4 #ih and #ii4 ,two small 
balls of lead1 b and b1 are fastened to 
the ends of a light rod e1 which is     
suspended from the point ,f by means of 
the thread ,,fe4 ,two large balls of    
lead1 ,w and ,w1 are placed on a        
turn-table1 so that one of them shall be
 just in front of one of the small      
balls1 and the other just behind the    
other small ball4 ,the pull of the large
 balls turns the rod around a little so 

as to bring the small balls nearer the  
large ones4 ,the small balls move       
towards the large ones till they are    
stopped by the torsion of the thread1   
which is then equal to the pull of the  
large balls4 ,the deflection of the rod 
is carefully measured4 ,the table is    
then turned into the position indicated 
by the dotted lines in ,fig4 #ii1 so as 
to reverse the position of the large    
balls with reference to the small ones4 
,the rod is now deflected in the        
opposite direction1 and the amount of   
deflection is again carefully measured4 
,the second measurement is made as a    
check upon the accuracy of the first4   
,the force required to twist the thread 
as much as it was twisted by the        
deflection of the rod is ascertained by 
measurement4 ,this gives the pull of the
 two large balls upon the two small     
ones4 ,we next calculate what this pull 
would be were the balls as far apart as 
the small balls are from the centre of  
the earth4 ,we can then form the        

following proportion3 the pull of the   
large balls upon the small ones is to   
the pull of the earth upon the small    
ones as the mass of the large balls is  
to the mass of the earth1 or as the     
weight of the large balls is to the     
weight of the earth4 ,of course1 the    
pull of the earth upon the small balls  
is the weight of the small balls4 ,in   
this way it has been ascertained that   
the mass of the earth is about #e4f     
times that of a globe of water of the   
same size4 ,in other words1 the mean    
density of the earth is about #e4f4     
,illustration3 ,fig4 #ih4               
,illustration3 ,fig4 #ii4  ,the weight  
of the earth in pounds may be found by  
multiplying the number of cubic feet in 
it by #fb-a_/b 7the weight1 in pounds1  
of one cubic foot of water71 and this   
product by #e4f4  ,illustration3 ,fig4  
#ajj4  #hf4 ,cause of ,precession4--,we 
have seen that the earth is flattened at
 the poles3 in other words1 the earth   
has the form of a sphere1 with a        

protuberant ring around its equator4    
,this equatorial ring is inclined to the
 plane of the ecliptic at an angle of   
about #bc-a_/b4 ,in ,fig4 #ajj this ring
 is represented as detached from the    
enclosed sphere4 ,s represents the sun1 
and ,sc the ecliptic4 ,as the point ,a  
of the ring is nearer the sun than the  
point ,b is1 the sun's pull upon ,a is  
greater than upon ,b3 hence the sun     
tends to pull the ring over into the    
plane of the ecliptic2 but the rotation 
of the earth tends to keep the ring in  
the same plane4 ,the struggle between   
these two tendencies causes the earth1  
to which the ring is attached1 to wabble
 like a spinning-top1 whose rotation    
tends to keep it erect1 while gravity   
tends to pull it over4 ,the handle of   
the top has a gyratory motion1 which    
causes it to describe a curve4 ,the axis
 of the heavens corresponds to the      
handle of the top4                      
,,ii4 ,,the ,,moon4                     
,distance1 ,size1 and ,motions4   #hg4  

,the ,distance of the ,moon4--,the moon 
is the nearest of the heavenly bodies4  
,its distance from the centre of the    
earth is only about sixty times the     
radius of the earth1 or1 in round       
numbers1 two hundred and forty thousand 
miles4      ,the ordinary method of     
finding the distance of one of the      
nearer     heavenly bodies is first to  
ascertain its horizontal parallax4 ,this
     enables us to form a right-angled  
triangle1 the lengths of whose     sides
 are easily computed1 and the length of 
whose hypothenuse is     the distance of
 the body from the centre of the earth4 
,illustration3 ,fig4 #aja4              
,horizontal parallax has already been   
defined 7#cb7 as the     displacement of
 a heavenly body when on the horizon1   
caused by its     being seen from the   
surface1 instead of the centre1 of the  
earth4     ,this displacement is due to 
the fact that the body is seen in a     
different direction from the surface of 
the earth from that in which     it     

would be seen from the centre4          
,horizontal parallax might be           
defined as the difference in the        
directions in which a body on the       
horizon would be seen from the surface  
and from the centre of the     earth4   
,thus1 in ,fig4 #aja1 ,c is the centre  
of the earth1 ,a a     point on the     
surface1 and ,b a body on the horizon of
 ,a4 ,,ab is     the direction in which 
the body would be seen from ,a1 and ,,cb
 the     direction in which it would be 
seen from ,c4 ,the difference of        
these directions1 or the angle ,,abc1 is
 the parallax of the body4      ,the    
triangle ,,bac is right-angled at ,a2   
the side ,,ac is the     radius of the  
earth1 and the hypothenuse is the       
distance of the body     from the centre
 of the earth4 ,when the parallax ,,abc 
is known1 the     length of ,,cb can    
easily by found by trigonometrical      
computation4      ,we have seen 7#cb7   
that the parallax of a heavenly body    
grows less     and less as the body     

passes from the horizon towards the     
zenith4 ,the     parallax of a body and 
its altitude are1 however1 so related1  
that1     when we know the parallax at  
any altitude1 we can readily compute    
the horizontal parallax4      ,the usual
 method of finding the parallax of one  
of the nearer     heavenly bodies is    
first to find its parallax when on the  
meridian1     as seen from two places on
 the earth which differ considerably in 
latitude3 then to calculate what would  
be the parallax of the body     as seen 
from one of these places and the centre 
of the earth3 and     then finally to   
calculate what would be the parallax    
were the body     on the horizon4       
,illustration3 ,fig4 #ajb4      ,thus1  
we should ascertain the parallax of the 
body ,b 7,fig4 #ajb7 as     seen from ,a
 and ,d1 or the angle ,,abd4 ,we should 
then calculate     its parallax as seen 
from ,a and ,c1 or the angle ,,abc4     
,finally     we should calculate what   
its parallax would be were the body on  

the     horizon1 or the angle ,,ab',c4  
,the simplest method of finding the     
parallax of a body ,b 7,fig4 #ajb7      
as seen from the two points ,a and ,d is
 to compare its direction     at each   
point with that of the same fixed star  
near the body4 ,the     star is so      
distant1 that it will be seen in the    
same direction from     both points3    
hence1 if the direction of the body     
differs from that     of the star #b as 
seen from one point1 and #b #f' as seen 
from the     other point1 the two lines 
,,ab and ,,db must differ in direction  
by     #f'2 in other words1 the angle   
,,abd would be #f'4      ,the method    
just described is the usual method of   
finding the     parallax of the moon4   
#hh4 ,the ,apparent ,size of the        
,moon4--,the apparent size of a body is 
the visual angle subtended by it2 that  
is1 the angle formed by two lines drawn 
from the eye to two opposite points on  
the outline of the body4 ,the apparent  
size of a body depends upon both its    

magnitude and its distance4  ,the       
apparent size1 or angular diameter1 of  
the moon is about thirty-one minutes4   
,this is ascertained by means of the    
wire micrometer already described 7#ai74
 ,the instrument is adjusted so that its
 longitudinal wire shall pass through   
the centre of the moon1 and its         
transverse wires shall be tangent to the
 limbs of the moon on each side1 at the 
point where they are cut by the         
longitudinal wire4 ,the micrometer screw
 is then turned till the wires are      
brought together4 ,the number of turns  
of the screw needed to accomplish this  
will indicate the arc between the wires1
 or the angular diameter of the moon4   
,illustration3 ,fig4 #ajc4  ,in order to
 be certain that the longitudinal wire  
shall pass through the centre of the    
moon1 it is best to take the moon when  
its disc is in the form of a crescent1  
and to place the longitudinal wire      
against the points1 or cusps1 of the    
crescent1 as shown in ,fig4 #ajc4       

,illustration3 ,fig4 #ajd4  #hi4 ,the   
,real ,size of the ,moon4--,the real    
diameter of the moon is a little over   
one-fourth of that of the earth1 or a   
little more than two thousand miles4    
,the comparative sizes of the earth and 
moon are shown in ,fig4 #ajd4           
,illustration3 ,fig4 #aje4      ,the    
distance and apparent size of the moon  
being known1 her real     diameter is   
found by means of a triangle formed as  
shown in ,fig4     #aje4 ,c represents  
the centre of the moon1 ,,cb the        
distance of the     moon from the earth1
 and ,,ca the radius of the moon4 ,,bac 
is a     triangle1 right-angled at ,a4  
,the angle ,,abc is half the apparent   
diameter of the moon4 ,with the angles  
,a and ,b1 and the side ,,cb     known1 
it is easy to find the length of ,,ac by
 trigonometrical     computation4 ,twice
 ,,ac will be the diameter of the moon4 
,the volume of the moon is about        
one-fiftieth of that of the earth4  #ij4
 ,apparent ,size of the ,moon on the    

,horizon and in the ,zenith44--,the moon
 is nearly four thousand miles farther  
from the observer when she is on the    
horizon than when she is in the zenith4 
,this is evident from ,fig4 #ajf4 ,c is 
the centre of the earth1 ,m the moon on 
the horizon1 ,m' the moon in the zenith1
 and ,o the point of observation4 ,,om  
is the distance of the moon when she is 
on the horizon1 and ,,om' the distance  
of the moon from the observer when she  
is in the zenith4 ,,cm is equal to      
,,cm'1 and ,,om is about the length of  
,,cm2 but ,,om' is about four thousand  
miles shorter than ,,cm'3 hence ,,om' is
 about four thousand miles shorter than 
,,om4  ,illustration3 ,fig4 #ajf4       
,notwithstanding the moon is much nearer
 when at the zenith than at     the     
horizon1 it seems to us much larger at  
the horizon4      ,this is a pure       
illusion1 as we become convinced when we
 measure the     disc with accurate     
instruments1 so as to make the result   
independent     of our ordinary way of  

judging4 ,when the moon is near the     
horizon1     it seems placed beyond all 
the objects on the surface of the earth 
in that direction1 and therefore farther
 off than at the zenith1     where no   
intervening objects enable us to judge  
of its distance4 ,in     any case1 an   
object which keeps the same apparent    
magnitude seems to     us1 through the  
instinctive habits of the eye1 the      
larger in     proportion as we judge it 
to be more distant4      #ia4 ,the      
,apparent ,size of the ,moon increased  
by ,irradiation4--,in     the case of   
the moon1 the word apparent means much  
more than it     does in the case of    
other celestial bodies4 ,indeed1 its    
brightness     causes our eyes to play  
us false4 ,as is well known1 the        
crescent of     the new moon seems part 
of a much larger sphere than that which 
it     has been said1 time out of mind1 
to hold in its arms4 ,the bright        
portion of the moon as seen with our    
measuring instruments1 as well     as   

when seen with the naked eye1 covers a  
larger space in the field     of the    
telescope than it would if it were not  
so bright4 ,this effect     of          
irradiation1 as it is called1 must be   
allowed for in exact     measurements of
 the diameter of the moon4              
,illustration3 ,fig4 #ajg4  #ib4        
,apparent ,size of the ,moon in         
,different ,parts of her ,orbit4--,owing
 to the eccentricity of the moon's      
orbit1 her distance from the earth      
varies somewhat from time to time4 ,this
 variation causes a corresponding       
variation in her apparent size1 which is
 illustrated in ,fig4 #ajg4  #ic4 ,the  
,mass of the ,moon4--,the moon is       
considerably less dense than the earth1 
its mass being only about one-eightieth 
of that of the earth2 that is1 while it 
would take only about fifty moons to    
make the bulk of the earth1 it would    
take about eighty to make the mass of   
the earth4      ,one method of finding  
the mass of the moon is to compare her  

effect     in producing the tides with  
that of the sun4 ,we first calculate    
what would be the moon's effect in      
producing the tides1 were she as     far
 off as the sun4 ,we then form the      
following proportion3 as the     sun's  
effect in producing the tides is to the 
moon's effect at the     same distance1 
so is the mass of the sun to the mass of
 the moon4      ,the method of finding  
the mass of the sun will be given       
farther on4  #id4 ,the ,orbital ,motion 
of the ,moon4--,if we watch the moon    
from night to night1 we see that she    
moves eastward quite rapidly among the  
stars4 ,when the new moon is first      
visible1 it appears near the horizon in 
the west1 just after sunset4 ,a week    
later the moon will be on the meridian  
at the same hour1 and about a week later
 still on the eastern horizon4 ,the moon
 completes the circuit of the heavens in
 a period of about thirty days1 moving  
eastward at the rate of about twelve    
degrees a day4 ,this eastward motion of 

the moon is due to the fact that she is 
revolving around the earth from west to 
east4  ,illustration3 ,fig4 #ajh4  #ie4 
,the ,aspects of the ,moon4--,as the    
moon revolves around the earth1 she     
comes into different positions with     
reference to the earth and sun4 ,these  
different positions of the moon are     
called the aspects of the moon4 ,the    
four chief aspects of the moon are shown
 in ,fig4 #ajh4 ,when the moon is at ,m1
 she appears in the opposite part of the
 heavens to the sun1 and is said to be  
in opposition2 when at ,m' and at ,m'''1
 she appears ninety degrees away from   
the sun1 and is said to be in           
quadrature2 when at ,m''1 she appears in
 the same part of the heavens as the    
sun1 and is said to be in conjunction4  
#if4 ,the ,sidereal and ,synodical      
,periods of the ,moon4--,the sidereal   
period of the moon is the time it takes 
her to pass around from a star to that  
star again1 or the time it takes her to 
make a complete revolution around the   

earth4 ,this is a period of about       
twenty-seven days and a third4 ,it is   
sometimes called the sidereal month4    
,the synodical period of the moon is the
 time that it takes the moon to pass    
from one aspect around to the same      
aspect again4 ,this is a period of about
 twenty-nine days and a half1 and it is 
sometimes called the synodical month4   
,illustration3 ,fig4 #aji4  ,the reason 
why the synodical period is longer than 
the sidereal period will appear from    
,fig4 #aji4 ,s represents the position  
of the sun1 ,e that of the earth1 and   
the small circle the orbit of the moon  
around the earth4 ,the arrow in the     
small circle represents the direction   
the moon is revolving around the earth1 
and the arrow in the arc between ,e and 
,e' indicates the direction of the      
earth's motion in its orbit4 ,when the  
moon is at ,m#a1 she is in conjunction4 
,as the moon revolves around the earth1 
the earth moves forward in its orbit4   
,when the moon has come round to m#a1 so

 that m#cm#a is parallel with ,m#c,m#a1 
she will have made a complete or        
sidereal revolution around the earth2   
but she will not be in conjunction again
 till she has come round to ,m1 so as   
again to be between the earth and sun4  
,that is to say1 the moon must make more
 than a complete revolution in a        
synodical period4  ,illustration3 ,fig4 
#aaj4      ,the greater length of the   
synodical period is also evident from   
,fig4     #aaj4 ,t represents the earth1
 and ,l the moon4 ,the arrows indicate  
the direction in which each is moving4  
,when the earth is at ,t1 and     the   
moon at ,l1 the latter is in            
conjunction4 ,when the earth has        
reached ,t'1 and the moon ,l'1 the      
latter has made a sidereal              
revolution2 but she will not be in      
conjunction again till the earth     has
 reached ,t''1 and the moon ,l''4  #ig4 
,the ,phases of the ,moon4--,when the   
new moon appears in the west1 it has the
 form of a crescent1 with its convex    

side towards the sun1 and its horns     
towards the east4 ,as the moon advances 
towards quadrature1 the crescent grows  
thicker and thicker1 till it becomes a  
half-circle at first quarter4 ,when it  
passes quadrature1 it begins to become  
convex also on the side away from the   
sun1 or gibbous in form4 ,as it         
approaches opposition1 it becomes more  
and more nearly circular1 until at      
opposition it is a full circle4 ,from   
full moon to last quarter it is again   
gibbous1 and at last quarter a          
half-circle4 ,from last quarter to new  
moon it is again crescent2 but the horns
 of the crescent are now turned towards 
the west4 ,the successive phases of the 
moon are shown in ,fig4 #aaa4           
,illustration3 ,fig4 #aaa4  #ih4 ,cause 
of the ,phases of the ,moon4--,take a   
globe1 half of which is colored white   
and the other half black in such a way  
that the line which separates the white 
and black portions shall be a great     
circle which passes through the poles of

 the globe1 and rotate the globe slowly1
 so as to bring the white half gradually
 into view4 ,when the white part first  
comes into view1 the line of separation 
between it and the black part1 which we 
may call the terminator1 appears        
concave1 and its projection on a plane  
perpendicular to the line of vision is a
 concave line4 ,as more and more of the 
white portion comes into view1 the      
projection of the terminator becomes    
less and less concave4 ,when half of the
 white portion comes into view1 the     
terminator is projected as a straight   
line4 ,when more than half of the white 
portion comes into view1 the terminator 
begins to appear as a convex line1 and  
this line becomes more and more convex  
till the whole of the white half comes  
into view1 when the terminator becomes  
circular4  ,illustration3 ,fig4 #aab4   
,the moon is of itself a dark1 opaque   
globe2 but the half that is towards the 
sun is always bright1 as shown in ,fig4 
#aab4 ,this bright half of the moon     

corresponds to the white half of the    
globe in the preceding illustration4 ,as
 the moon revolves around the earth1    
different portions of this illumined    
half are turned towards the earth4 ,at  
new moon1 when the moon is in           
conjunction1 the bright half is turned  
entirely away from the earth1 and the   
disc of the moon is black and invisible4
 ,between new moon and first quarter1   
less than half of the illumined side is 
turned towards the earth1 and we see    
this illumined portion projected as a   
crescent4 ,at first quarter1 just half  
of the illumined side is turned towards 
the earth1 and we see this half         
projected as a half-circle4 ,between    
first quarter and full1 more than half  
of the illumined side is turned towards 
the earth1 and we see it as gibbous4 ,at
 full1 the whole of the illumined side  
is turned towards us1 and we see it as a
 full circle4 ,from full to new moon    
again1 the phases occur in the reverse  
order4  #ii4 ,the ,form of the ,moon's  

,orbit4--,the orbit of the moon around  
the earth is an ellipse of slight       
eccentricity4 ,the form of this ellipse 
is shown in ,fig4 #aac4 ,c is the centre
 of the ellipse1 and ,e the position of 
the earth at one of its foci4 ,the      
eccentricity of the ellipse is only     
about one-eighteenth4 ,it is impossible 
for the eye to distinguish such an      
ellipse from a circle4  ,illustration3  
,fig4 #aac4  #ajj4 ,the ,inclination of 
the ,moon's ,orbit4--,the plane of the  
moon's orbit is inclined to the ecliptic
 by an angle of about five degrees4 ,the
 two points where the moon's orbit cuts 
the ecliptic are called her nodes4 ,the 
moon's nodes have a westward motion     
corresponding to that of the equinoxes1 
but much more rapid4 ,they complete the 
circuit of the ecliptic in about        
nineteen years4  ,the moon's latitude   
ranges from #e north to #e south2 and   
since1 owing to the motion of her nodes1
 the moon is1 during a period of        
nineteen years1 #e north and #e south of

 every part of the ecliptic1 her        
declination will range from #bc-a_/b  #e
  #bh-a_/b north to #bc-a_/b  #e        
#bh-a_/b south4  #aja4 ,the ,meridian   
,altitude of the ,moon4--,the meridian  
altitude of any body is its altitude    
when on the meridian4 ,in our latitude1 
the meridian altitude of any point on   
the equinoctial is forty-nine degrees4  
,the meridian altitude of the summer    
solstice is #di  #bc-a_/b  #gb-a_/b1 and
 that of the winter solstice is #di -   
#bc-a_/b  #be-a_/b4 ,the greatest       
meridian altitude of the moon is        
#gb-a_/b  #e  #gg-a_/b1 and its least   
meridian altitude1 #be-a_/b - #e        
#bj-a_/b4  ,when the moon's meridian    
altitude is greater than the elevation  
of the equinoctial1 it is said to run   
high1 and when less1 to run low4 ,the   
full moon runs high when the sun is     
south of the equinoctial1 and low when  
the sun is north of the equinoctial4    
,this is because the full moon is always
 in the opposite part of the heavens to 

the sun4      #ajb4 ,wet and ,dry       
,moon4--,at the time of new moon1 the   
cusps of the     crescent sometimes lie 
in a line which is nearly perpendicular 
with     the horizon1 and sometimes in a
 line which is nearly parallel with     
the horizon4 ,in the former case the    
moon is popularly described as a     wet
 moon1 and in the latter case as a dry  
moon4  ,illustration3 ,fig4 #aad4       
,the great circle which passes through  
the centre of the sun and moon     will 
pass through the centre of the crescent1
 and be perpendicular     to the line   
joining the cusps4 ,now the ecliptic    
makes the least     angle with the      
horizon when the vernal equinox is on   
the eastern     horizon and the autumnal
 equinox is on the western4 ,in our     
latitude1     as we have seen1 this     
angle is #be-a_/b3 hence in our         
latitude1 if     the moon were at new on
 the ecliptic when the sun is at the    
autumnal     equinox1 as shown at ,m#c  
7,fig4 #aad71 the great circle passing  

through the centre of the sun and moon  
would be the ecliptic1 and at     ,new  
,york would be inclined to the horizon  
at an angle of #be-a_/b4 ,if     the    
moon happened to be #e south of the     
ecliptic at this time1 as at     ,m#d1  
the great circle passing through the    
centre of the sun and     moon would    
make an angle of only #bj-a_/b with the 
horizon4 ,in either     of these cases  
the line joining the cusps would be     
nearly     perpendicular to the horizon4
  ,illustration3 ,fig4 #aae4      ,if   
the moon were at new on the ecliptic    
when the sun is near the     vernal     
equinox1 as shown at ,m#a 7,fig4 #aae71 
the great circle     passing through the
 centres of the sun and moon would make 
an angle     of #gb-a_/b with the       
horizon at ,new ,york2 and were the moon
 #e north     of the ecliptic at that   
time1 as shown at ,m#b1 this great      
circle     would make an angle of       
#gg-a_/b with the horizon4 ,in either of
 these     cases1 the line joining the  

cusps would be nearly parallel with the 
horizon4      ,at different times1 the  
line joining the cusps may have every   
possible inclination to the horizon     
between the extreme cases shown     in  
,figs4 #aad and #aae4  #ajc4 ,daily     
,retardation of the ,moon's             
,rising4--,the moon rises1 on the       
average1 about fifty minutes later each 
day4 ,this is owing to her eastward     
motion4 ,as the moon makes a complete   
revolution around the earth in about    
twenty-seven days1 she moves eastward at
 the rate of about thirteen degrees a   
day1 or about twelve degrees a day      
faster than the sun4 ,were the moon1    
therefore1 on the horizon at any hour   
to-day1 she would be some twelve degrees
 below the horizon at the same hour     
to-morrow4 ,now1 as the horizon moves at
 the rate of one degree in four minutes1
 it would take it some fifty minutes to 
come up to the moon so as to bring her  
upon the horizon4 ,hence the daily      
retardation of the moon's rising is     

about fifty minutes2 but it varies      
considerably in different parts of her  
orbit4      ,there are two reasons for  
this variation in the daily             
retardation3--      7#a7 ,the moon moves
 at a varying rate in her orbit2 her    
speed being     greatest at perigee1 and
 least at apogee3 hence1 other things   
being     equal1 the retardation is     
greatest when the moon is at perigee1   
and     least when she is at apogee4    
,illustration3 ,fig4 #aaf4              
,illustration3 ,fig4 #aag4      7#b7    
,the moon moves at a varying angle to   
the horizon4 ,the moon     moves nearly 
in the plane of the ecliptic1 and of    
course she passes     both equinoxes    
every lunation4 ,when she is near the   
autumnal     equinox1 her path makes the
 greatest angle with the eastern        
horizon1     and when she is near the   
vernal equinox1 the least angle3 hence  
the     moon moves away from the horizon
 fastest when she is near the           
autumnal equinox1 and slowest when she  

is near the vernal equinox4     ,this   
will be evident from ,figs4 #aaf and    
#aag4 ,in each figure1 ,,sn             
represents a portion of the eastern     
horizon1 and ,ec1 ,e'c'1 a     portion  
of the ecliptic4 ,,ae1 in ,fig4 #aaf1   
represents the autumnal     equinox1 and
 ,,aem the daily motion of the moon4    
,,ve1 in ,fig4 #aag1     represents the 
vernal equinox1 and ,,vem' the motion of
 the moon for     one day4 ,in the first
 case this motion would carry the moon  
away     from the horizon the distance  
,,am1 and in the second case the        
distance ,a',m'4 ,now1 it is evident    
that ,,am is greater than     ,a',m'3   
hence1 other things being equal1 the    
greatest retardation of     the moon's  
rising will be when the moon is near the
 autumnal     equinox1 and the least    
retardation when the moon is near the   
vernal     equinox4  ,the least         
retardation at ,new ,york is            
twenty-three minutes1 and the greatest  
an hour and seventeen minutes4 ,the     

greatest and least retardations vary    
somewhat from month to month2 since they
 depend not only upon the position of   
the moon in her orbit with reference to 
the equinoxes1 but also upon the        
latitude of the moon1 and upon her      
nearness to the earth4  ,illustration3  
,fig4 #aah4  ,the direction of the      
moon's motion with reference to the     
ecliptic is shown in ,fig4 #aah1 which  
shows the moon's motion for one day in  
,july1 #ahgf4  #ajd4 ,the ,harvest      
,moon--,the long and short retardations 
in the rising of the moon1 though they  
occur every month1 are not likely to    
attract attention unless they occur at  
the time of full moon4 ,the long        
retardations for full moon occur when   
the moon is near the autumnal equinox at
 full4 ,as the full moon is always      
opposite to the sun1 the sun must in    
this case be near the vernal equinox3   
hence the long retardations for full    
moon occur in the spring1 the greatest  
retardation being in ,march4  ,the least

 retardations for full moon occur when  
the moon is near the vernal equinox at  
full3 the sun must then be near the     
autumnal equinox4 ,hence the least      
retardations for full moon occur in the 
months of ,august1 ,september1 and      
,october4 ,the retardation is1 of       
course1 least for ,september2 and the   
full moon of this month rises night     
after night less than half an hour later
 than the previous night4 ,the full moon
 of ,september is called the ,harvest   
,moon1 and that of ,october the         
,hunter's ,moon4  #aje4 ,the ,rotation  
of the ,moon4--,a careful examination of
 the spots on the disc of the moon      
reveals the fact that she always        
presents the same side to the earth4 ,in
 order to do this1 she must rotate on   
her axis while making a revolution      
around the earth1 or in about           
twenty-seven days4  #ajf4 ,librations of
 the ,moon4--,the moon appears to rock  
slowly to and fro1 so as to allow us to 
see alternately a little farther around 

to the right and the left1 or above and 
below1 than we otherwise could4 ,this   
apparent rocking of the moon is called  
libration4 ,the moon has three          
librations3--  7#a7 ,libration in       
,latitude4--,this libration enables us  
to see alternately a little way around  
on the northern and southern limbs of   
the moon4      ,this libration is due to
 the fact that the axis of the moon is  
not     quite perpendicular to the plane
 of her orbit4 ,the deviation from      
the perpendicular is six degrees and a  
half4 ,as the axis of the     moon1 like
 that of the earth1 maintains the same  
direction1 the     poles of the moon    
will be turned alternately six degrees  
and a half     toward and from the      
earth4  7#b7 ,libration in              
,longitude4--,this libration enables us 
to see alternately a little farther     
around on the eastern and western limbs 
of the moon4  ,illustration3 ,fig4 #aai4
      ,it is due to the fact that the   
moon's axial motion is uniform1 while   

her orbital motion is not4 ,at perigee  
her orbital motion will be in           
advance of her axial motion1 while at   
apogee the axial motion will     be in  
advance of the orbital4 ,in ,fig4 #aai1 
,e represents the earth1     ,m the     
moon1 the large arrow the direction of  
the moon's motion in     her orbit1 and 
the small arrow the direction of her    
motion of     rotation4 ,when the moon  
is at ,m1 the line ,,ab1 drawn          
perpendicular to ,,em1 represents the   
circle which divides the     visible    
from the invisible portion of the moon4 
,while the moon is     passing from ,m  
to ,m'1 the moon performs less than a   
quarter of a     rotation1 so that ,,ab 
is no longer perpendicular to ,,em'4 ,an
     observer on the earth can now see  
somewhat beyond ,a on the western       
limb of the moon1 and not quite up to ,b
 on the eastern limb4 ,while     the    
moon is passing from ,m' to ,m''1 her   
axial motion again     overtakes her    
orbital motion1 so that the line ,,ab   

again becomes     perpendicular to the  
line joining the centre of the moon to  
the     centre of the earth4 ,exactly   
the same side is now turned towards the 
earth as when the moon was at ,m4 ,while
 the moon passes from ,m''     to ,m'''1
 her axial motion gets in advance of her
 orbital motion1     so that ,,ab is    
again inclined to the line joining the  
centres of     the earth and moon4 ,a   
portion of the eastern limb of the moon 
beyond     ,b is now brought into view  
to the earth1 and a portion of the      
western limb at ,a is carried out of    
view4 ,while the moon is     passing    
from ,m''' to ,m1 the orbital motion    
again overtakes the     axial motion1   
and ,,ab is again perpendicular to ,,me4
  7#c7 ,parallactic ,libration4--,while 
an observer at the centre of the earth  
would get the same view of the moon1    
whether she were on the eastern horizon1
 in the zenith1 or on the western       
horizon1 an observer on the surface of  
the earth does not get exactly the same 

view in these three cases4 ,when the    
moon is on the eastern horizon1 an      
observer on the surface of the earth    
would see a little farther around on the
 western limb of the moon than when she 
is in the zenith1 and not quite so far  
around on the eastern limb4 ,on the     
contrary1 when the moon is on the       
western horizon1 an observer on the     
surface of the earth sees a little      
farther around on the eastern limb of   
the moon than when she is in the zenith1
 and not quite so far around on her     
western limb4  ,illustration3 ,fig4     
#abj4      ,this will be evident from   
,fig4 #abj4 ,e is the centre of the     
earth1     and ,o a point on its        
surface4 ,,ab is a line drawn through   
the     centre of the moon1             
perpendicular to a line joining the     
centres of     the moon and the earth4  
,this line marks off the part of the    
moon     turned towards the centre of   
the earth1 and remains essentially the  
same during the day4 ,,cd is a line     

drawn through the centre of the     moon
 perpendicular to a line joining the    
centre of the moon and the     point of 
observation4 ,this line marks off the   
part of the moon     turned towards ,o4 
,when the moon is in the zenith1 ,,cd   
coincides     with ,,ab2 but1 when the  
moon is on the horizon1 ,,cd is inclined
 to     ,,ab4 ,when the moon is on the  
eastern horizon1 an observer at ,o      
sees a little beyond ,b1 and not quite  
to ,a2 and1 when she is on     the      
western horizon1 he sees a little beyond
 ,a1 and not quite to     ,b4 ,b is on  
the western limb of the moon1 and ,a on 
her eastern     limb4      ,since this  
libration is due to the point from which
 the moon is     viewed1 it is called   
parallactic libration2 and1 since it    
occurs     daily1 it is called diurnal  
libration4  ,illustration3 ,fig4 #aba4  
#ajg4 ,portion of the ,lunar ,surface   
brought into ,view by                   
,libration4--,the area brought into view
 by the first two librations     is     

between one-twelfth and one-thirteenth  
of the whole lunar     surface1 or      
nearly one-sixth of the hemisphere of   
the moon which is     turned away from  
the earth when the moon is at her state 
of mean     libration4 ,of course a     
precisely equal portion of the          
hemisphere     turned towards us during 
mean libration is carried out of view by
     the lunar librations4      ,if we  
add to each of these areas a fringe     
about one degree wide1 due     to the   
diurnal libration1 and which we may call
 the parallactic     fringe1 we shall   
find that the total area brought into   
view is     almost exactly one-eleventh 
part of the whole surface of the moon4  
,a     similar area is carried out of   
view2 so that the whole region thus     
swayed out of and into view amounts to  
two-elevenths of the moon's     surface4
 ,this area is shown in ,fig4 #aba1     
which is a side view of the     moon4   
,illustration3 ,fig4 #abb4      #ajh4   
,the ,moon's ,path through              

,space4--,were the earth stationary1    
the moon would describe an ellipse      
around it similar to that of ,fig4      
#aac2 but1 as the earth moves forward in
 her orbit at the same time     that the
 moon revolves around it1 the moon is   
made to describe a     sinuous path1 as 
shown by the continuous line in ,fig4   
#abb4 ,this     feature of the moon's   
path is greatly exaggerated in the upper
     portion of the diagram4 ,the form  
of her path is given with a greater     
degree of accuracy in the lower part of 
the figure 7the broken line             
represents the path of the earth72 but  
even here there is     considerable     
exaggeration4 ,the complete serpentine  
path of the moon     around the sun is  
shown1 greatly exaggerated1 in ,fig4    
#abc1 the     broken line being the path
 of the earth4  ,illustration3 ,fig4    
#abc4      ,the path described by the   
moon through space is much the same as  
that described by a point on the        
circumference of a wheel which is       

rolled over another wheel4 ,if we place 
a circular disk against the     wall1   
and carefully roll along its edge       
another circular disk 7to     which a   
piece of lead pencil has been fastened  
so as to mark upon     the wall71 the   
curve described will somewhat resemble  
that described     by the moon4 ,this   
curve is called an epicycloid1 and it   
will be     seen that at every point it 
is concave towards the centre of the    
larger disk4 ,in the same way the moon's
 orbit is at every point     concave    
towards the sun4  ,illustration3 ,fig4  
#abd4      ,the exaggeration of the     
sinuosity in ,fig4 #abc will be more    
evident     when it is stated1 that1 on 
the scale of ,fig4 #abd1 the whole of   
the     serpentine curve would lie      
within the breadth of the fine circular 
line ,,mm'4  #aji4 ,the ,lunar          
,day4--,the lunar day is twenty-nine    
times and a half as long as the         
terrestrial day4 ,near the moon's       
equator the sun shines without          

intermission nearly fifteen of our days1
 and is absent for the same length of   
time4 ,consequently1 the vicissitudes of
 temperature to which the surface is    
exposed must be very great4 ,during the 
long lunar night the temperature of a   
body on the moon's surface would        
probably fall lower than is ever known  
on the earth1 while during the day it   
must rise higher than anywhere on our   
planet4  ,illustration3 ,fig4 #abe4     
,it might seem1 that1 since the moon    
rotates on her axis in about            
twenty-seven days1 the lunar day ought  
to be twenty-seven days long1           
instead of twenty-nine4 ,there is1      
however1 a solar1 as well as a          
sidereal1 day at the moon1 as on the    
earth2 and the solar day at the     moon
 is longer than the sidereal day1 for   
the same reason as on the     earth4    
,during the solar day the moon must make
 both a synodical     rotation and a    
synodical revolution4 ,this will be     
evident from     ,fig4 #abe1 in which is

 shown the path of the moon during one  
complete     lunation4 ,e1 ,e'1 ,e''1   
etc41 are the successive positions of   
the earth2 and #a1 #b1 #c1 #d1 #e1 the  
successive positions of the moon4       
,the small arrows indicate the direction
 of the moon's rotation4 ,the     moon  
is full at #a and #e4 ,at #a1 ,a1 at the
 centre of the moon's     disk1 will    
have the sun1 which lies in the         
direction ,,as1 upon the     meridian4  
,before ,a will again have the sun on   
the meridian1 the     moon must have    
made a synodical revolution2 and1 as    
will be seen by     the dotted lines1   
she must have made more than a complete 
rotation4     ,the rotation which brings
 the point ,a into the same relation to 
the earth and sun is called a synodical 
rotation4      ,it will also be evident 
from this diagram that the moon must    
make a     synodical rotation during a  
synodical revolution1 in order always to
     present the same side to the earth4
  #aaj4 ,the ,earth as seen from the    

,moon4--,to an observer on the moon1 the
 earth would be an immense moon1 going  
through the same phases that the moon   
does to us2 but1 instead of rising and  
setting1 it would only oscillate to and 
fro through a few degrees4 ,on the other
 side of the moon it would never be seen
 at all4 ,the peculiarities of the      
moon's motions which cause the          
librations1 and make a spot on the      
moon's disk seem to an observer on the  
earth to oscillate to and fro1 would    
cause the earth as a whole to appear to 
a lunar observer to oscillate to and fro
 in the heavens in a similar manner4    
,it is a well-known fact1 that1 at the  
time of new moon1 the dark part of the  
moon's surface is partially illumined1  
so that it becomes visible to the naked 
eye4 ,this must be due to the light     
reflected to the moon from the earth4   
,since at new moon the moon is between  
the earth and sun1 it follows1 that1    
when it is new moon at the earth1 it    
must be full earth at the moon3 hence1  

while the bright crescent is enjoying   
full sunlight1 the dark part of its     
surface is enjoying the light of the    
full earth4 ,fig4 #abf represents the   
full earth as seen from the moon4       
,illustration3 ,fig4 #abf4              
,the ,atmosphere of the ,moon4   #aaa4  
,the ,moon has no ,appreciable          
,atmosphere4--,there are several reasons
 for believing that the moon has little 
or no atmosphere4  7#a7 ,had the moon an
 atmosphere1 it would be indicated at   
the time of a solar eclipse1 when the   
moon passes over the disk of the sun4   
,if the atmosphere were of any          
considerable density1 it would absorb a 
part of the sun's rays1 so as to produce
 a dusky border in front of the moon's  
disk1 as shown in ,fig4 #abg4 ,in       
reality no such dusky border is ever    
seen2 but the limb of the moon appears  
sharp1 and clearly defined1 as in ,fig4 
#abh4  ,illustration3 ,fig4 #abg4       
,illustration3 ,fig4 #abh4  ,if the     
atmosphere were not dense enough to     

produce this dusky border1 its          
refraction would be sufficient to       
distort the delicate cusps of the sun's 
crescent in the manner shown at the top 
of ,fig4 #abe2 but no such distortion is
 ever observed4 ,the cusps always appear
 clear and sharp1 as shown at the bottom
 of the figure3 hence it would seem that
 there can be no atmosphere of          
appreciable density at the moon4  7#b7  
,the absence of an atmosphere from the  
moon is also shown by the absence of    
twilight and of diffused daylight4      
,upon the earth1 twilight continues     
until the sun is eighteen degrees below 
the horizon2 that is1 day and night are 
separated by a belt twelve hundred miles
 in breadth1 in which the transition    
from light to darkness is gradual4 ,we  
have seen 7#ff7 that this twilight      
results from the refraction and         
reflection of light by our atmosphere2  
and1 if the moon had an atmosphere1 we  
should notice a similar gradual         
transition from the bright to the dark  

portions of her surface4 ,such1 however1
 is not the case4 ,the boundary between 
the light and darkness1 though          
irregular1 is sharply defined4 ,close to
 this boundary the unillumined portion  
of the moon appears just as dark as at  
any distance from it4  ,the shadows on  
the moon are also pitchy black1 without 
a trace of diffused daylight4           
,illustration3 ,fig4 #abi4      7#c7    
,the absence of an atmosphere is also   
proved by the absence of     refraction 
when the moon passes between us and the 
stars4 ,let ,,ab     7,fig4 #abi7       
represent the disk of the moon1 and ,,cd
 an atmosphere     supposed to surround 
it4 ,let ,,sae represent a straight line
 from     the earth1 touching the moon  
at ,a1 and let ,s be a star situated    
in the direction of this line4 ,if the  
moon had no atmosphere1 this     star   
would appear to touch the edge of the   
moon at ,a2 but1 if the     moon had an 
atmosphere1 a star behind the edge of   
the moon1 at ,s'1     would be visible  

at the earth2 for the ray ,s',a would be
 bent by     the atmosphere into the    
direction ,,ae'4 ,so1 also1 on the      
opposite     side of the moon1 a star   
might be seen at the earth1 although    
really     behind the edge of the moon3 
hence1 if the moon had an atmosphere1   
the time during which a star would be   
concealed by the moon would be     less 
than if it had no atmosphere1 and the   
amount of this effect     must be       
proportional to the density of the      
atmosphere4      ,the moon1 in her      
orbital course across the heavens1 is   
continually     passing before1 or      
occulting1 some of the stars that so    
thickly     stud her apparent path2 and 
when we see a star thus pass behind the 
lunar disk on one side1 and come out    
again on the other side1 we are         
virtually observing the setting and     
rising of that star upon the     moon4  
,the moon's apparent diameter has been  
measured over and over     again1 and is
 known with great accuracy2 the rate of 

her motion     across the sky is also   
known with perfect accuracy3 hence it is
 easy     to calculate how long the moon
 will take to travel across a part of   
the sky exactly equal in length to her  
own diameter4 ,supposing1     then1 that
 we observe a star pass behind the moon1
 and out again1 it     is clear1 that1  
if there is no atmosphere1 the interval 
of time     during which it remains     
occulted ought to be exactly equal to   
the     computed time which the moon    
would take to pass over the star4 ,if1  
however1 from the existence of a lunar  
atmosphere1 the star     disappears too 
late1 and re-appears too soon1 as we    
have seen it     would1 these two       
intervals will not agree2 the computed  
time will be     greater than the       
observed time1 and the difference will  
represent     the amount of refraction  
the star's light has sustained or       
suffered1     and hence the extent of   
atmosphere it has had to pass through4  
,comparisons of these two intervals of  

time have been repeatedly     made1 the 
most extensive being executed under the 
direction of the     ,astronomer ,royal 
of ,england1 several years ago1 and     
based upon no     less than two hundred 
and ninety-six occultation observations4
 ,in     this determination the measured
 or telescopic diameter of the moon     
was compared with the diameter deduced  
from the occultations2 and it     was   
found that the telescopic diameter was  
greater than the     occultation        
diameter by two seconds of angular      
measurement1 or by     about a          
thousandth part of the whole diameter of
 the moon4 ,this     discrepancy is     
probably due1 in part at least1 to      
irradiation     7#ia71 which augments   
the apparent size of the moon1 as seen  
in the     telescope as well as with the
 naked eye2 but1 if the whole two       
seconds were caused by atmospheric      
refraction1 this would imply a          
horizontal refraction of one second1    
which is only one     two-thousandth of 

the earth's horizontal refraction4 ,it  
is possible     that an atmosphere      
competent to produce this refraction    
would not     make itself visible in any
 other way4      ,but an atmosphere two 
thousand times rarer than our air can   
scarcely     be regarded as an          
atmosphere at all4 ,the contents of an  
air-pump     receiver can seldom be     
rarefied to a greater extent than to    
about a     thousandth of the density of
 air at the earth's surface2 and the    
lunar atmosphere1 if it exists at all1  
is thus proved to be twice as           
attenuated as what we commonly call a   
vacuum4                           ,the  
,surface of the ,moon4   ,illustration3 
,fig4 #acj4  #aab4 ,dusky ,patches on   
the ,disk of the ,moon4--,with the naked
 eye1 large dusky patches are seen on   
the moon1 in which popular fancy has    
detected a resemblance to a human face4 
,with a telescope of low power1 these   
dark patches appear as smooth as water1 
and they were once supposed to be seas4 

,this theory was the origin of the name 
mare 7,latin for sea71 which is still   
applied to the larger of these plains2  
but1 if there were water on the surface 
of the moon1 it could not fail to       
manifest its presence by its vapor1     
which would form an appreciable         
atmosphere4 ,moreover1 with a high      
telescopic power1 these plains present a
 more or less uneven surface2 and1 as   
the elevations and depressions are found
 to be permanent1 they cannot1 of       
course1 belong to the surface of water4 
,the chief of these plains are shown in 
,fig4 #acj4 ,they are ,mare             
,crisium1 ,mare ,foecunditatis1 ,mare   
,nectaris1 ,mare     ,tranquillitatis1  
,mare ,serenitatis1 ,mare ,imbrium1     
,mare     ,frigoris1 and ,oceanus       
,procellarum4 ,all these plains can     
easily be     recognized on the surface 
of the full moon with the unaided eye4  
#aac4 ,the ,terminator of the           
,moon4--,the terminator of the moon is  
the line which separates the bright and 

dark portions of its disk4 ,when viewed 
with a telescope of even moderate power1
 the terminator is seen to be very      
irregular and uneven4 ,many bright      
points are seen just outside of the     
terminator in the dark portion of the   
disk1 while all along in the            
neighborhood of the terminator are      
bright patches and dense shadows4 ,these
 appearances are shown in ,figs4 #aca   
and #acb1 which represent the moon near 
the first and last quarters4 ,they      
indicate that the surface of the moon is
 very rough and uneven4  ,illustration3 
,fig4 #aca4  ,illustration3 ,fig4 #acb4 
,as it is always either sunrise or      
sunset along the terminator1 the bright 
spots outside of it are clearly the tops
 of mountains1 which catch the rays of  
the sun while their bases are in the    
shade4 ,the bright patches in the       
neighborhood of the terminator are the  
sides of hills and mountains which are  
receiving the full light of the sun1    
while the dense shadows near by are cast

 by these elevations4  #aad4 ,height of 
the ,lunar ,mountains4--,there are two  
methods of finding the height of lunar  
mountains3--  7#a7 ,we may measure the  
length of the shadows1 and then         
calculate the height of the mountains   
that would cast such shadows with the   
sun at the required height above the    
horizon4      ,the length of a shadow   
may be obtained by the following method3
 the     longitudinal wire of the       
micrometer 7#ai7 is adjusted so as to   
pass     through the shadow whose length
 is to be measured1 and the             
transverse wires are placed one at each 
end of the shadow1 as shown     in ,fig4
 #acc4 ,the micrometer screw is then    
turned till the wires are     brought   
together1 so as to ascertain the length 
of the arc between     them4 ,we may    
then form the proportion3 the number of 
seconds in the     semi-diameter of the 
moon is to the number of seconds in the 
length     of the shadow1 as the length 
of the moon's radius in miles to the    

length of the shadow in miles4          
,illustration3 ,fig4 #acc4      ,the    
height of the sun above the horizon is  
ascertained by measuring     the angular
 distance of the mountain from the      
terminator4  7#b7 ,we may measure the   
distance of a bright point from the     
terminator1 and then construct a        
right-angled triangle1 as shown in ,fig4
 #acd4 ,a solution of this triangle will
 enable us to ascertain the height of   
the mountain whose top is just catching 
the level rays of the sun4              
,illustration3 ,fig4 #acd4      ,b is   
the centre of the moon1 ,m the top of   
the mountain1 and     ,,sam a ray of    
sunlight which just grazes the          
terminator at ,a1 and     then strikes  
the top of the mountain at ,m4 ,the     
triangle ,,bam is     right-angled at   
,a4 ,,ba is the radius of the moon1 and 
,,am is     known by measurement2 ,,bm1 
the hypothenuse1 may then be found by   
computation4 ,,bm is evidently equal to 
the radius of the moon     plus the     

height of the mountain4  ,by one or the 
other of these methods1 the heights of  
the lunar mountains have been found with
 a great degree of accuracy4 ,it is     
claimed that the heights of the lunar   
mountains are more accurately known than
 those of the mountains on the earth4   
,compared with the size of the moon1    
lunar mountains attain a greater height 
than those on the earth4  #aae4 ,general
 ,aspect of the ,lunar ,surface4--,a    
cursory examination of the moon with a  
low power is sufficient to show the     
prevalence of crater-like inequalities  
and the general tendency to circular    
shape which is apparent in nearly all   
the surface markings2 for even the large
 seas and the smaller patches of the    
same character repeat in their outlines 
the round form of the craters4 ,it is   
along the terminator that we see these  
crater-like spots to the best advantage2
 as it is there that the rising or      
setting sun casts long shadows over the 
lunar landscape1 and brings elevations  

into bold relief4 ,they vary greatly in 
size2 some being so large as to bear a  
sensible proportion to the moon's       
diameter1 while the smallest are so     
minute as to need the most powerful     
telescopes and the finest conditions of 
atmosphere to perceive them4            
,illustration3 ,fig4 #ace4  ,the        
prevalence of ring-shaped mountains and 
plains willbe evident from ,fig4 #ace1  
which is from a photograph of a model of
 the moon constructed by ,nasmyth4      
,this same feature is nearly as marked  
in ,figs4 #aca and #acb1 which are      
copies of ,rutherfurd's photographs of  
the moon4  #aaf4 ,lunar ,craters4--,the 
smaller saucer-shaped formations on the 
surface of the moon are called craters4 
,they are of all sizes1 from a mile to a
 hundred and fifty miles in diameter2   
and they are supposed to be of volcanic 
origin4 ,a high telescopic power shows  
that these craters vary remarkably1 not 
only in size1 but also in structure and 
arrangement4 ,some are considerably     

elevated above the surrounding surface1 
others are basins hollowed out of that  
surface1 and with low surrounding       
ramparts2 some are like walled plains1  
while the majority have their lowest    
depression considerably below the       
surrounding surface2 some are isolated  
upon the plains1 others are thickly     
crowded together1 overlapping and       
intruding upon each other2 some have    
elevated peaks or cones in their        
centres1 and some are without these     
central cones1 while others1 again1     
contain several minute craters instead2 
some have their ramparts whole and      
perfect1 others have them broken or     
deformed1 and many have them divided    
into terraces1 especially on their inner
 sides4  ,a typical lunar crater is     
shown in ,fig4 #acf4  ,illustration3    
,fig4 #acf4  ,it is not generally       
believed that any active volcanoes exist
 on the moon at the present time1 though
 some observers have thought they       
discerned indications of such volcanoes4

  ,illustration3 ,fig4 #acg4  #aag4     
,copernicus4--,this is one of the       
grandest of lunar craters 7,fig4 #acg74 
,although its diameter 7forty-six miles7
 is exceeded by others1 yet1 taken as a 
whole1 it forms one of the most         
impressive and interesting objects of   
its class4 ,its situation1 near the     
centre of the lunar disk1 renders all   
its wonderful details conspicuous1 as   
well as those of objects immediately    
surrounding it4 ,its vast rampart rises 
to upwards of twelve thousand feet above
 the level of the plateau1 nearly in the
 centre of which stands a magnificent   
group of cones1 three of which attain a 
height of more than twenty-four hundred 
feet4  ,many ridges1 or spurs1 may be   
observed leading away from the outer    
banks of the great rampart4 ,around the 
crater1 extending to a distance of more 
than a hundred miles on every side1     
there is a complex network of bright    
streaks1 which diverge in all           
directions4 ,these streaks do not appear

 in the figure1 nor are they seen upon  
the moon1 except at and near the full   
phase4 ,they show conspicuously1        
however1 by their united lustre on the  
full moon4  ,this crater is seen just to
 the south-west of the large dusky plain
 in the upper part of ,fig4 #acb4 ,this 
plain is ,mare ,imbrium1 and the        
mountain-chain seen a little to the     
right of ,copernicus is named the       
,apennines4 ,copernicus is also seen in 
,fig4 #ace1 a little to the left of the 
same range4  ,under circumstances       
specially favorable1 myriads of         
comparatively minute but perfectly      
formed craters may be observed for more 
than seventy miles on all sides around  
,copernicus4 ,the district on the       
south-east side is specially rich in    
these thickly scattered craters1 which  
we have reason to suppose stand over or 
upon the bright streaks4  #aah4 ,dark   
,chasms4--,dark cracks1 or chasms1 have 
been observed on various parts of the   
moon's surface4 ,they sometimes occur   

singly1 and sometimes in groups4 ,they  
are often seen to radiate from some     
central cone1 and they appear to be of  
volcanic origin4 ,they have been called 
canals and rills4  ,illustration3 ,fig4 
#ach4  ,one of the most remarkable      
groups of these chasms is that to the   
west of the crater named ,triesneker4   
,the crater and the chasms are shown in 
,fig4 #ach4 ,several of these great     
cracks obviously diverge from a small   
crater near the west bank of the great  
one1 and they subdivide as they extend  
from the apparent point of divergence1  
while they are crossed by others4 ,these
 cracks1 or chasms1 are nearly a mile   
broad at the widest part1 and1 after    
extending full a hundred miles1 taper   
away till they become invisible4        
,illustration3 ,fig4 #aci4  #aai4       
,mountain-,ranges4--,there are          
comparatively few mountain-ranges on the
 moon4 ,the three most conspicuous are  
those which partially enclose ,mare     
,imbrium2 namely1 the ,apennines on the 

south1 and the ,caucasus and the ,alps  
on the east and north-east4 ,the        
,apennines are the most extended of     
these1 having a length of about four    
hundred and fifty miles4 ,they rise     
gradually1 from a comparatively level   
surface towards the south-west1 in the  
form of innumerable small elevations1   
which increase in number and height     
towards the north-east1 where they      
culminate in a range of peaks whose     
altitude and rugged aspect must form one
 of the most terribly grand and romantic
 scenes which imagination can conceive4 
,the north-east face of the range       
terminates abruptly in an almost        
vertical precipice2 while over the plain
 beneath1 intensely black spire-like    
shadows are cast1 some of which at      
sunrise extend full ninety miles1 till  
they lose themselves in the general     
shading due to the curvature of the     
lunar surface4 ,many of the peaks rise  
to heights of from eighteen thousand to 
twenty thousand feet above the plain at 

their north-east base 7,fig4 #aci74     
,illustration3 ,fig4 #adj4  ,fig4 #adj  
represents an ideal lunar landscape near
 the base of such a lunar range4 ,owing 
to the absence of an atmosphere1 the    
stars will be visible in full daylight4 
,illustration3 ,fig4 #ada4  #abj4 ,the  
,valley of the ,alps4--,the range of the
 ,alps is shown in ,fig4 #ada4 ,the     
great crater at the north end of this   
range is named ,plato4 ,it is seventy   
miles in diameter4  ,the most remarkable
 feature of the ,alps is the valley near
 the centre of the range4 ,it is more   
than seventy-five miles long1 and about 
six miles wide at the broadest part4    
,when examined under favorable          
circumstances1 with a high magnifying   
power1 it is seen to be a vast          
flat-bottomed valley1 bordered by       
gigantic mountains1 some of which attain
 heights of ten thousand feet or more4  
,illustration3 ,fig4 #adb4  #aba4       
,isolated ,peaks4--,there are           
comparatively few isolated peaks to be  

found on the surface of the moon4 ,one  
of the most remarkable of these is that 
known as ,pico1 and shown in ,fig4 #adb4
 ,its height exceeds eight thousand     
feet1 and it is about three times as    
long at the base as it is broad4 ,the   
summit is cleft into three peaks1 as is 
shown by the three-peaked shadow it     
casts on the plain4  #abb4 ,bright      
,rays4--,about the time of full moon1   
with a telescope of moderate power1 a   
number of bright lines may be seen      
radiating from several of the lunar     
craters1 extending often to the distance
 of hundreds of miles4 ,these streaks do
 not arise from any perceptible         
difference of level of the surface1 they
 have no very definite outline1 and they
 do not present any sloping sides to    
catch more sunlight1 and thus shine     
brighter1 than the general surface4     
,indeed1 one great peculiarity of them  
is1 that they come out most forcibly    
when the sun is shining perpendicularly 
upon them3 hence they are best seen when

 the moon is at full1 and they are not  
visible at all at those regions upon    
which the sun is rising or setting4     
,they are not diverted by elevations in 
their path1 but traverse in their course
 craters1 mountains1 and plains alike1  
giving a slight additional brightness to
 all objects over which they pass1 but  
producing no other effect upon them4    
,they look as if1 after the whole       
surface of the moon had assumed its     
final configuration1 a vast brush       
charged with a whitish pigment had been 
drawn over the globe in straight lines1 
radiating from a central point1 leaving 
its trail upon every thing it touched1  
but obscuring nothing4  ,illustration3  
,fig4 #adc4  ,the three most conspicuous
 craters from which these lines radiate 
are ,tycho1 ,copernicus1 and ,kepler4   
,tycho is seen at the bottom of ,figs4  
#adc and #acj4 ,kepler is a little to   
the left of ,copernicus in the same     
figures4  ,it has been thought that     
these bright streaks are chasms which   

have been filled with molten lava1      
which1 on cooling1 would afford a smooth
 reflecting surface on the top4  #abc4  
,tycho4--,this crater is fifty-four     
miles in diameter1 and about sixteen    
thousand feet deep1 from the highest    
ridge of the rampart to the surface of  
the plateau1 whence rises a central cone
 five thousand feet high4 ,it is one of 
the most conspicuous of all the lunar   
craters2 not so much on account of its  
dimensions as from its being the centre 
from whence diverge those remarkable    
bright streaks1 many of which may be    
traced over a thousand miles of the     
moon's surface 7,fig4 #adc74 ,tycho     
appears to be an instance of a vast     
disruptive action which rent the solid  
crust of the moon into radiating        
fissures1 which were subsequently filled
 with molten matter1 whose superior     
luminosity marks the course of the      
cracks in all directions from the crater
 as their common centre4 ,so numerous   
are these bright streaks when examined  

by the aid of the telescope1 and they   
give to this region of the moon's       
surface such increased luminosity1 that1
 when viewed as a whole1 the locality   
can be distinctly seen at full moon by  
the unassisted eye1 as a bright patch of
 light on the southern portion of the   
disk4                     ,,iii4        
,,inferior ,,and ,,superior ,,planets4  
,inferior ,planets4   #abd4 ,the        
,inferior ,planets4--,the inferior      
planets are those which lie between the 
earth and the sun1 and whose orbits are 
included by that of the earth4 ,they are
 ,mercury and ,venus4  ,illustration3   
,fig4 #add4  #abe4 ,aspects of an       
,inferior ,planet4--,the four chief     
aspects of an inferior planet as seen   
from the earth are shown in ,fig4 #add1 
in which ,s represents the sun1 ,p the  
planet1 and ,e the earth4  ,when the    
planet is between the earth and the sun1
 as at ,p1 it is said to be in inferior 
conjunction4  ,when it is in the same   
direction as the sun1 but beyond it1 as 

at ,p''1 it is said to be in superior   
conjunction4  ,when the planet is at    
such a point in its orbit that a line   
drawn from the earth to it would be     
tangent to the orbit1 as at ,p' and     
,p'''1 it is said to be at its greatest 
elongation4  ,illustration3 ,fig4 #ade4 
#abf4 ,apparent ,motion of an ,inferior 
,planet4--,when the planet is at ,p1 if 
it could be seen at all1 it would appear
 in the heavens at ,a4 ,as it moves from
 ,p to ,p'1 it will appear to move in   
the heavens from ,a to ,b4 ,then1 as it 
moves from ,p' to ,p''1 it will appear  
to move back again from ,b to ,a4 ,while
 it moves from ,p'' to ,p'''1 it will   
appear to move from ,a to ,c2 and1 while
 moving from ,p''' to ,p1 it will appear
 to move back again from ,c to ,a4 ,thus
 the planet will appear to oscillate to 
and fro across the sun from ,b to ,c1   
never getting farther from the sun than 
,b on the west1 or ,c on the east3      
hence1 when at these points1 it is said 
to be at its greatest western and       

eastern elongations4 ,this oscillating  
motion of an inferior planet across the 
sun1 combined with the sun's motion     
among the stars1 causes the planet to   
describe a path among the stars similar 
to that shown in ,fig4 #ade4            
,illustration3 ,fig4 #adf4  #abg4       
,phases of an ,inferior ,planet4--,an   
inferior planet1 when viewed with a     
telescope1 is found to present a        
succession of phases similar to those of
 the moon4 ,the reason of this is       
evident from ,fig4 #adf4 ,as an inferior
 planet passes around the sun1 it       
presents sometimes more and sometimes   
less of its bright hemisphere to the    
earth4 ,when the earth is at ,t1 and    
,venus at superior conjunction1 the     
planet turns the whole of its bright    
hemisphere towards the earth1 and       
appears full2 it then becomes gibbous1  
half1 and crescent4 ,when it comes into 
inferior conjunction1 it turns its dark 
hemisphere towards the earth3 it then   
becomes crescent1 half1 gibbous1 and    

full again4  #abh4 ,the ,sidereal and   
,synodical ,periods of an ,inferior     
,planet4--,the time it takes a planet to
 make a complete revolution around the  
sun is called the sidereal period of the
 planet2 and the time it takes it to    
pass from one aspect around to the same 
aspect again1 its synodical period4     
,illustration3 ,fig4 #adg4  ,the        
synodical period of an inferior planet  
is longer than its sidereal period4     
,this will be evident from an           
examination of ,fig4 #adg4 ,s is the    
position of the sun1 ,e that of the     
earth1 and ,p that of the planet at     
inferior conjunction4 ,before the planet
 can be in inferior conjunction again1  
it must pass entirely around its orbit1 
and overtake the earth1 which has in the
 mean time passed on in its orbit to    
,e'4  ,while the earth is passing from  
,e to ,e'1 the planet passes entirely   
around its orbit1 and from ,p to ,p' in 
addition4 ,now the arc ,,pp' is just    
equal to the arc ,,ee'3 hence the planet

 has to pass over the same arc that the 
earth does1 and #cfj more4 ,in other    
words1 the planet has to gain #cfj on   
the earth4  ,the synodical period of the
 planet is found by direct observation4 
#abi4 ,the ,length of the ,sidereal     
,period4--,the length of the            
sidereal period of an inferior planet   
may be found by the following           
computation3--          ,let a denote   
the synodical period of the planet1     
,let b denote the sidereal period of the
 earth1         ,let x denote the       
sidereal period of the planet4          
,then #cfj_/;b  the daily motion of the 
earth1         ,and #cfj_/x  the daily  
motion of the planet1         ,and      
#cfj_/x - #cfj_/;b  the daily gain of   
the planet3         ,also #cfj_/;a  the 
daily gain of the planet3         ,hence
 #cfj_/x - #cfj_/;b  #cfj_/;a4          
,dividing by #cfj1 we have #a_/x -      
#a_/;b  #a_/;a2         ,clearing of    
fractions1 we have ab - ax  bx3         
,transposing and collecting1 we have 7a 

b7x  ab3          ,therefore x  ab_/ab4 
#acj4 ,the ,relative ,distance of an    
,inferior ,planet4--,by the     relative
 distance of a planet1 we mean its      
distance from the sun     compared with 
the earth's distance from the sun4 ,the 
relative     distance of an inferior    
planet may be found by the following    
method3--  ,illustration3 ,fig4 #adh4   
,let ,v1 in ,fig4 #adh1 represent the   
position of ,venus at its     greatest  
elongation from the sun1 ,s the position
 of the sun1 and     ,e that of the     
earth4 ,the line ,,ev will evidently be 
tangent to a     circle described about 
the sun with a radius equal to the      
distance     of ,venus from the sun at  
the time of this greatest elongation4   
,draw     the radius ,,sv and the line  
,,se4 ,since ,,sv is a radius1 the angle
     at ,v is a right angle4 ,the angle 
at ,e is known by measurement1     and  
the angle at ,s is equal to #ij- the    
angle ,e4 ,in the     right-angled      
triangle ,,evs1 we then know the three  

angles1 and we     wish to find the     
ratio of the side ,,sv to the side ,,se4
      ,the ratio of these lines may be  
found by trigonometrical computation    
as follows3--          ,,vs 3 ,,es  sin 
,,sev 3 #a4      ,substitute the value  
of the sine of ,,sev1 and we have       
,,vs 3 ,,es  4#gbc 3 #a4      ,hence the
 relative distances of ,venus and of the
 earth from the sun     are 4#gbc and   
#a4                                     
,superior ,planets4   #aca4 ,the        
,superior ,planets4--,the superior      
planets are those which lie beyond the  
earth4 ,they are ,mars1 the ,asteroids1 
,jupiter1 ,saturn1 ,uranus1 and         
,neptune4  ,illustration3 ,fig4 #adi4   
#acb4 ,apparent ,motion of a ,superior  
,planet4--,in order to deduce the       
apparent motion of a superior planet    
from the real motions of the earth and  
planet1 let ,s 7,fig4 #adi7 be the place
 of the sun2 #a1 #b1 #c1 etc41 the orbit
 of the earth2 a1 b1 c1 etc41 the orbit 
of ,mars2 and ,,cgl a part of the starry

 firmament4 ,let the orbit of the earth 
be divided into twelve equal parts1 each
 described in one month2 and let ab1 bc1
 cd1 etc41 be the spaces described by   
,mars in the same time4 ,suppose the    
earth to be at the point #a when ,mars  
is at the point a1 ,mars will then      
appear in the heavens in the direction  
of #a a4 ,when the earth is at #c1 and  
,mars at c1 he will appear in the       
heavens at ,c4 ,when the earth arrives  
at #d1 ,mars will arrive at d1 and will 
appear in the heavens at ,d4 ,while the 
earth moves from #d to #e and from #e to
 #f1 ,mars will appear to have advanced 
among the stars from ,d to ,e and from  
,e to ,f1 in the direction from west to 
east4 ,during the motion of the earth   
from #f to #g and from #g to #h1 ,mars  
will appear to go backward from ,f to ,g
 and from ,g to ,h1 in the direction    
from east to west4 ,during the motion of
 the earth from #h to #i and from #i to 
#aj1 ,mars will appear to advance from  
,h to ,i and from ,i to ,k1 in the      

direction from west to east1 and the    
motion will continue in the same        
direction until near the succeeding     
opposition4  ,the apparent motion of a  
superior planet projected on the heavens
 is thus seen to be similar to that of  
an inferior planet1 except that1 in the 
latter case1 the retrogression takes    
place near inferior conjunction1 and in 
the former it takes place near          
opposition4  ,illustration3 ,fig4 #aej4 
#acc4 ,aspects of a ,superior           
,planet4--,the four aspects of a        
superior planet are shown in ,fig4 #aej1
 in which ,s is the position of the sun1
 ,e that of the earth1 and ,p that of   
the planet4  ,when the planet is on the 
opposite side of the earth to the sun1  
as at ,p1 it is said to be in           
opposition4 ,the sun and the planet will
 then appear in opposite parts of the   
heavens1 the sun appearing at ,c1 and   
the planet at ,a4  ,when the planet is  
on the opposite side of the sun to the  
earth1 as at ,p''1 it is said to be in  

superior conjunction4 ,it will then     
appear in the same part of the heavens  
as the sun1 both appearing at ,c4  ,when
 the planet is at ,p' and ,p'''1 so that
 a line drawn from the earth through the
 planet will make a right angle with a  
line drawn from the earth to the sun1 it
 is said to be in quadrature4 ,at ,p' it
 is in its western quadrature1 and at   
,p''' in its eastern quadrature4        
,illustration3 ,fig4 #aea4  #acd4       
,phases of a ,superior ,planet4--,mars  
is the only one of the superior planets 
that has appreciable phases4 ,at        
quadrature1 as will appear from ,fig4   
#aea1 ,mars does not present quite the  
same side to the earth as to the sun3   
hence1 near these parts of its orbit1   
the planet appears slightly gibbous4    
,elsewhere in its orbit1 the planet     
appears full4  ,all the other superior  
planets are so far away from the sun and
 earth1 that the sides which they turn  
towards the sun and the earth in every  
part of their orbit are so nearly the   

same1 that no change in the form of     
their disks can be detected4  #ace4 ,the
 ,synodical ,period of a ,superior      
,planet4--,during a synodical period of 
a superior planet the earth must gain   
one revolution1 or #cfj1 on the planet1 
as will be evident from an examination  
of ,fig4 #aeb1 in which ,s represents   
the sun1 ,e the earth1 and ,p the planet
 at opposition4 ,before the planet can  
be in opposition again1 the earth must  
make a complete revolution1 and overtake
 the planet1 which has in the mean time 
passed on from ,p to ,p'4               
,illustration3 ,fig4 #aeb4  ,in the case
 of most of the superior planets the    
synodical period is shorter than the    
sidereal period2 but in the case of     
,mars it is longer1 since ,mars makes   
more than a complete revolution before  
the earth overtakes it4  ,the synodical 
period of a superior planet is found by 
direct observation4      #acf4 ,the     
,sidereal ,period of a ,superior        
,planet4--,the sidereal     period of a 

superior planet is found by a method of 
computation     similar to that for     
finding the sidereal period of an       
inferior     planet3--          ,let a  
denote the synodical period of the      
planet1         ,let b denote the       
sidereal period of the earth1           
,let x denote the sidereal period of the
 planet4         ,then will #cfj_/;b    
daily motion of the earth1         ,and 
#cfj_/x  daily motion of the planet2    
,also #cfj_/;b - #cfj_/x  daily gain of 
the earth4         ,but #cfj_/;a  daily 
gain of the earth3         ,hence       
#cfj_/;b - #cfj_/x  #cfj_/;a            
#a_/;b - #a_/x  #a_/;a          ax - ab 
bx          7a-b7x  ab          x       
ab_/7a-b74  ,illustration3 ,fig4 #aec4  
#acg4 ,the ,relative ,distance of a     
,superior ,planet4--,let ,s1 e1     and 
m1 in ,fig4 #aec1 represent the relative
 positions of the sun1     the earth1   
and ,mars1 when the latter planet is in 
opposition4 ,let     ,e and ,m represent
 the relative positions of the earth and

 ,mars     the day after opposition4 ,at
 the first observation ,mars will be    
seen     in the direction em,a1 and at  
the second observation in the           
direction ,,ema4      ,but the fixed    
stars are so distant1 that if a line1   
e,a1 were drawn     to a fixed star at  
the first observation1 and a line1 ,,eb1
 drawn     from the earth to the same   
fixed star at the second observation1   
these two lines would be sensibly       
parallel2 that is1 the fixed star       
would be seen in the direction of the   
line e,a at the first     observation1  
and in the direction of the line ,,eb1  
parallel to     e,a1 at the second      
observation4 ,but if ,mars were seen in 
the     direction of the fixed star at  
the first observation1 it would         
appear back1 or west1 of that star at   
the second observation by the           
angular distance ,,bea2 that is1 the    
planet would have retrograded     that  
angular distance4 ,now1 this            
retrogression of ,mars during one       

day1 at the time of opposition1 can be  
measured directly by     observation4   
,this measurement gives us the value of 
the angle ,,bea2     but we know the    
rate at which both the earth and ,mars  
are moving in     their orbits1 and from
 this we can easily find the angular    
distance     passed over by each in one 
day4 ,this gives us the angles ,,esa and
     ,,msa4 ,we can now find the        
relative length of the lines ,,ms and   
,,es 7which represent the distances of  
,mars and of the earth from     the     
sun71 both by construction and by       
trigonometrical computation4      ,since
 ,,eb and e,a are parallel1 the angle   
,,eas is equal to ,,bea4          ,,sea 
#ahj - 7,,esa  ,,eas7         ,,esm     
,,esa - ,,msa         ,,ems  #ahj -     
7,,sea  ,,esm74      ,we have then      
,,ms 3 ,,es  sin ,,sea 3 sin ,,ems4     
,substituting the values of the sines1  
and reducing the ratio to its     lowest
 terms1 we have          ,,ms 3 ,,es    
#a4ebd 3 #a4      ,thus we find that the

 relative distances of ,mars and the    
earth from     the sun are #a4ebd and   
#a4 ,by the simple observation of its   
greatest     elongation1 we are able to 
determine the relative distances of an  
inferior planet and the earth from the  
sun2 and1 by the equally     simple     
observation of the daily retrogression  
of a superior planet1     we can find   
the relative distances of such a planet 
and the earth     from the sun4         
,,iv4 ,,the ,,sun4                      
,i4 ,,magnitude ,,and ,,distance ,,of   
,,the ,,sun4   ,illustration3 ,fig4     
#aed4  #ach4 ,the ,volume of the        
,sun4--,the apparent diameter of the sun
 is about #cb'1 being a little greater  
than that of the moon4 ,the real        
diameter of the sun is #hff1djj miles1  
or about a hundred and nine times that  
of the earth4  ,as the diameter of the  
moon's orbit is only about #dhj1jjj     
miles1 or some sixty times the diameter 
of the earth1 it follows that the       
diameter of the sun is nearly double    

that of the moon's orbit3 hence1 were   
the centre of the sun placed at the     
centre of the earth1 the sun would      
completely fill the moon's orbit1 and   
reach nearly as far beyond it in every  
direction as it is from the earth to the
 moon4 ,the circumference of the sun as 
compared with the moon's orbit is shown 
in ,fig4 #aed4  ,the volume of the sun  
is #a1cje1jjj times that of the earth4  
#aci4 ,the ,mass of the ,sun4--,the sun 
is much less dense than the earth4 ,the 
mass of the sun is only #ccj1jjj times  
that of the earth1 and its density only 
about a fourth that of the earth4       
,to find the mass of the sun1 we first  
ascertain the distance the     earth    
would draw the moon towards itself in a 
given time1 were the     moon at the    
distance of the sun1 and then form the  
proportion3 as     the distance the     
earth would draw the moon towards itself
 is to the     distance that the sun    
draws the earth towards itself in the   
same     time1 so is the mass of the    

earth to the mass of the sun4  ,although
 the mass of the sun is over three      
hundred thousand times that of the      
earth1 the pull of gravity at the       
surface of the sun is only about        
twenty-eight times as great as at the   
surface of the earth4 ,this is because  
the distance from the surface of the sun
 to its centre is much greater than from
 the surface to the centre of the earth4
  ,illustration3 ,fig4 #aee4  #adj4     
,size of the ,sun ,compared with that of
 the ,planets4--,the size of the sun    
compared with that of the larger planets
 is shown in ,fig4 #aee4 ,the mass of   
the sun is more than seven hundred and  
fifty times that of all of the planets  
and moons in the solar system4 ,in ,fig4
 #aef is shown the apparent size of the 
sun as seen from the different planets4 
,the apparent diameter of the sun       
decreases as the distance from it       
increases1 and the disk of the sun      
decreases as the square of the distance 
from it increases4  ,illustration3 ,fig4

 #aef4  #ada4 ,the ,distance of the     
,sun4--,the mean distance of the sun    
from the earth is about #ib1hjj1jjj     
miles4 ,owing to the eccentricity of the
 earth's orbit1 the distance of the sun 
varies somewhat2 being about #c1jjj1jjj 
miles less in ,january1 when the earth  
is at perihelion1 than in ,june1 when   
the earth is at aphelion4      ,but1    
though the distance of the sun can      
easily be stated in     figures1 it is  
not possible to give any real idea of a 
space so     enormous3 it is quite      
beyond our power of conception4 ,if one 
were to     try to walk such a distance1
 supposing that he could walk four miles
     an hour1 and keep it up for ten    
hours every day1 it would take          
sixty-eight years and a half to make a  
single million of miles1 and     more   
than sixty-three hundred years to       
traverse the whole4      ,if some       
celestial railway could be imagined1 the
 journey to the     sun1 even if our    
trains ran sixty miles an hour day and  

night and     without a stop1 would     
require over a hundred and seventy-five 
years4     ,sensation1 even1 would not  
travel so far in a human lifetime4 ,to  
borrow the curious illustration of      
,professor ,mendenhall1 if we could     
imagine an infant with an arm long      
enough to enable him to touch the       
sun and burn himself1 he would die of   
old age before the pain could     reach 
him2 since1 according to the experiments
 of ,helmholtz and     others1 a nervous
 shock is communicated only at the rate 
of about a     hundred feet per second1 
or #a1fcg miles a day1 and would need   
more     than a hundred and fifty years 
to make the journey4 ,sound would do    
it in about fourteen years1 if it could 
be transmitted through     celestial    
space2 and a cannon-ball in about nine1 
if it were to move     uniformly with   
the same speed as when it left the      
muzzle of the gun4     ,if the earth    
could be suddenly stopped in her orbit1 
and allowed to     fall unobstructed    

toward the sun1 under the accelerating  
influence     of his attraction1 she    
would reach the centre in about four    
months4     ,i have said if she could be
 stopped2 but such is the compass of her
     orbit1 that1 to make its circuit in
 a year1 she has to move nearly         
nineteen miles a second1 or more than   
fifty times faster than the     swiftest
 rifle-ball2 and1 in moving twenty      
miles1 her path deviates     from       
perfect straightness by less than an    
eighth of an inch4 ,and     yet1 over   
all the circumference of this tremendous
 orbit1 the sun     exercises his       
dominion1 and every pulsation of his    
surface receives     its response from  
the subject earth4 7,professor ,c4 ,a4  
,young3 ,the     ,sun47      #adb4      
,method of ,finding the ,sun's          
,distance4--,there are several          
methods of finding the sun's distance4  
,the simplest method is that     of     
finding the actual distance of one of   
the nearer planets by     observing its 

displacement in the sky as seen from    
widely separated     points on the      
earth4 ,as the relative distances of the
 planets from     each other and from   
the sun are well known1 we can easily   
deduce the     actual distance of the   
sun if we can find that of any of the   
planets4 ,the two planets usually chosen
 for this method are ,mars and          
,venus4      7#a7 ,the displacement of  
,mars in the sky1 as seen from two      
observatories which differ considerably 
in latitude1 is1 of course1     greatest
 when ,mars is nearest the earth4 ,now1 
it is evident than     ,mars will be    
nearer the earth when in opposition than
 when in any     other part of its      
orbit2 and the planet will be least     
distant from     the earth when it is at
 its perihelion point1 and the earth is 
at     its aphelion point1 at the time  
of opposition4 ,this method1 then1      
can be used to the best advantage1 when1
 at the time of opposition1     ,mars is
 near its perihelion1 and the earth near

 its aphelion4 ,these     favorable     
oppositions occur about once in fifteen 
years1 and the     last one was in      
#ahgg4  ,illustration3 ,fig4 #aeg4      
,suppose two observers situated at ,n'  
and ,s' 7,fig4 #aeg71 near the     poles
 of the earth4 ,the one at ,n' would see
 ,mars in the sky at     ,n1 and the one
 at ,s' would see it at ,s4 ,the        
displacement would     be the angle     
,,nms4 ,each observer measures carefully
 the distance of     ,mars from the same
 fixed star near it4 ,the difference of 
these     distances gives the           
displacement of the planet1 or the angle
 ,,nms4     ,these observations were    
made with the greatest care in #ahgg4   
7#b7 ,venus is nearest the earth at the 
time of inferior conjunction2     but it
 can then be seen only in the daytime4  
,it is1 therefore1     impossible to    
ascertain the displacement of ,venus1 as
 seen from     different stations1 by   
comparing her distances from a fixed    
star4     ,occasionally1 at the time of 

inferior conjunction1 ,venus passes     
directly across the sun's disk4 ,the    
last of these transits of     ,venus    
occurred in #ahgd1 and the next will    
occur in #ahhb4 ,it will     then be    
over a hundred years before another will
 occur4  ,illustration3 ,fig4 #aeh4     
,suppose two observers1 ,a and ,b 7,fig4
 #aeh71 near the poles of the     earth 
at the time of a transit of ,venus4 ,the
 observer at ,a would     see ,venus    
crossing the sun at ,v#b1 and the one at
 ,b would see     it crossing at ,v#a4  
,any observation made upon ,venus1 which
 would     give the distance and        
direction of ,venus from the centre of  
the sun1     as seen from each station1 
would enable us to calculate the angular
     distance between the two chords    
described across the sun4 ,this1 of     
course1 would give the displacement of  
,venus on the sun's disk4 ,this         
method was first employed at the last   
transits of ,venus which     occurred   
before #ahgd2 namely1 those of #agfa and

 #agfi4      ,there are three methods of
 observation employed to ascertain the  
apparent direction and distance of      
,venus from the centre of the sun1      
called respectively the contact method1 
the micrometric method1     and the     
photographic method4      7a7 ,in the   
contact method1 the observation consists
 in noting     the exact time when      
,venus crosses the sun's limb4 ,to      
ascertain this     it is necessary to   
observe the exact time of external and  
internal     contact4 ,this observation1
 though apparently simple1 is really    
very     difficult4 ,with reference to  
this method ,professor ,young says1--   
,the difficulties depend in part upon   
the imperfections of optical            
instruments and the human eye1 partly   
upon the essential nature of     light  
leading to what is known as diffraction1
 and partly upon the     action of the  
planet's atmosphere4 ,the two           
first-named causes     produce what is  
called irradiation1 and operate to make 

the apparent     diameter of the planet1
 as seen on the solar disk1 smaller than
 it     really is2 smaller1 too1 by an  
amount which varies with the size of    
the telescope1 the perfection of its    
lenses1 and the tint and     brightness 
of the sun's image4 ,the edge of the    
planet's image is     also rendered     
slightly hazy and indistinct4           
,illustration3 ,fig4 #aei4      ,the    
planet's atmosphere also causes its disk
 to be surrounded by a     narrow ring  
of light1 which becomes visible long    
before the planet     touches the sun1  
and1 at the moment of internal contact1 
produces an     appearance1 of which the
 accompanying figure is intended to give
 an     idea1 though on an exaggerated  
scale4 ,the planet moves so slowly as   
to occupy more than twenty minutes in   
crossing the sun's limb2 so     that    
even if the planet's edge were perfectly
 sharp and definite1     and the sun's  
limb undistorted1 it would be very      
difficult to     determine the precise  

second at which contact occurs4 ,but1 as
 things     are1 observers with         
precisely similar telescopes1 and side  
by side1     often differ from each     
other five or six seconds2 and1 where   
the     telescopes are not similar1 the 
differences and uncertainties are       
much greater4444 ,astronomers1          
therefore1 at present are pretty much   
agreed that such observations can be of 
little value in removing the            
remaining uncertainty of the parallax1  
and are disposed to put more            
reliance upon the micrometric and       
photographic methods1 which are     free
 from these peculiar difficulties1      
though1 of course1 beset with           
others1 which1 however1 it is hoped will
 prove less formidable4      7b7 ,of the
 micrometric method1 as employed at the 
last transit1     ,professor ,young     
speaks as follows3--      ,the          
micrometric method requires the use of a
 heliometer1--an     instrument common  
only in ,germany1 and requiring much    

skill and     practice in its use in    
order to obtain with it accurate        
measures4 ,at     the late transit1 a   
single ,english party1 two or three of  
the     ,russian parties1 and all five  
of the ,german1 were equipped with      
these instruments2 and at some of the   
stations extensive series of            
measures were made4 ,none of the        
results1 however1 have appeared as      
yet2 so that it is impossible to say how
 greatly1 if at all1 this     method    
will have the advantage in precision    
over the contact     observations4      
7c7 ,the following observations1 with   
reference to the     photographic       
method1 are also taken from ,professor  
,young3--      ,the ,americans and      
,french placed their main reliance upon 
the     photographic method1 while the  
,english and ,germans also provided for 
its use to a certain extent4 ,the great 
advantage of this method is1     that it
 makes it possible to perform the       
necessary measurements     7upon whose  

accuracy every thing depends7 at leisure
 after the     transit1 without hurry1  
and with all possible precautions4 ,the 
field-work consists merely in obtaining 
as many and as good pictures     as     
possible4 ,a principal objection to the 
method lies in the     difficulty of    
obtaining good pictures1 i4e41 pictures 
free from     distortion1 and so        
distinct and sharp as to bear high      
magnifying     power in the microscopic 
apparatus used for their measurement4   
,the     most serious difficulty1       
however1 is involved in the accurate    
determination of the scale of the       
picture2 that is1 of the number of      
seconds of arc corresponding to a linear
 inch upon the plate4     ,besides this1
 we must know the exact ,greenwich time 
at which each     picture is taken1 and 
it is also extremely desirable that the 
orientation of the picture should be    
accurately determined2 that     is1 the 
north and south1 the east and west      
points of the solar image     on the    

finished plate4 ,there has been a good  
deal of anxiety lest     the image1     
however accurate and sharp when first   
produced1 should     alter1 in course of
 time1 through the contraction of the   
collodion     film on the glass plate2  
but the experiments of ,rutherfurd1     
,huggins1     and ,paschen1 seem to show
 that this danger is imaginary4444 ,the 
,americans placed the photographic      
telescope exactly in line with a        
meridian instrument1 and so determined1 
with the extremest     precision1 the   
direction in which it was pointed4      
,knowing this and     the time at which 
any picture was taken1 it becomes       
possible1 with     the help of the      
plumb-line image1 to determine precisely
 the     orientation of the picture1--an
 advantage possessed by the ,american   
pictures alone1 and making their value  
nearly twice as great as     otherwise  
it would have been4      ,the figure    
below is a representation of one of the 
,american     photographs reduced about 

one-half4 ,v is the image of ,venus1    
which1 on the actual plate1 is about a  
seventh of an inch in     diameter2 aa' 
is the image of the plumb-line4 ,the    
centre of the     reticle is marked with
 a cross4  ,illustration3 ,fig4 #afj4   
,the ,english photographs proved to be  
of little value1 and the     results of 
the measurements and calculations upon  
the ,american     pictures have not yet 
been published4 ,there is a growing     
apprehension that no photographic method
 can be relied upon4  ,the most recent  
determinations by various methods       
indicate that the sun's distance is such
 that his parallax is about eighty-eight
 seconds4 ,this would make the linear   
value of a second at the surface of the 
sun about four hundred and fifty miles4 
,illustration3 ,plate #a4               
,,ii4 ,,physical ,,and ,,chemical       
,,condition ,,of ,,the ,,sun4           
,physical ,condition of the ,sun4       
#adc4 ,the ,sun ,composed mainly of     
,gas4--,it is now generally believed    

that the sun is mainly a ball of gas1 or
 vapor1 powerfully condensed at the     
centre by the weight of the             
superincumbent mass1 but kept from      
liquefying by its exceedingly high      
temperature4  ,the gaseous interior of  
the sun is surrounded by a layer of     
luminous clouds1 which constitutes its  
visible surface1 and which is called its
 photosphere4 ,here and there in the    
photosphere are seen dark spots1 which  
often attain an immense magnitude4      
,these clouds float in the solar        
atmosphere1 which extends some distance 
beyond them4  ,the luminous surface of  
the sun is surrounded by a rose-colored 
stratum of gaseous matter1 called the   
chromosphere4 ,here and there great     
masses of this chromospheric matter rise
 high above the general level4 ,these   
masses are called prominences4  ,outside
 of the chromosphere is the corona1 an  
irregular halo of faint1 pearly light1  
mainly composed of filaments and        
streamers1 which radiate from the sun to

 enormous distances1 often more than a  
million of miles4  ,in ,fig4 #afa is    
shown a section of the sun1 according to
 ,professor ,young4  ,the accompanying  
lithographic plate gives a general view 
of the photosphere with its spots1 and  
of the chromosphere and its prominences4
  #add4 ,the ,temperature of the        
,sun4--,those who have investigated the 
subject of the temperature of the sun   
have come to very different conclusions2
 some placing it as high as four million
 degrees ,fahrenheit1 and others as low 
as ten thousand degrees4 ,professor     
,young thinks that ,rosetti's estimate  
of eighteen thousand degrees as the     
effective temperature of the sun's      
surface is probably not far from        
correct4 ,by this is meant the          
temperature that a uniform surface of   
lampblack of the size of the sun must   
have in order to radiate as much heat as
 the sun does4 ,the most intense        
artificial heat does not exceed four    
thousand degrees ,fahrenheit4           

,illustration3 ,fig4 #afa4  #ade4 ,the  
,amount of ,heat ,radiated by the       
,sun4--,a unit of heat is the amount of 
heat required to raise a pound of water 
one degree in temperature4 ,it takes    
about a hundred and forty-three units of
 heat to melt a pound of ice without    
changing its temperature4 ,a cubic foot 
of ice weighs about fifty-seven pounds4 
,according to ,sir ,william ,herschel1  
were all the heat radiated by the sun   
concentrated on a cylinder of ice       
forty-five miles in diameter1 it would  
melt it off at the rate of about a      
hundred and ninety thousand miles a     
second4  ,professor ,young gives the    
following illustration of the energy of 
solar radiation3 ,if we could build up a
 solid column of ice from the earth to  
the sun1 two miles and a quarter in     
diameter1 spanning the inconceivable    
abyss of ninety-three million miles1 and
 if then the sun should concentrate his 
power upon it1 it would dissolve and    
melt1 not in an hour1 nor a minute1 but 

in a single second4 ,one swing of the   
pendulum1 and it would be water2 seven  
more1 and it would be dissipated in     
vapor4  ,illustration3 ,fig4 #afb4      
,this heat would be sufficient to melt a
 layer of ice nearly fifty feet thick   
all around the sun in a minute4 ,to     
develop this heat would require the     
hourly consumption of a layer of        
anthracite coal1 more than sixteen feet 
thick1 over the entire surface of the   
sun2 and the mechanical equivalent of   
this heat is about ten thousand         
horse-power on every square foot of the 
sun's surface4  #adf4 ,the ,brightness  
of the ,sun's ,surface4--,the sun's     
surface is a hundred and ninety thousand
 times as bright as a candle-flame1 a   
hundred and forty-six times as bright as
 the calcium-light1 and about three     
times and a half as bright as the       
voltaic arc4  ,the sun's disk is much   
less bright near the margin than near   
the centre1 a point on the limb of the  
sun being only about a fourth as bright 

as one near the centre of the disk4     
,this diminution of brightness towards  
the margin of the disk is due to the    
increase in the absorption of the solar 
atmosphere as we pass from the centre   
towards the margin of the sun's disk2   
and this increased absorption is due to 
the fact1 that the rays which reach us  
from near the margin have to traverse a 
much greater thickness of the solar     
atmosphere than those which reach us    
from the centre of the disk4 ,this will 
be evident from ,fig4 #afb1 in which the
 arrows mark the paths of rays from     
different parts of the solar disk4      
,the ,spectroscope4   ,illustration3    
,fig4 #afc4  #adg4 ,the ,spectroscope as
 an ,astronomical ,instrument4--,the    
spectroscope is now continually employed
 in the study of the physical condition 
and chemical constitution of the sun and
 of the other heavenly bodies4 ,it has  
become almost as indispensable to the   
astronomer as the telescope4  #adh4 ,the
 ,dispersion ,spectroscope4--,the       

essential parts of the dispersion       
spectroscope are shown in ,fig4 #afc4   
,these are the collimator tube1 the     
prism1 and the telescope4 ,the          
collimator tube has a narrow slit at one
 end1 through which the light to be     
examined is admitted1 and somewhere     
within the tube a lens for condensing   
the light4 ,the light is dispersed on   
passing through the prism3 it then      
passes through the objective of the     
telescope1 and forms within the tube an 
image of the spectrum1 which is examined
 by means of the eye-piece4 ,the power  
of the spectroscope is increased by     
increasing the number of prisms1 which  
are arranged so that the light shall    
pass through one after another in       
succession4 ,such an arrangement of     
prisms is shown in ,fig4 #afd4 ,one end 
of the collimator tube is seen at the   
left1 and one end of the telescope at   
the right4 ,sometimes the prisms are    
made long1 and the light is sent twice  
through the same train of prisms1 once  

through the lower1 and once through the 
upper1 half of the prisms4 ,this is     
accomplished by placing a rectangular   
prism against the last prism of the     
train1 as shown in ,fig4 #afe4          
,illustration3 ,fig4 #afd4              
,illustration3 ,fig4 #afe4  #adi4 ,the  
,micrometer ,scale4--,various devices   
are employed to obtain an image of a    
micrometer scale in the tube of the     
telescope beside that of the spectrum4  
,illustration3 ,fig4 #aff4  ,one of the 
simplest of these methods is shown in   
,fig4 #aff4 ,a is the telescope1 ,b the 
collimator1 and ,c the micrometer tube4 
,the opening at the outer end of ,c     
contains a piece of glass which has a   
micrometer scale marked upon it4 ,the   
light from the candle shines through    
this glass1 falls upon the surface of   
the prism ,p1 and is thence reflected   
into the telescope1 where it forms an   
enlarged image of the micrometer scale  
alongside the image of the spectrum4    
,illustration3 ,fig4 #afg4  #aej4 ,the  

,comparison of ,spectra4--,in order to  
compare two spectra1 it is desirable to 
be able to see them side by side in the 
telescope4 ,the images of two spectra   
may be obtained side by side in the     
telescope tube by the use of a little   
rectangular prism1 which covers one-half
 of the slit of the collimator tube1 as 
shown in ,fig4 #afg4 ,the light from one
 source is admitted directly through the
 uncovered half of the slit1 while the  
light from the other source is sent     
through the covered portion of the slit 
by reflection from the surface of the   
rectangular prism4 ,this arrangement and
 its action will be readily understood  
from ,fig4 #afg4  ,illustration3 ,fig4  
#afh4  #aea4 ,direct-,vision            
,spectroscope4--,a beam of light may be 
dispersed1 without any ultimate         
deflection from its course1 by combining
 prisms of crown and flint glass with   
equal refractive1 but unequal dispersive
 powers4 ,such a combination of prisms  
is called a direct-vision combination4  

,one of three prisms is shown in ,fig4  
#afh1 and one of five prisms in ,fig4   
#afi4  ,illustration3 ,fig4 #afi4       
,illustration3 ,fig4 #agj4  ,a          
direct-vision spectroscope 7,fig4 #agj7 
is one in which a direct-vision         
combination of prisms is employed4 ,c is
 the collimator tube1 ,p the train of   
prisms1 ,f the telescope1 and r the     
comparison prism4  ,illustration3 ,fig4 
#aga4  #aeb4 ,the                       
,telespectroscope4--,the spectroscope1  
when used for astronomical work1 is     
usually combined with a telescope4 ,the 
compound instrument is called a         
telespectroscope4 ,the spectroscope is  
mounted at the end of the telescope in  
such a way that the image formed by the 
object-glass of the telescope falls upon
 the slit at the end of the collimator  
tube4 ,a telespectroscope of small      
dispersive power is shown in ,fig4 #aga2
 a being the object-glass of the        
telescope1 cc the tube of the telescope1
 and e the comparison prism at the end  

of the collimator tube4 ,a more powerful
 instrument is shown in ,fig4 #agb4 ,a  
is the telescope1 ,c the collimator tube
 of the spectroscope1 ,p the train of   
prisms1 and ,e the telescope tube4 ,fig4
 #agc shows a still more powerful       
spectroscope attached to the great      
,newall refractor 7#ah74  ,illustration3
 ,fig4 #agb4  ,illustration3 ,fig4 #agc4
  #aec4 ,the ,diffraction               
,spectroscope4--,a diffraction          
spectroscope is one in which the        
spectrum is produced by reflection of   
the light from a finely ruled surface1  
or grating1 as it is called1 instead of 
by dispersion in passing through a      
prism4 ,the essential parts of this     
instrument are shown in ,fig #agd4 ,this
 spectroscope may be attached to the    
telescope in the same manner as the     
dispersion spectroscope4 ,when the      
spectroscope is thus used1 the eye-piece
 of the telescope is removed4           
,illustration3 ,fig4 #agd4              
,spectra4   #aed4 ,continuous           

,spectra4--,light from an incandescent  
solid or liquid which has suffered no   
absorption in the medium which it has   
traversed gives a spectrum consisting of
 a continuous colored band1 in which the
 colors1 from the red to the violet1    
pass gradually and imperceptibly into   
one another4 ,the spectrum is entirely  
free from either light or dark lines1   
and is called a continuous spectrum4    
#aee4 ,bright-,lined ,spectra4--,light  
from a luminous gas or vapor gives a    
spectrum composed of bright lines       
separated by dark spaces1 and known as a
 bright-lined spectrum4 ,it has been    
found that the lines in the spectrum of 
a substance in the state of a gas or    
vapor are the most characteristic thing 
about the substance1 since no two vapors
 give exactly the same lines3 hence1    
when we have once become acquainted with
 the bright-lined spectrum of any       
substance1 we can ever after recognize  
that substance by the spectrum of its   
luminous vapor4 ,even when several      

substances are mixed1 they may all be   
recognized by the bright-lined spectrum 
of the mixture1 since the lines of all  
the substances will be present in the   
spectrum of the mixture4 ,this method of
 identifying substances by their spectra
 is called spectrum analysis4  ,the     
bright-lined spectra of several         
substances are given in the             
frontispiece4 ,the number of lines in   
the spectra of the elements varies      
greatly4 ,the spectrum of sodium is one 
of the simplest1 while that of iron is  
one of the most complex4 ,the latter    
contains over six hundred lines4 ,though
 no two vapors give identical spectra1  
there are many cases in which one or    
more of the spectral lines of one       
element coincide in position with lines 
of other elements4      #aef4 ,methods  
of rendering ,gases and ,vapors         
,luminous4--,in order to     study the  
spectra of vapors and gases it is       
necessary to have some     means of     
converting solids and liquids into      

vapor1 and also of     rendering the    
vapors and gases luminous4 ,there are   
four methods of     obtaining luminous  
vapors and gases in common use4         
,illustration3 ,fig4 #age4  7#a7 ,by    
means of the ,bunsen ,flame4--,this is a
 very hot but an almost non-luminous    
flame4 ,if any readily volatilized      
substance1 such as the compounds of     
sodium1 calcium1 strontium1 etc41 is    
introduced into this flame on a fine    
platinum wire1 it is volatilized in the 
flame1 and its vapor is rendered        
luminous1 giving the flame its own      
peculiar color4 ,the flame thus colored 
may be examined by the spectroscope4    
,the arrangement of the flame is shown  
in ,fig4 #age4  ,illustration3 ,fig4    
#agf4  7#b7 ,by means of the ,voltaic   
,arc4--,an electric lamp is shown in    
,fig4 #agf4 ,when this lamp is to be    
used for obtaining luminous vapors1 the 
lower carbon is made larger than the    
upper one1 and hollowed out at the top  
into a little cup4 ,the substance to be 

volatilized is placed in this cup1 and  
the current is allowed to pass4 ,the    
heat of the voltaic arc is much more    
intense than that of the ,bunsen flame3 
hence substances that cannot be         
volatilized in the flame are readily    
volatilized in the arc1 and the vapor   
formed is raised to a very high         
temperature4  7#c7 ,by means of the     
,spark from an ,induction ,coil4--,the  
arrangement of the coil for obtaining   
luminous vapors is shown in ,fig4 #agg4 
,illustration3 ,fig4 #agg4  ,the        
terminals of the coil between which the 
spark is to pass are brought quite close
 together4 ,when we wish to vaporize any
 metal1 as iron1 the terminals are made 
of iron4 ,on the passage of the spark1 a
 little of the iron at the ends of the  
terminals is evaporated2 and the vapor  
is rendered luminous in the space       
traversed by the spark4 ,a condenser is 
usually placed in the circuit4 ,with the
 coil1 the temperature may be varied at 
pleasure2 and the vapor may be raised   

even to a higher temperature than with  
the electric lamp4 ,to obtain a low     
temperature1 the coil is used without   
the condenser4 ,by using a larger and   
larger condenser1 the temperature may be
 raised higher and higher4  ,by means of
 the induction coil we may also heat    
gases to incandescence4 ,it is only     
necessary to allow the spark to pass    
through a space filled with the gas4    
,illustration3 ,fig4 #agh4  7#d7 ,by    
means of a ,vacuum ,tube4--,the form of 
the vacuum tube commonly used for this  
purpose is shown in ,fig4 #agh4 ,the gas
 to be examined1 and which is contained 
in the tube1 has very slight density3   
but upon the passage of the discharge   
from an induction coil or a ,holtz      
machine1 through the tube1 the gas in   
the capillary part of the tube becomes  
heated to a high temperature1 and is    
then quite brilliant4  #aeg4 ,reversed  
,spectra4--,if the light from an        
incandescent cylinder of lime1 or from  
the incandescent point of an electric   

lamp1 is allowed to pass through        
luminous sodium vapor1 and is then      
examined with a spectroscope1 the       
spectrum will be found to be a bright   
spectrum crossed by a single dark line  
in the position of the yellow line of   
the sodium vapor4 ,the spectrum of      
sodium vapor is reversed1 its bright    
lines becoming dark and its dark spaces 
bright4 ,with a spectroscope of any     
considerable power1 the yellow line of  
sodium vapor is resolved into a double  
line4 ,with a spectroscope of the same  
power1 the dark sodium line of the      
reversed spectrum is seen to be a double
 line4  ,it is found to be generally    
true1 that the spectrum of the light    
from an incandescent solid or liquid    
which has passed through a luminous     
vapor on its way to the spectroscope is 
made up of a bright ground crossed by   
dark lines2 there being a dark line for 
every bright line that the vapor alone  
would give4  #aeh4 ,explanation of      
,reversed ,spectra4--,it has been found 

that gases absorb and quench rays of the
 same degree of refrangibility as those 
which they themselves emit1 and no      
others4 ,when a solid is shining through
 a luminous vapor1 this absorbs and     
quenches those rays from the solid which
 have the same degrees of refrangibility
 as those which it is itself emitting3  
hence the lines of the spectrum receive 
light from the vapor alone1 while the   
spaces between the lines receive light  
from the solid4 ,now1 solids and        
liquids1 when heated to incandescence1  
give a very much brighter light than    
vapors and gases at the same            
temperature3 hence the lines of a       
reversed spectrum1 though receiving     
light from the vapor or gas1 appear dark
 by contrast4  #aei4 ,effect of         
,increasing the ,power of the           
,spectroscope upon the ,brilliancy of a 
,spectrum4--,an increase in the power of
 a spectroscope diminishes the          
brilliancy of a continuous spectrum1    
since it makes the colored band longer1 

and therefore spreads the light out over
 a greater extent of surface2 but1 in   
the case of a bright-lined spectrum1 an 
increase of power in the spectroscope   
produces scarcely any alteration in the 
brilliancy of the lines1 since it merely
 separates the lines farther without    
making the lines themselves any wider4  
,in the case of a reversed spectrum1 an 
increase of power in the spectroscope   
dilutes the light in the spaces between 
the lines without diluting that of the  
lines3 hence lines which appear dark in 
a spectroscope of slight dispersive     
power may appear bright in an instrument
 of great dispersive power4  #afj4      
,change of the ,spectrum with the       
,density of the ,luminous ,vapor4--,it  
has been found1 that1 as the density of 
a luminous vapor is diminished1 the     
lines in its spectrum become fewer and  
fewer1 till they are finally reduced to 
one4 ,on the other hand1 an increase of 
density causes new lines to appear in   
the spectrum1 and the old lines to      

become thicker4  #afa4 ,change of the   
,spectrum with the ,temperature of the  
,luminous ,vapor4--,it has also been    
found that the appearance of a          
bright-lined spectrum changes           
considerably with the temperature of the
 luminous vapor4 ,in some cases1 an     
increase of temperature changes the     
relative intensities of the lines2 in   
other cases1 it causes new lines to     
appear1 and old lines to disappear4  ,in
 the case of a compound vapor1 an       
increase of temperature causes the      
colored bands 7which are peculiar to the
 spectrum of the compound7 to disappear1
 and to be replaced by the spectral     
lines of the elements of which the      
compound is made up4 ,the heat appears  
to dissociate the compound2 that is1 to 
resolve it into its constituent         
elements4 ,in this case1 each elementary
 vapor would give its own spectral      
lines4 ,as the compound is not          
completely dissociated at once1 it is   
possible1 of course1 for one or more of 

the spectral lines of the elementary    
vapors to co-exist in the spectrum with 
the bands of the compound4  ,it has been
 found1 that1 in some cases1 the spectra
 of the elementary gases change with the
 temperature of the gas2 and ,lockyer   
thinks he has discovered conclusive     
evidence1 in the spectra of the sun and 
stars1 that many of the substances      
regarded as elementary are really       
resolved into simpler substances by the 
intense heat of the sun2 in other words1
 that our so-called elements are really 
compounds4                              
,chemical ,constitution of the ,sun4    
#afb4 ,the ,solar ,spectrum4--,the solar
 spectrum is crossed transversely by a  
great number of fine dark lines1 and    
hence it belongs to the class of        
reversed spectra4  ,these lines were    
first studied and mapped by ,fraunhofer1
 and from him they have been called     
,fraunhofer's lines4  ,illustration3    
,fig4 #agi4      ,a reduced copy of     
,fraunhofer's map is shown in ,fig4     

#agi4 ,a few of     the most prominent  
of the dark solar lines are designated  
by the     letters of the alphabet4 ,the
 other lines are usually designated by  
the numbers at which they are found on  
the scale which accompanies     the map4
 ,this scale is usually drawn at the top
 of the map1 as will     be seen in some
 of the following diagrams4 ,the two    
most elaborate     maps of the solar    
spectrum are those of ,kirchhoff and    
,angstrm4 ,the     scale on ,kirchhoff's
 map is an arbitrary one1 while that of 
,angstrm     is based upon the          
wave-lengths of the rays of light which 
would fall     upon the lines in the    
spectrum4  ,illustration3 ,fig4 #ahj4   
,the appearance of the spectrum varies  
greatly with the power of the           
spectroscope employed4 ,fig4 #ahj shows 
a portion of the spectrum as     it     
appears in a spectroscope of a single   
prism3 while ,fig4 #aha shows     the b 
group of lines alone1 as they appear in 
a powerful     diffraction spectroscope4

  ,illustration3 ,fig4 #aha4  #afc4 ,the
 ,telluric ,lines4--,there are many     
lines of the solar spectrum which vary  
considerably in intensity as the sun    
passes from the horizon to the meridian1
 being most intense when the sun is     
nearest the horizon1 and when his rays  
are obliged to pass through the greatest
 depth of the earth's atmosphere4 ,these
 lines are of atmospheric origin1 and   
are due to the absorption of the aqueous
 vapor in our atmosphere4 ,they are the 
same lines that are obtained when a     
candle or other artificial light is     
examined with a spectroscope through a  
long tube filled with steam4 ,since     
these lines are due to the absorption of
 our own atmosphere1 they are called    
telluric lines4 ,a map of these lines is
 shown in ,fig4 #ahb4  ,illustration3   
,fig4 #ahb4  #afd4 ,the ,solar          
,lines4--,after deducting the telluric  
lines1 the remaining lines of the solar 
spectrum are of solar origin4 ,they must
 be due to absorption which takes place 

in the sun's atmosphere4 ,they are1 in  
fact1 the reversed spectra of the       
elements which exist in the solar       
atmosphere in the state of vapor3 hence 
we conclude that the luminous surface of
 the sun is surrounded with an          
atmosphere of luminous vapors4 ,the     
temperature of this atmosphere1 at least
 near the surface of the sun1 must be   
sufficient to enable all the elements   
known on the earth to exist in it as    
vapors4  ,illustration3 ,fig4 #ahc4     
#afe4 ,chemical ,constitution of the    
,sun's ,atmosphere4--,to find whether   
any element which exists on the earth is
 present in the solar atmosphere1 we    
have merely to ascertain whether the    
bright lines of its gaseous spectrum are
 matched by dark lines in the solar     
spectrum when the two spectra are placed
 side by side4 ,in ,fig4 #ahc1 we have  
in ,no4 #a a portion of the red end of  
the solar spectra1 and in ,no4 #b the   
spectrum of sodium vapor1 both as       
obtained in the same spectroscope by    

means of the comparison prism4 ,it will 
be seen that the double sodium line is  
exactly matched by a double dark line of
 the solar spectrum3 hence we conclude  
that sodium vapor is present in the     
sun's atmosphere4 ,fig4 #ahd shows the  
matching of a great number of the bright
 lines of iron vapor by dark lines in   
the solar spectrum4 ,this matching of   
the iron lines establishes the fact that
 iron vapor is present in the solar     
atmosphere4  ,illustration3 ,fig4 #ahd4 
,the following table 7given by          
,professor ,young7 contains a list of   
all the elements which have1 up to the  
present time1 been detected     with    
certainty in the sun's atmosphere4 ,it  
also gives the number of     bright     
lines in the spectrum of each element1  
and the number of     those lines which 
have been matched by dark lines in the  
solar     spectrum3--                   
,elements4      ,bright ,lines          
,reversed4 ,observer4                   
,lines4              #a4 ,iron          

#fjj             #dfj ,kirchhoff4       
#b4 ,titanium       #bjf                
#aah ,thalen4              #c4 ,calcium 
#hi              #ge ,kirchhoff4        
#d4 ,manganese       #ge                
#eg ,angstrm4              #e4 ,nickel  
#ea              #cc ,kirchhoff4        
#f4 ,cobalt          #hf                
#ai ,thalen4              #g4 ,chromium 
#ga              #ah ,kirchhoff4        
#h4 ,barium          #bf                
#aa ,kirchhoff4              #i4 ,sodium
           #i               #i          
,kirchhoff4              #aj4 ,magnesium
       #g               #g ,kirchhoff4  
#aa4 ,copper8        #ae                
#g8 ,kirchhoff4              #ab4       
,hydrogen        #e               #e    
,angstrm4              #ac4 ,palladium  
#bi               #e ,lockyer4          
#ad4 ,vanadium       #ed                
#d ,lockyer4              #ae4          
,molybdenum     #bg               #d    
,lockyer4              #af4 ,strontium  
#gd               #d ,lockyer4          

#ag4 ,lead           #da                
#c ,lockyer4              #ah4 ,uranium 
#ba               #c ,lockyer4          
#ai4 ,aluminium      #ad                
#b ,angstrm4              #bj4 ,cerium  
#fd               #b ,lockyer4          
#ba4 ,cadmium        #bj                
#b ,lockyer4              #bb4 ,oxygen a
       #db     #ab  bright ,h4 ,draper4 
,oxygen b            #d              #d8
 ,schuster4      ,in addition to the    
above elements1 it is probable that     
several other     elements are present  
in the sun's atmosphere2 since at least 
one of     their bright lines has been  
found to coincide with dark lines of the
     solar spectrum4 ,there are1        
however1 a large number of elements1 no 
traces of which have yet been detected2 
and1 in the cases of the     elements   
whose presence in the solar atmosphere  
has been     established1 the matching  
of the lines is far from complete in the
     majority of the cases1 as will be  
seen from the above table4 ,this        

want of complete coincidence of the     
lines is undoubtedly due to the     very
 high temperature of the solar          
atmosphere4 ,we have already seen       
that the lines of the spectrum change   
with the temperature2 and1 as     the   
temperature of the sun is far higher    
than any that we can     produce by     
artificial means1 we might reasonably   
expect that it     would cause the      
disappearance from the spectrum of many 
lines which     we find to be present at
 our highest temperature4      ,lockyer 
maintains that the reason why no trace  
of the spectral lines     of certain of 
our so-called elements is found in the  
solar     atmosphere is1 that these     
substances are not really elementary1   
and     that the intense heat of the sun
 resolves them into simpler             
constituents4                           
,motion at the ,surface of the ,sun4    
#aff4 ,change of ,pitch caused by       
,motion of ,sounding ,body4--,when a    
sounding body is moving rapidly towards 

us1 the pitch of its note becomes       
somewhat higher than when the body is   
stationary2 and1 when such a body is    
moving rapidly from us1 the pitch of its
 note is lowered somewhat4 ,we have a   
good illustration of this change of     
pitch at a country railway station on   
the passage of an express-train4 ,the   
pitch of the locomotive whistle is      
considerably higher when the train is   
approaching the station than when it is 
leaving it4  #afg4 ,explanation of the  
,change of ,pitch produced by           
,motion4--,the pitch of sound depends   
upon the rapidity with which the        
pulsations of sound beat upon the drum  
of the ear4 ,the more rapidly the       
pulsations follow each other1 the higher
 is the pitch3 hence the shorter the    
sound-waves 7provided the sound is all  
the while travelling at the same rate71 
the higher the pitch of the sound4 ,any 
thing1 then1 which tends to shorten the 
waves of sound tends also to raise its  
pitch1 and any thing which tends to     

lengthen these waves tends to lower its 
pitch4  ,when a sounding body is moving 
rapidly forward1 the sound-waves are    
crowded together a little1 and therefore
 shortened2 when it is moving backward1 
the sound-waves are drawn out1 or       
lengthened a little4      ,the effect of
 the motion of a sounding body upon the 
length of its     sonorous waves will be
 readily seen from the following        
illustration3     ,suppose a number of  
persons stationed at equal intervals in 
a line     on a long platform capable of
 moving backward and forward4 ,suppose  
the men are four feet apart1 and all    
walking forward at the same     rate1   
and that the platform is stationary1 and
 that1 as the men     leave the         
platform1 they keep on walking at the   
same rate3 the men     will evidently be
 four feet apart in the line in front of
 the     platform1 as well as on it4    
,suppose next1 that the platform is     
moving forward at the rate of one foot  
in the interval between two     men's   

leaving the platform1 and that the men  
continue to walk as     before3 it is   
evident that the men will then be three 
feet apart in     the line after they   
have left the platform4 ,the forward    
motion of     the platform has the      
effect of crowding the men together a   
little4     ,were the platform moving   
backward at the same rate1 the men would
 be     five feet apart after they had  
left the platform4 ,the backward        
motion of the platform has the effect of
 separating the men from one            
another4      ,the distance between the 
men in this illustration corresponds to 
the     length of the sound-wave1 or the
 distance between its two ends4 ,were   
a person to stand beside the line1 and  
count the men that passed him     in the
 three cases given above1 he would find 
that more persons     would pass him in 
the same time when the platform is      
moving forward     than when it is      
stationary1 and fewer persons would pass
 him in the     same time when the      

platform is moving backward than when it
 is     stationary4 ,in the same way1   
when a sounding body is moving rapidly  
forward1 the sound-waves beat more      
rapidly upon the ear of a person     who
 is standing still than when the body is
 at rest1 and less     rapidly when the 
sounding body is moving rapidly         
backward4      ,were the platform       
stationary1 and were the person who is  
counting     the men to be walking along
 the line1 either towards or away from  
the platform1 the effect upon the number
 of men passing him in a     given time 
would be precisely the same as it would 
be were the     person stationary1 and  
the platform moving either towards or   
away     from him at the same rate4 ,so 
the change in the rapidity with which   
pulsations of sound beat upon the ear is
 precisely the same whether     the ear 
is stationary and the sounding body     
moving1 or the sounding     body is     
stationary and the ear moving4  #afh4   
,change of ,refrangibility due to the   

,motion of a ,luminous                  
,body4--,refrangibility in light        
corresponds to pitch in sound1 and      
depends upon the length of the luminous 
waves4 ,the shorter the luminous waves1 
the greater the refrangibility of the   
waves4 ,very rapid motion of a luminous 
body has the same effect upon the length
 of the luminous waves that motion of a 
sounding body has upon the length of the
 sonorous waves4 ,when a luminous body  
is moving very rapidly towards us1 its  
luminous waves are shortened a little1  
and its light becomes a little more     
refrangible2 when the luminous body is  
moving rapidly from us1 its luminous    
waves are lengthened a little1 and its  
light becomes a little less refrangible4
  ,illustration3 ,fig4 #ahe4  #afi4     
,displacement of ,spectral ,lines4--,in 
examining the spectra of the stars1 we  
often find that certain of the dark     
lines are displaced somewhat1 either    
towards the red or the violet end of the
 spectrum4 ,as the dark lines are in the

 same position as the bright lines of   
the absorbing vapor would be1 a         
displacement of the lines towards the   
red end of the spectrum indicates a     
lowering of the refrangibility of the   
rays1 due to a motion of the luminous   
vapor away from us2 and a displacement  
of the lines towards the violet end of  
the spectrum indicates an increase of   
refrangibility1 due to a motion of the  
luminous vapor towards us4 ,from the    
amount of the displacement of the lines1
 it is possible to calculate the        
velocity at which the luminous gas is   
moving4 ,in ,fig4 #ahe is shown the     
displacement of the ,f line in the      
spectrum of ,sirius4 ,this is one of the
 hydrogen lines4 ,,rv is the spectrum1  
,r being the red1 and ,v the violet end4
 ,the long vertical line is the bright  
,f line of hydrogen1 and the short dark 
line to the left of it is the position  
of the ,f line in the spectrum of       
,sirius4 ,it is seen that this line is  
displaced somewhat towards the red end  

of the spectrum4 ,this indicates that   
,sirius must be moving from us2 and the 
amount of the displacement indicates    
that the star must be moving at the rate
 of some twenty-five or thirty miles a  
second4  ,illustration3 ,fig4 #ahf4     
#agj4 ,contortion of ,lines on the ,disk
 of the ,sun4--,certain of the dark     
lines seen on the centre of the sun's   
disk often appear more or less          
distorted1 as shown in ,fig4 #ahf1 which
 represents the contortion of the       
hydrogen line as seen at various times4 
#a and #b indicate a rapid motion of    
hydrogen away from us1 or a down-rush at
 the sun2 #c and #d 7in which the line  
at the centre is dark on one side1 and  
bent towards the red end of the         
spectrum1 and bright on the other side  
with a distortion towards the violet end
 of the spectrum7 indicate a down-rush  
of cool hydrogen side by side with an   
up-rush of hot and bright hydrogen2 #e  
indicates local down-rushes associated  
with quiescent hydrogen4  ,the contorted

 lines1 which indicate a violently      
agitated state of the sun's atmosphere1 
appear in the midst of other lines which
 indicate a quiescent state4 ,this is   
owing to the fact that the absorption   
which produces the dark lines takes     
place at various depths in the solar    
atmosphere4 ,there may be violent       
commotion in the lower layers of the    
sun's atmosphere1 and comparative quiet 
in the upper layers4 ,in this case1 the 
lines which are due to absorption in the
 lower layers would indicate this       
disturbance by their contortions2 while 
the lines produced by absorption in the 
upper layers would be free from         
contortion4  ,it often happens1 too1    
that the contortions are confined to one
 set of lines of an element1 while other
 lines of the same element are entirely 
free from contortions4 ,this is         
undoubtedly due to the fact that        
different layers of the solar atmosphere
 differ greatly in temperature2 so that 
the same element would give one set of  

lines at one depth1 and another set at  
another depth3 hence commotion in the   
solar atmosphere at any particular depth
 would be indicated by the contortion of
 those lines of the element only which  
are produced by the temperature at that 
particular depth4  ,a remarkable case of
 contortion witnessed by ,professor     
,young is shown in ,fig4 #ahg4 ,three   
successive appearances of the ,c line   
are shown4 ,the second view was taken   
three minutes after the first1 and the  
third five minutes after the second4    
,the contortion in this case indicated a
 velocity ranging from two hundred to   
three hundred miles a second4           
,illustration3 ,fig4 #ahg4  #aga4       
,contortion of ,lines on the ,sun's     
,limb4--,when the spectroscope is       
directed to the centre of the sun's     
disk1 the distortion of the lines       
indicates only vertical motion in the   
sun's atmosphere2 but1 when the         
spectroscope is directed to the limb of 
the sun1 displacements of the lines     

indicate horizontal motions in the sun's
 atmosphere4 ,when a powerful           
spectroscope is directed to the margin  
of the sun's disk1 so that the slit of  
the collimator tube shall be            
perpendicular to the sun's limb1 one or 
more of the dark lines on the disk are  
seen to be prolonged by a bright line1  
as shown in ,fig4 #ahh4 ,but this       
prolongation1 instead of being straight 
and narrow1 as shown in the figure1 is  
often widened and distorted in various  
ways1 as shown in ,fig4 #ahi4 ,in the   
left-hand portion of the diagram1 the   
line is deflected towards the red end of
 the spectrum2 this indicates a violent 
wind on the sun's surface blowing away  
from us4 ,in the right-hand portion of  
the diagram1 the line is deflected      
towards the violet end of the spectrum2 
this indicates a violent wind blowing   
towards us4 ,in the middle portion of   
the figure1 the line is seen to be bent 
both ways2 this indicates a cyclone1 on 
one side of which the wind would be     

blowing from us1 and on the other side  
towards us4  ,illustration3 ,fig4 #ahh4 
,illustration3 ,fig4 #ahi4  ,the        
distortions of the solar lines indicate 
that the wind at the surface of the sun 
often blows with a velocity of from one 
hundred to three hundred miles a second4
 ,the most violent wind known on the    
earth has velocity of a hundred miles an
 hour4                     ,,iii4 ,,the 
,,photosphere ,,and ,,sun ,,spots4      
,the ,photosphere4   ,illustration3     
,fig4 #aij4  #agb4 ,the ,granulation of 
the ,photosphere4--,when the surface of 
the sun is examined with a good         
telescope under favorable atmospheric   
conditions1 it is seen to be composed of
 minute grains of intense brilliancy and
 of irregular form1 floating in a darker
 medium1 and arranged in streaks and    
groups1 as shown in ,fig4 #aij4 ,with a 
rather low power1 the general effect of 
the surface is much like that of rough  
drawing-paper1 or of curdled milk seen  
from a little distance4 ,with a high    

power and excellent atmospheric         
conditions1 the grains are seen to be   
irregular1 rounded masses1 some hundreds
 of miles in diameter1 sprinkled upon a 
less brilliant background1 and appearing
 somewhat like snow-flakes sparsely     
scattered over a grayish cloth4 ,fig4   
#aia is a representation of these grains
 according to ,secchi4  ,illustration3  
,fig4 #aia4  ,with a very powerful      
telescope and the very best atmospheric 
conditions1 the grains themselves are   
resolved into granules1 or little       
luminous dots1 not more than a hundred  
miles or so in diameter1 which1 by their
 aggregation1 make up the grains1 just  
as they1 in their turn1 make up the     
coarser masses of the solar surface4    
,professor ,langley estimates that these
 granules constitute about one-fifth of 
the sun's surface1 while they emit at   
least three-fourths of its light4  #agc4
 ,shape of the ,grains4--,the grains    
differ considerably in shape at         
different times and on different parts  

of the sun's surface4 ,nasmyth1 in      
#ahfa1 described them as willow-leaves  
in shape1 several thousand miles in     
length1 but narrow and with pointed     
ends4 ,he figured the surface of the sun
 as a sort of basket-work formed by the 
interweaving of such filaments4 ,to     
others they have appeared to have the   
form of rice-grains4 ,on portions of the
 sun's disk the elementary structure is 
often composed of long1 narrow1         
blunt-ended filaments1 not so much like 
willow-leaves as like bits of straw     
lying roughly parallel to each other1--a
 thatch-straw formation1 as it has been 
called4 ,this is specially common in the
 immediate neighborhood of the spots4   
#agd4 ,nature of the ,grains4--,the     
grains are1 undoubtedly1 incandescent   
clouds floating in the sun's atmosphere1
 and composed of partially condensed    
metallic vapors1 just as the clouds of  
our atmosphere are composed of partially
 condensed aqueous vapor4 ,rain on the  
sun is composed of white-hot drops of   

molten iron and other metals2 and these 
drops are often driven with the wind    
with a velocity of over a hundred miles 
a second4  ,as to the forms of the      
grains1 ,professor ,young says1 ,if one 
were to speculate as to the explanation 
of the grains and thatch-straws1 it     
might be that the grains are the upper  
ends of long filaments of luminous      
cloud1 which1 over most of the sun's    
surface1 stand approximately vertical1  
but in the neighborhood of a spot are   
inclined so as to lie nearly horizontal4
 ,this is not certain1 though3 it may be
 that the cloud-masses over the more    
quiet portions of the solar surface are 
really1 as they seem1 nearly globular1  
while near the spots they are drawn out 
into filamentary forms by atmospheric   
currents4  #age4 ,facul4--,the facul are
 irregular streaks of greater brightness
 than the general surface1 looking much 
like the flecks of foam on the surface  
of a stream below a waterfall4 ,they are
 sometimes from five to twenty thousand 

miles in length1 covering areas         
immensely larger than a terrestrial     
continent4  ,these facul are elevated   
regions of the solar surface1 ridges and
 crests of luminous matter1 which rise  
above the general level of the sun's    
surface1 and protrude through the denser
 portions of the solar atmosphere4 ,when
 one of these passes over the edge of   
the sun's disk1 it can be seen to       
project1 like a little tooth4 ,any      
elevation on the sun to be perceptible  
at all must measure at least half a     
second of an arc1 or two hundred and    
twenty-five miles4  ,the facul are most 
numerous in the neighborhood of the     
spots1 and much more conspicuous near   
the limb of the sun than near the centre
 of the disk4 ,fig4 #aib gives the      
general appearance of the facul1 and the
 darkening of the limb of the sun4 ,near
 the spots1 the facul often undergo very
 rapid change of form1 while elsewhere  
on the disk they change rather slowly1  
sometimes undergoing little apparent    

alteration for several days4            
,illustration3 ,fig4 #aib4  #agf4 ,why  
the ,facul are most ,conspicuous near   
the ,limb of the ,sun4--,the reason why 
the facul are most conspicuous near the 
limb of the sun is this3 ,the luminous  
surface of the sun is covered with an   
atmosphere1 which1 though not very thick
 compared with the diameter of the sun1 
is still sufficient to absorb a good    
deal of light4 ,light coming from the   
centre of the sun's disk penetrates this
 atmosphere under the most favorable    
conditions1 and is but slightly reduced 
in amount4 ,the edges of the disk1 on   
the other hand1 are seen through a much 
greater thickness of atmosphere2 and the
 light is reduced by absorption some    
seventy-five per cent4 ,suppose1 now1 a 
facula were sufficiently elevated to    
penetrate quite through this atmosphere4
 ,its light would be undimmed by        
absorption on any part of the sun's     
disk2 but at the centre of the disk it  
would be seen against a background      

nearly as bright as itself1 while at the
 margin it would be seen against one    
only a quarter as bright4 ,it is evident
 that the light of any facula1 owing to 
the elevation1 would be reduced less    
rapidly as we approach the edge of the  
disk than that of the general surface of
 the sun1 which lies at a lower level4  
,sun-,spots4   #agg4 ,general           
,appearance of ,sun-,spots4--,the       
general appearance of a well-formed     
sun-spot is shown in ,fig4 #aic4 ,the   
spot consists of a very dark central    
portion of irregular shape1 called the  
umbra1 which is surrounded by a less    
dark fringe1 called the penumbra4 ,the  
penumbra is made up1 for the most part1 
of filaments directed radially inward4  
,illustration3 ,fig4 #aic4  ,there is   
great variety in the details of form in 
different sun-spots2 but they are       
generally nearly circular during the    
middle period of their existence4       
,during the period of their development 
and of their disappearance they are much

 more irregular in form4  ,there is     
nothing like a gradual shading-off of   
the penumbra1 either towards the umbra  
on the one side1 or towards the         
photosphere on the other4 ,the penumbra 
is separated from both the umbra and the
 photosphere by a sharp line of         
demarcation4 ,the umbra is much brighter
 on the inner than on the outer edge1   
and frequently the photosphere is       
excessively bright at the margin of the 
penumbra4 ,the brightness of the inner  
penumbra seems to be due to the crowding
 together of the penumbral filaments    
where they overhang the edge of the     
umbra4  ,there is a general antithesis  
between the irregularities of the outer 
and inner edges of the penumbra4 ,where 
an angle of the penumbral matter crowds 
in upon the umbra1 it is generally      
matched by a corresponding outward      
extension into the photosphere1 and vice
 versa4  ,the umbra of the spot is far  
from being uniformly dark4 ,many of the 
penumbral filaments terminate in little 

detached grains of luminous matter2 and 
there are also fainter veils of a       
substance less brilliant1 but sometimes 
rose-colored1 which seem to float above 
the umbra4 ,the umbra itself is made up 
of masses of clouds which are really    
intensely brilliant1 and which appear   
dark only by contrast with the intenser 
brightness of the solar surface4 ,among 
these clouds are often seen one or more 
minute circular spots much darker than  
the rest of the umbra4 ,these darker    
portions are called nuclei4 ,they seem  
to be the mouths of tubular orifices    
penetrating to unknown depths4 ,the     
faint veils mentioned above continually 
melt away1 and are replaced by others in
 some different position4 ,the bright   
granules at the tips of the penumbral   
filaments seem to sink and dissolve1    
while fresh portions break off to       
replace them4 ,there is a continual     
indraught of luminous matter over the   
whole extent of the penumbra4  ,at      
times1 though very rarely1 patches of   

intense brightness suddenly break out1  
remain visible for a few minutes1 and   
move over the spot with velocities as   
great as a hundred miles a second4  ,the
 spots change their form and size quite 
perceptibly from day to day1 and        
sometimes even from hour to hour4  #agh4
 ,duration of ,sun-,spots4--,the average
 life of a sun-spot is two or three     
months3 the longest on record is that of
 a spot observed in #ahdj and #ahda1    
which lasted eighteen months4 ,there are
 cases1 however1 where the disappearance
 of a spot is very soon followed by the 
appearance of another at the same point2
 and sometimes this alternate           
disappearance and re-appearance is      
several times repeated4 ,while some     
spots are thus long-lived1 others endure
 only a day or two1 and sometimes only a
 few hours4  #agi4 ,groups of           
,spots4--,the spots usually appear not  
singly1 but in groups4 ,a large spot is 
often followed by a train of smaller    
ones to the east of it1 many of which   

are apt to be irregular in form and very
 imperfect in structure1 sometimes with 
no umbra at all1 often with a penumbra  
only on one side4 ,in such cases1 when  
any considerable change of form or      
structure shows itself in the principal 
spot1 it seems to rush westward over the
 solar surface1 leaving its attendants  
trailing behind4 ,when a large spot     
divides into two or more1 as often      
happens1 the parts usually seem to repel
 each other1 and fly apart with great   
velocity4  #ahj4 ,size of the           
,spots4--,the spots are sometimes of    
enormous size4 ,groups have often been  
observed covering areas of more than a  
hundred thousand miles square1 and      
single spots occasionally measure from  
forty to fifty thousand miles in        
diameter1 the umbra being twenty-five or
 thirty thousand miles across4 ,a spot1 
however1 measuring thirty thousand miles
 over all1 may be considered a large    
one4 ,such a spot can easily be seen    
without a telescope when the brightness 

of the sun's surface is reduced by      
clouds or nearness to the horizon1 or by
 the use of colored glass4 ,during the  
years #ahga and #ahgb spots were visible
 to the naked eye for a considerable    
portion of the time4 ,the largest spot  
yet recorded was observed in #aheh4 ,it 
had a breadth of more than a hundred and
 forty-three thousand miles1 or nearly  
eighteen times the diameter of the      
earth1 and covered about a thirty-sixth 
of the whole surface of the sun4        
,illustration3 ,fig4 #aid4  ,fig4 #aid  
represents a group of sun-spots observed
 by ,professor ,langley1 and drawn on   
the same scale as the small circle in   
the upper left-hand corner1 which       
represents the surface of half of our   
globe4  ,illustration3 ,fig4 #aie4      
#aha4 ,the ,penumbral ,filaments4--,not 
unfrequently the penumbral filaments are
 curved spirally1 indicating a cyclonic 
action1 as shown in ,fig4 #aie4 ,in such
 cases the whole spot usually turns     
slowly around1 sometimes completing an  

entire revolution in a few days4 ,more  
frequently1 however1 the spiral motion  
lasts but a short time2 and             
occasionally1 after continuing for a    
while in one direction1 the motion is   
reversed4 ,very often in large spots we 
observe opposite spiral movements in    
different portions of the umbra1 as     
shown in ,figs4 #aif and #aig4          
,illustration3 ,fig4 #aif4  ,neighboring
 spots show no tendency to rotate in the
 same direction4 ,the number of spots in
 which a decided cyclonic motion 7like  
that shown in ,fig4 #aih7 appears is    
comparatively small1 not exceeding two  
or three per cent of the whole4         
,illustration3 ,fig4 #aig4              
,illustration3 ,fig4 #aih4              
,illustration3 ,plate #b4  ,plate ,,ii4 
represents a typical sun-spot as        
delineated by ,professor ,langley4 ,at  
the left-hand and upper portions of this
 great spot the filaments present the   
ordinary appearance1 while at the lower 
edge1 and upon the great overhanging    

branch1 they are arranged very          
differently4 ,the feathery brush below  
the branch1 closely resembling a        
frost-crystal on a window-pane1 is as   
rare as it is curious1 and has not been 
satisfactorily explained4               
,illustration3 ,fig4 #aii4  #ahb4 ,birth
 and ,decay of ,sun-,spots4--,the       
formation of a spot is sometimes        
gradual1 requiring days or even weeks   
for its full development2 and sometimes 
a single day suffices4 ,generally1 for  
some time before its appearance1 there  
is an evident disturbance of the solar  
surface1 indicated especially by the    
presence of many brilliant facul1 among 
which pores1 or minute black dots1 are  
scattered4 ,these enlarge1 and between  
them appear grayish patches1 in which   
the photospheric structure is unusually 
evident1 as if they were caused by a    
dark mass lying below a thin veil of    
luminous filaments4 ,this veil seems to 
grow gradually thinner1 and finally     
breaks open1 giving us at last the      

complete spot with its penumbra4 ,some  
of the pores coalesce with the principal
 spot1 some disappear1 and others form  
the attendant train before described    
7#agi74 ,the spot when once formed      
usually assumes a circular form1 and    
remains without striking change until it
 disappears4 ,as its end approaches1 the
 surrounding photosphere seems to crowd 
in1 and overwhelm the penumbra4 ,bridges
 of light 7,fig4 #aii71 often much      
brighter than the average of the solar  
surface1 push across the umbra2 the     
arrangement of the penumbra filaments   
becomes confused2 and1 as ,secchi       
expresses it1 the luminous matter of the
 photosphere seems to tumble pell-mell  
into the chasm1 which disappears1 and   
leaves a disturbed surface marked with  
facul1 which1 in their turn1 gradually  
subside4  ,illustration3 ,fig4 #bjj4    
#ahc4 ,motion of ,sun-,spots4--,the     
spots have a regular motion across the  
disk of the sun from east to west1      
occupying about twelve days in the      

transit4 ,a spot generally appears first
 on or near the east limb1 and1 after   
twelve or fourteen days1 disappears at  
the west limb4 ,at the end of another   
fourteen days1 or more1 it re-appears at
 the east limb1 unless1 in the mean     
time1 it has vanished from sight        
entirely4 ,this motion of the spots is  
indicated by the arrow in ,fig4 #bjj4   
,the interval between two successive    
appearances of the same spot on the     
eastern edge of the sun is about        
twenty-seven days4  ,illustration3 ,fig4
 #bja4  #ahd4 ,the ,rotation of the     
,sun4--,the spots are evidently carried 
around by the rotation of the sun on its
 axis4 ,it is evident1 from ,fig4 #bja1 
that the sun will need to make more than
 a complete rotation in order to bring a
 spot again upon the same part of the   
disk as seen from the earth4 ,s         
represents the sun1 and ,e the earth4   
,the arrows indicate the direction of   
the sun's rotation4 ,when the earth is  
at ,e1 a spot at a would be seen at the 

centre of the solar disk4 ,while the sun
 is turning on its axis1 the earth moves
 in its orbit from ,e to ,e'3 hence the 
sun must make a complete rotation1 and  
turn from a to a' in addition1 in order 
to bring the spot again to the centre of
 the disk4 ,to carry the spot entirely  
around1 and then on to a'1 requires     
about twenty-seven days4 ,from this     
synodical period of the spot1 as it     
might be called1 it has been calculated 
that the sun must rotate on its axis in 
about twenty-five days4  ,illustration3 
,fig4 #bjb4  #ahe4 ,the ,inclination of 
the ,sun's ,axis4--,the paths described 
by sun-spots across the solar disk vary 
with the position of the earth in its   
orbit1 as shown in ,fig4 #bjb4 ,we      
therefore conclude that the sun's axis  
is not perpendicular to the plane of the
 earth's orbit4 ,the sun rotates on its 
axis from west to east1 and the axis    
leans about seven degrees from the      
perpendicular to the earth's orbit4     
#ahf4 ,the ,proper ,motion of the       

,spots4--,when the period of the sun's  
rotation is deduced from the motion of  
spots in different solar latitudes1     
there is found to be considerable       
variation in the results obtained4 ,thus
 spots near the equator indicate that   
the sun rotates in about twenty-five    
days2 while those in latitude #bj       
indicate a period about eighteen hours  
longer2 and those in latitude #cj a     
period of twenty-seven days and a half4 
,strictly speaking1 the sun1 as a whole1
 has no single period of rotation2 but  
different portions of its surface       
perform their revolutions in different  
times4 ,the equatorial regions not only 
move more rapidly in miles per hour than
 the rest of the solar surface1 but they
 complete the entire rotation in shorter
 time4  ,illustration3 ,fig4 #bjc4      
,there appears to be a peculiar         
surface-drift in the equatorial regions 
of the sun1 the cause of which is       
unknown1 but which gives the spots a    
proper motion2 that is1 a motion of     

their own1 independent of the rotation  
of the sun4  ,illustration3 ,fig4 #bjd4 
#ahg4 ,distribution of the              
,sun-,spots4--,the sun-spots are not    
distributed uniformly over the sun's    
surface1 but occur mainly in two zones  
on each side of the equator1 and between
 the latitudes of #aj and #cj1 as shown 
in ,fig4 #bjc4 ,on and near the equator 
itself they are comparatively rare4     
,there are still fewer beyond #ce of    
latitude1 and only a single spot has    
ever been recorded more than #de from   
the solar equator4  ,fig4 #bjd shows the
 distribution of the sun-spots observed 
by ,carrington during a period of eight 
years4 ,the irregular line on the       
left-hand side of the figure indicates  
by its height the comparative frequency 
with which the spots occurred in        
different latitudes4 ,in ,fig4 #bje the 
same thing is indicated by different    
degrees of darkness in the shading of   
the belts4  ,illustration3 ,fig4 #bje4  
#ahh4 ,the ,periodicity of the          

,spots4--,careful observations of the   
solar spots indicate a period of about  
eleven years in the spot-producing      
activity of the sun4 ,during two or     
three years the spots increase in number
 and in size2 then they begin to        
diminish1 and reach a minimum five or   
six years after the maximum4 ,another   
period of about six years brings the    
return of the maximum4 ,the intervals   
are1 however1 somewhat irregular4       
,illustration3 ,fig4 #bjf4  ,fig4 #bjf  
gives a graphic representation of the   
periodicity of the sun-spots4 ,the      
height of the curve shows the frequency 
of the sun-spots in the years given at  
the bottom of the figure4 ,it appears1  
from an examination of this sun-spot    
curve1 that the average interval from a 
minimum to the next following maximum is
 only about four years and a half1 while
 that from a maximum to the next        
following minimum is six years and      
six-tenths4 ,the disturbance which      
produces the sun-spots is developed     

suddenly1 but dies away gradually4      
#ahi4 ,connection between ,sun-,spots   
and ,terrestrial ,magnetism4--,the      
magnetic needle does not point steadily 
in the same direction1 but is subject to
 various disturbances1 some of which are
 regular1 and others irregular4  7#a7   
,one of the most noticeable of the      
regular magnetic changes is the         
so-called diurnal oscillation4 ,during  
the early part of the day the north pole
 of the needle moves toward the west in 
our latitude1 returning to its mean     
position about ten ,p4,m41 and remaining
 nearly stationary during the night4    
,the extent of this oscillation in the  
,united ,states is about fifteen minutes
 of arc in summer1 and not quite half as
 much in winter2 but it differs very    
much in different localities and at     
different times1 and the average diurnal
 oscillation in any locality increases  
and decreases pretty regularly during a 
period of about eleven years4 ,the      
maximum and minimum of this period of   

magnetic disturbance are found to       
coincide with the maximum and minimum of
 the sun-spot period4 ,this is shown in 
,fig4 #bjf1 in which the dotted lines   
indicate the variations in the intensity
 of the magnetic disturbance4  7#b7     
,occasionally so-called magnetic storms 
occur1 during which the compass-needle  
is sometimes violently disturbed1       
oscillating five degrees1 or even ten   
degrees1 within an hour or two4 ,these  
storms are generally accompanied by an  
aurora1 and an aurora is always         
accompanied by magnetic disturbance4 ,a 
careful comparison of aurora            
observations with those of sun-spots    
shows an almost perfect parallelism     
between the curves of auroral and       
sun-spot frequency4  7#c7 ,a number of  
observations render it very probable    
that every intense disturbance of the   
solar surface is propagated to our      
terrestrial magnetism with the speed of 
light4  ,illustration3 ,fig4 #bjg4      
,fig4 #bjg shows certain of the solar   

lines as they were observed by          
,professor ,young on ,aug4 #c1 #ahgb4   
,the contortions of the ,f line         
indicated an intense disturbance in the 
atmosphere of the sun4 ,there were three
 especially notable paroxysms in this   
distortion1 occurring at a quarter of   
nine1 half-past ten1 and ten minutes of 
twelve1 ,a4,m4  ,illustration3 ,fig4    
#bjh4  ,fig4 #bjh shows the curve of    
magnetic disturbance as traced at       
,greenwich on the same day4 ,it will be 
seen from the curve that it was a day of
 general magnetic disturbance4 ,at the  
times of the three paroxysms1 which are 
given at the bottom of the figure1 it   
will be observed that there is a        
peculiar shivering of the magnetic      
curve4  #aij4 ,the ,spots are           
,depressions in the ,photosphere4--,this
 fact was first clearly brought out by  
,dr4 ,wilson of ,glasgow1 in #agfi1 from
 observations upon the penumbra of a    
spot in ,november of that year4 ,he     
found1 that when the spot appeared at   

the eastern limb1 or edge of the sun1   
just moving into sight1 the penumbra was
 well marked on the side of the spot    
nearest to the edge of the disk2 while  
on the other edge of the spot1 towards  
the centre of the sun1 there was no     
penumbra visible at all1 and the umbra  
itself was almost hidden1 as if behind a
 bank4 ,when the spot had moved a day's 
journey toward the centre of the disk1  
the whole of the umbra came into sight1 
and the penumbra on the inner edge of   
the spot began to be visible as a narrow
 line4 ,after the spot was well advanced
 upon the disk1 the penumbra was of the 
same width all around the spot4 ,when   
the spot approached the sun's western   
limb1 the same phenomena were repeated1 
but in the inverse order4 ,the penumbra 
on the inner edge of the spot narrowed  
much faster than that on the outer1     
disappeared entirely1 and finally seemed
 to hide from sight much of the umbra   
nearly a whole day before the spot      
passed from view around the limb4 ,this 

is precisely what would occur 7as ,fig4 
#bji clearly shows7 if the spot were a  
saucer-shaped depression in the solar   
surface1 the bottom of the saucer       
corresponding to the umbra1 and the     
sloping sides to the penumbra4          
,illustration3 ,fig4 #bji4              
,illustration3 ,fig4 #baj4  #aia4       
,sun-,spot ,spectrum4--,when the image  
of a sun-spot is thrown upon the slit of
 the spectroscope1 the spectrum is seen 
to be crossed longitudinally by a       
continuous dark band1 showing an        
increased general absorption in the     
region of the sun-spot4 ,many of the    
spectral lines are greatly thickened1 as
 shown in ,fig4 #baj4 ,this thickening  
of the lines shows that the absorption  
is taking place at a greater depth4 ,new
 lines and shadings often appear1 which 
indicate1 that1 in the cooler nucleus of
 the spot1 certain compound vapors      
exist1 which are dissociated elsewhere  
on the sun's surface4 ,these lines and  
shadings are shown in ,fig4 #baa4       

,illustration3 ,fig4 #baa4  ,it often   
happens that certain of the spectral    
lines are reversed in the spectrum of   
the spot1 a thin bright line appearing  
over the centre of a thick dark one1 as 
shown in ,fig4 #bab4 ,these reversals   
are due to very bright vapors floating  
over the spot4  ,illustration3 ,fig4    
#bab4  ,at times1 also1 the spectrum of 
a spot indicates violent motion in the  
overlying gases by distortion and       
displacement of the lines4 ,this        
phenomenon occurs oftener at points near
 the outer edge of the penumbra than    
over the centre of the spot2 but        
occasionally the whole neighborhood is  
violently agitated4 ,in such cases1     
lines in the spectrum side by side are  
often affected in entirely different    
ways1 one being greatly displaced while 
its neighbor is not disturbed in the    
least1 showing that the vapors which    
produce the lines are at different      
levels in the solar atmosphere1 and     
moving independently of each other4     

,illustration3 ,fig4 #bac4  #aib4 ,the  
,cause and ,nature of                   
,sun-,spots4--,according to ,professor  
,young1 the arrangement and relations of
 the photospheric clouds in the         
neighborhood of a spot are such as are  
represented in ,fig4 #bac4 ,over the    
sun's surface generally1 these clouds   
probably have the form of vertical      
columns1 as at aa4 ,just outside the    
spot1 the level of the photosphere is   
the most part1 overtopped by eruptions  
of hydrogen and usually raised into     
facul1 as at bb4 ,these facul are1 for  
metallic vapors1 as indicated by the    
shaded clouds4444 ,while the great      
clouds of hydrogen are found everywhere 
upon the sun1 these spiky1 vivid        
outbursts of metallic vapors seldom     
occur except just in the neighborhood of
 a spot1 and then only during its season
 of rapid change4 ,in the penumbra of   
the spot the photospheric filaments     
become more or less nearly horizontal1  
as at pp2 in the umbra at u it is quite 

uncertain what the true state of affairs
 may be4 ,we have conjecturally         
represented the filaments there as      
vertical also1 but depressed and carried
 down by a descending current4 ,of      
course1 the cavity is filled by the     
gases which overlie the photosphere2 and
 it is easy to see1 that1 looked at from
 above1 such a cavity and arrangement of
 the luminous filaments would present   
the appearances actually observed4      
,professor ,young also suggests that the
 spots may be depressions in the        
photosphere caused by the diminution of 
upward pressure from below1 in          
consequence of eruptions in the         
neighborhood2 the spots thus being1 so  
to speak1 sinks in the photosphere4     
,undoubtedly the photosphere is not a   
strictly continuous shell or crust2 but 
it is heavy as compared with the        
uncondensed vapors in which it lies1    
just as a rain-cloud in our terrestrial 
atmosphere is heavier than the air2 and 
it is probably continuous enough to have

 its upper level affected by any        
diminution of pressure below4 ,the      
gaseous mass below the photosphere      
supports its weight and the weight of   
the products of condensation1 which must
 always be descending in an             
inconceivable rain and snow of molten   
and crystallized material4 ,to all      
intents and purposes1 though nothing but
 a layer of clouds1 the photosphere thus
 forms a constricting shell1 and the    
gases beneath are imprisoned and        
compressed4 ,moreover1 at a high        
temperature the viscosity of gases is   
vastly increased1 so that quite probably
 the matter of the solar nucleus        
resembles pitch or tar in its           
consistency more than what we usually   
think of as a gas4 ,consequently1 any   
sudden diminution of pressure would     
propagate itself slowly from the point  
where it occurred4 ,putting these things
 together1 it would seem1 that1 whenever
 a free outlet is obtained through the  
photosphere at any point1 thus          

decreasing the inward pressure1 the     
result would be the sinking of a portion
 of the photosphere somewhere in the    
immediate neighborhood1 to restore the  
equilibrium2 and1 if the eruption were  
kept up for any length of time1 the     
depression in the photosphere would     
continue till the eruption ceased4 ,this
 depression1 filled with the overlying  
gases1 would constitute a spot4         
,moreover1 the line of fracture 7if we  
may call it so7 at the edges of the sink
 would be a region of weakness in the   
photosphere1 so that we should expect a 
series of eruptions all around the spot4
 ,for a time the disturbance1 therefore1
 would grow1 and the spot would enlarge 
and deepen1 until1 in spite of the      
viscosity of the internal gases1 the    
equilibrium of pressure was gradually   
restored beneath4 ,so far as we know the
 spectroscopic and visual phenomena1    
none of them contradict this hypothesis4
 ,there is nothing in it1 however1 to   
account for the distribution of the     

spots in solar latitudes1 nor for their 
periodicity4                    ,,iv4   
,,the ,,chromosphere ,,and              
,,prominences4   #aic4 ,the ,sun's      
,outer ,atmosphere4--,what we see of the
 sun under ordinary circumstances is but
 a fraction of his total bulk4 ,while by
 far the greater portion of the solar   
mass is included within the photosphere1
 the larger portion of his volume lies  
without1 and constitutes a gaseous      
envelope whose diameter is at least     
double1 and its bulk therefore          
sevenfold1 that of the central globe4   
,this outer envelope1 though mainly     
gaseous1 is not spherical1 but has an   
exceedingly irregular and variable      
outline4 ,it seems to be made up1 not of
 regular strata of different density1   
like our atmosphere1 but rather of      
flames1 beams1 and streamers1 as        
transient and unstable as those of the  
aurora borealis4 ,it is divided into two
 portions by a boundary as definite1    
though not so regular1 as that which    

separates them both from the            
photosphere4 ,the outer and far more    
extensive portion1 which in texture and 
rarity seems to resemble the tails of   
comets1 is known as the coronal         
atmosphere1 since to it is chiefly due  
the corona1 or glory1 which surrounds   
the darkened sun during an eclipse4     
#aid4 ,the ,chromosphere4--,at the base 
of the coronal atmosphere1 and in       
contact with the photosphere1 is what   
resembles a sheet of scarlet fire4 ,it  
appears as if countless jets of heated  
gas were issuing through vents over the 
whole surface1 clothing it with flame1  
which heaves and tosses like the blaze  
of a conflagration4 ,this is the        
chromosphere1 or color-sphere4 ,it owes 
its vivid redness to the predominance of
 hydrogen in the flames4 ,the average   
depth of the chromosphere is not far    
from ten or twelve seconds1 or five     
thousand or six thousand miles4  #aie4  
,the ,prominences4--,here and there     
masses of this hydrogen1 mixed with     

other substances1 rise far above the    
general level into the coronal regions1 
where they float like clouds1 or are    
torn to pieces by conflicting currents4 
,these cloud-masses are known as solar  
prominences1 or protuberances4  #aif4   
,magnitude and ,distribution of the     
,prominences4--,the prominences differ  
greatly in magnitude4 ,of the #b1gfg    
observed by ,secchi1 #a1ifd attained an 
altitude of eighteen thousand miles2    
#gea1 or nearly a fourth of the whole1  
reached a height of twenty-eight        
thousand miles2 several exceeded        
eighty-four thousand miles4 ,in rare    
instances they reach elevations as great
 as a hundred thousand miles4 ,a few    
have been seen which exceeded a hundred 
and fifty thousand miles2 and ,secchi   
has recorded one of three hundred       
thousand miles4  ,illustration3 ,fig4   
#bad4  ,the irregular lines on the      
right-hand side of ,fig4 #bad show the  
proportion of the prominences observed  
by ,secchi1 that were seen in different 

parts of the sun's surface4 ,the outer  
line shows the distribution of the      
smaller prominences1 and the inner      
dotted line that of the larger          
prominences4 ,by comparing these lines  
with those on the opposite side of the  
circle1 which show the distribution of  
the spots1 it will be seen1 that1 while 
the spots are confined mainly to two    
belts1 the prominences are seen in all  
latitudes4  #aig4 ,the ,spectrum of the 
,chromosphere4--,the spectrum of the    
chromosphere is comparatively simple4   
,there are eleven lines only which are  
always present2 and six of these are    
lines of hydrogen1 and the others1 with 
a single exception1 are of unknown      
elements4 ,there are sixteen other lines
 which make their appearance very       
frequently4 ,among these latter are     
lines of sodium1 magnesium1 and iron4   
,where some special disturbance is going
 on1 the spectrum at the base of the    
chromosphere is very complicated1       
consisting of hundreds of bright lines4 

,the majority of the lines1 however1 are
 seen only occasionally1 for a few      
minutes at a time1 when the gases and   
vapors1 which generally lie low 7mainly 
in the interstices of the clouds which  
constitute the photosphere71 and below  
its upper surface1 are elevated for the 
time being by some eruptive action4 ,for
 the most part1 the lines which appear  
only at such times are simply reversals 
of the more prominent dark lines of the 
ordinary solar spectrum4 ,but the       
selection of the lines seems most       
capricious3 one is taken1 and another   
left1 though belonging to the same      
element1 of equal intensity1 and close  
beside the first4 ,some of the main     
lines of the chromosphere and           
prominences are shown in ,fig4 #bae4    
,illustration3 ,fig4 #bae4  #aih4       
,method of ,studying the ,chromosphere  
and ,prominences4--,until recently1 the 
solar atmosphere could be seen only     
during a total eclipse of the sun2 but  
now the spectroscope enables us to study

 the chromosphere and the prominences   
with nearly the same facility as the    
spots and facul4  ,the protuberances are
 ordinarily invisible1 for the same     
reason that the stars cannot be seen in 
the daytime2 they are hidden by the     
intense light reflected from our own    
atmosphere4 ,if we could only get rid of
 this aerial illumination1 without at   
the same time weakening the light of the
 prominences1 the latter would become   
visible4 ,this the spectroscope enables 
us to accomplish4 ,since the air-light  
is reflected sunshine1 it of course     
presents the same spectrum as           
sunlight1--a continuous band of color   
crossed by dark lines4 ,now1 this sort  
of spectrum is weakened by increase of  
dispersive power 7#aei71 because the    
light is spread out into a longer       
ribbon1 and made to cover a greater     
area4 ,on the other hand1 the spectrum  
of the prominences1 being composed of   
bright lines1 undergoes no such         
diminution by increased dispersion4     

,illustration3 ,fig4 #baf4  ,when the   
spectroscope is used as a means of      
examining the prominences1 the slit is  
more or less widened4 ,the telescope is 
directed so that the image of that      
portion of the solar limb which is to be
 examined shall be tangent to the opened
 slit1 as in ,fig4 #baf1 which          
represents the slit-plate of the        
spectroscope of its actual size1 with   
the image of the sun in the proper      
position for observation4               
,illustration3 ,fig4 #bag4  ,if1 now1 a 
prominence exists at this part of the   
solar limb1 and if the spectroscope     
itself is so adjusted that the ,c line  
falls in the centre of the field of     
view1 then one will see something like  
,fig4 #bag4 ,the red portion of the     
spectrum will stretch athwart the field 
of view like a scarlet ribbon with a    
darkish band across it2 and in that band
 will appear the prominences1 like      
scarlet clouds1 so like our own         
terrestrial clouds1 indeed1 in form and 

texture1 that the resemblance is quite  
startling4 ,one might almost think he   
was looking out through a partly-opened 
door upon a sunset sky1 except that     
there is no variety or contrast of      
color2 all the cloudlets are of the same
 pure scarlet hue4 ,along the edge of   
the opening is seen the chromosphere1   
more brilliant than the clouds which    
rise from it or float above it1 and1 for
 the most part1 made up of minute       
tongues and filaments4  #aii4 ,quiescent
 ,prominences4--,the prominences differ 
as widely in form and structure as in   
magnitude4 ,the two principal classes   
are the quiescent1 cloud-formed1 or     
hydrogenous1 and the eruptive1 or       
metallic4  ,illustration3 ,plate #c4    
,the quiescent prominences resemble     
almost exactly our terrestrial clouds1  
and differ among themselves in the same 
manner4 ,they are often of enormous     
dimensions1 especially in horizontal    
extent1 and are comparatively permanent1
 often undergoing little change for     

hours and days4 ,near the poles they    
sometimes remain during a whole solar   
revolution of twenty-seven days4        
,sometimes they appear to lie upon the  
limb of the sun1 like a bank of clouds  
in the terrestrial horizon1 probably    
because they are so far from the edge   
that only their upper portions are in   
sight4 ,when fully seen1 they are       
usually connected to the chromosphere by
 slender columns1 generally smallest at 
the base1 and often apparently made up  
of separate filaments closely           
intertwined1 and expanding upward4      
,sometimes the whole under surface is   
fringed with pendent filaments4         
,sometimes they float entirely free from
 the chromosphere2 and in most cases the
 larger clouds are attended by detached 
cloudlets4 ,various forms of quiescent  
prominences are shown in ,plate ,,iii4  
,other forms are given in ,figs4 #bah   
and #bai4  ,illustration3 ,fig4 #bah4   
,their spectrum is usually very simple1 
consisting of the four lines of hydrogen

 and the orange ,d#c3 hence the         
appellation hydrogenous4 ,occasionally  
the sodium and magnesium lines also     
appear1 even near the tops of the       
clouds4  ,illustration3 ,fig4 #bai4     
#bjj4 ,eruptive ,prominences4--,the     
eruptive prominences ordinarily consist 
of brilliant spikes or jets1 which      
change very rapidly in form and         
brightness4 ,as a rule1 their altitude  
is not more than twenty thousand or     
thirty thousand miles2 but occasionally 
they rise far higher than even the      
largest of the quiescent protuberances4 
,their spectrum is very complicated1    
especially near their base1 and often   
filled with bright lines4 ,the most     
conspicuous lines are those of sodium1  
magnesium1 barium1 iron1 and titanium3  
hence ,secchi calls them metallic       
prominences4  ,illustration3 ,fig4 #bbj4
  ,they usually appear in the immediate 
vicinity of a spot1 never very near the 
solar poles4 ,they change with such     
rapidity1 that the motion can almost be 

seen with the eye4 ,sometimes1 in the   
course of fifteen or twenty minutes1 a  
mass of these flames1 fifty thousand    
miles high1 will undergo a total        
transformation2 and in some instances   
their complete development or           
disappearance takes no longer time4     
,sometimes they consist of pointed rays1
 diverging in all directions1 as        
represented in ,fig4 #bbj4 ,sometimes   
they look like flames1 sometimes like   
sheaves of grain1 sometimes like        
whirling water-spouts capped with a     
great cloud2 occasionally they present  
most exactly the appearance of jets of  
liquid fire1 rising and falling in      
graceful parabolas2 frequently they     
carry on their edges spirals like the   
volutes of an ,ionic column2 and        
continually they detach filaments1 which
 rise to a great elevation1 gradually   
expanding and growing fainter as they   
ascend1 until the eye loses them4       
,illustration3 ,fig4 #bba4  #bja4       
,change of ,form in ,prominences4--,fig4

 #bba represents a prominence as seen by
 ,professor ,young1 ,sept4 #g1 #ahga4   
,it was an immense quiescent cloud1 a   
hundred thousand miles long and         
fifty-four thousand miles high4 ,at a   
there was a brilliant lump1 somewhat in 
the form of a thunder-head4 ,on         
returning to the spectroscope less than 
half an hour afterwards1 he found that  
the cloud had been literally blown into 
shreds by some inconceivable uprush from
 beneath4 ,the prominence then presented
 the form shown in ,fig4 #bbb4 ,the     
dbris of the cloud had already attained 
a height of a hundred thousand miles4   
,while he was watching them for the next
 ten minutes1 they rose1 with a motion  
almost perceptible to the eye1 till the 
uppermost reached an altitude of two    
hundred thousand miles4 ,as the         
filaments rose1 they gradually faded    
away like a dissolving cloud4           
,illustration3 ,fig4 #bbb4  ,meanwhile  
the little thunder-head had grown and   
developed into what appeared to be a    

mass of rolling and ever-changing flame4
 ,figs4 #bbc and #bbd give the          
appearance of this portion of the       
prominence at intervals of fifteen      
minutes4 ,other similar eruptions have  
been observed4  ,illustration3 ,fig4    
#bbc4  ,illustration3 ,fig4 #bbd4       
,v4 ,,the ,,corona4   #bjb4 ,general    
,appearance of the ,corona4--,at the    
time of a total eclipse of the sun1 if  
the sky is clear1 the moon appears as a 
huge black ball1 the illumination at the
 edge of the disk being just sufficient 
to bring out its rotundity4 ,from behind
 it1 to borrow ,professor ,young's vivid
 description1 stream out on all sides   
radiant filaments1 beams1 and sheets of 
pearly light1 which reach to a distance 
sometimes of several degrees from the   
solar surface1 forming an irregular     
stellate halo1 with the black globe of  
the moon in its apparent centre4 ,the   
portion nearest the sun is of dazzling  
brightness1 but still less brilliant    
than the prominences which blaze through

 it like carbuncles4 ,generally this    
inner corona has a pretty uniform       
height1 forming a ring three or four    
minutes of arc in width1 separated by a 
somewhat definite outline from the outer
 corona1 which reaches to a much greater
 distance1 and is far more irregular in 
form4 ,usually there are several rifts1 
as they have been called1 like narrow   
beams of darkness1 extending from the   
very edge of the sun to the outer night1
 and much resembling the cloud-shadows  
which radiate from the sun before a     
thunder-shower2 but the edges of these  
rifts are frequently curved1 showing    
them to be something else than real     
shadows4 ,sometimes there are narrow    
bright streamers1 as long as the rifts1 
or longer4 ,these are often inclined1   
occasionally are even nearly tangential 
to the solar surface1 and frequently are
 curved4 ,on the whole1 the corona is   
usually less extensive and brilliant    
over the solar poles1 and there is a    
recognizable tendency to accumulations  

above the middle latitudes1 or          
spot-zones2 so that1 speaking roughly1  
the corona shows a disposition to assume
 the form of a quadrilateral or         
four-rayed star1 though in almost every 
individual case this form is greatly    
modified by abnormal streamers at some  
point or other4  ,illustration3 ,fig4   
#bbe4  #bjc4 ,the ,corona as seen at    
,recent ,eclipses4--,the corona can be  
seen only at the time of a total eclipse
 of the sun1 and then for only a few    
minutes4 ,its form varies considerably  
from one eclipse to another1 and        
apparently also during the same eclipse4
 ,at least1 different observers at      
different stations depict the same      
corona under very different forms4 ,fig4
 #bbe represents the corona of #aheg as 
observed by ,liais4 ,in this view the   
petal-like forms1 which have been       
noticed in the corona at other times1   
are especially prominent4               
,illustration3 ,fig4 #bbf4  ,fig4 #bbf  
shows the corona of #ahfj as it was     

observed by ,temple4  ,illustration3    
,fig4 #bbg4  ,fig4 #bbg shows the corona
 of #ahfg4 ,this is interesting as being
 a corona at the time of sun-spot       
minimum4  ,illustration3 ,fig4 #bbh4    
,fig4 #bbh represents the corona of     
#ahfh4 ,this is a larger and more       
irregular corona than usual4            
,illustration3 ,fig4 #bbi4  ,the corona 
of #ahfi is shown in ,fig4 #bbi4        
,illustration3 ,fig4 #bcj4  ,fig4 #bcj  
is a view of the corona of #ahga as seen
 by ,capt4 ,tupman4  ,illustration3     
,fig4 #bca4  ,fig4 #bca shows the same  
corona as seen by ,foenander4           
,illustration3 ,fig4 #bcb4  ,fig4 #bcb  
shows the same corona as photographed by
 ,davis4  ,illustration3 ,fig4 #bcc4    
,fig4 #bcc shows the corona of #ahgh    
made up from several views as combined  
by ,professor ,young4  #bjd4 ,the       
,spectrum of the ,corona4--,the chief   
line in the spectrum of the corona is   
the one usually designated as #adgd1 and
 now known as the coronal line4 ,it is  

seen as a dark line on the disk of the  
sun2 and a spectroscope of great        
dispersive power shows this dark line to
 be closely double1 the lower component 
being one of the iron lines1 and the    
upper the coronal line4 ,this dark line 
is shown at x1 ,fig4 #bcd4              
,illustration3 ,fig4 #bcd4  ,besides    
this bright line1 the hydrogen lines    
appear faintly in the spectrum of the   
corona4 ,the #adgd line has been        
sometimes traced with the spectroscope  
to an elevation of nearly twenty minutes
 above the moon's limb1 and the hydrogen
 lines nearly as far2 and the lines were
 just as strong in the middle of a dark 
rift as anywhere else4  ,the substance  
which produces the #adgd line is unknown
 as yet4 ,it seems to be something with 
a vapor-density far below that of       
hydrogen1 which is the lightest         
substance of which we have any          
knowledge4 ,it can hardly be an         
allotropic form of any terrestrial      
element1 as some scientists have        

suggested2 for in the most violent      
disturbances in prominences and near    
sun-spots1 when the lines of hydrogen1  
magnesium1 and other metals1 are        
contorted and shattered by the rush of  
the contending elements1 this line alone
 remains fine1 sharp1 and straight1 a   
little brightened1 but not otherwise    
affected4 ,for the present it remains1  
like a few other lines in the spectrum1 
an unexplained mystery4  ,besides bright
 lines1 the corona shows also a faint   
continuous spectrum1 in which have been 
observed a few of the more prominent    
dark lines of the solar spectrum4  ,this
 shows1 that1 while the corona may be in
 the main composed of glowing gas 7as   
indicated by the bright lines of its    
spectrum71 it also contains considerable
 matter in such a state as to reflect   
the sunlight1 probably in the form of   
dust or fog4                            
,v4 ,,eclipses4   ,illustration3 ,fig4  
#bce4  #bje4 ,the ,shadows of the ,earth
 and ,moon4--,the shadows cast by the   

earth and moon are shown in ,fig4 #bce4 
,each shadow is seen to be made up of a 
dark portion called the umbra1 and of a 
lighter portion called the penumbra4    
,the light of the sun is completely     
excluded from the umbra1 but only       
partially from the penumbra4 ,the umbra 
is in the form of a cone1 with its apex 
away from the sun2 though in the case of
 the earth's shadow it tapers very      
slowly4 ,the penumbra surrounds the     
umbra1 and increases in size as we      
recede from the sun4 ,the axis of the   
earth's shadow lies in the plane of the 
ecliptic1 which in the figure is the    
surface of the page4 ,as the moon's     
orbit is inclined five degrees to the   
plane of the ecliptic1 the axis of the  
moon's shadow will sometimes lie above1 
and sometimes below1 the ecliptic4 ,it  
will lie on the ecliptic only when the  
moon is at one of her nodes4  #bjf4     
,when there will be an ,eclipse of the  
,moon4--,the moon is eclipsed whenever  
it passes into the umbra of the earth's 

shadow4 ,it will be seen from the figure
 that the moon can pass into the shadow 
of the earth only when she is in        
opposition1 or at full4 ,owing to the   
inclination of the moon's orbit to the  
ecliptic1 the moon will pass either     
above or below the earth's shadow when  
she is at full1 unless she happens to be
 near her node at this time3 hence there
 is not an eclipse of the moon every    
month4  ,when the moon simply passes    
into the penumbra of the earth's shadow1
 the light of the moon is somewhat      
dimmed1 but not sufficiently to attract 
attention1 or to be denominated an      
eclipse4  ,illustration3 ,fig4 #bcf4    
#bjg4 ,the ,lunar ,ecliptic             
,limits4--,in ,fig4 #bcf the line ,,ab  
represents the plane of the ecliptic1   
and the line ,,cd the moon's orbit4 ,the
 large black circles on the line ,,ab   
represent sections of the umbra of the  
earth's shadow1 and the smaller circles 
on ,,cd represent the moon at full4 ,it 
will be seen1 that1 if the moon is full 

at ,e1 she will just graze the umbra of 
the earth's shadow4 ,in this case she   
will suffer no eclipse4 ,were the moon  
full at any point nearer her node1 as at
 ,f1 she would pass into the umbra of   
the earth's shadow1 and would be        
partially eclipsed4 ,were the moon full 
at ,g1 she would pass through the centre
 of the earth's shadow1 and be totally  
eclipsed4  ,it will be seen from the    
figure that full moon must occur when   
the moon is within a certain distance   
from her node1 in order that there may  
be a lunar eclipse2 and this space is   
called the lunar ecliptic limits4  ,the 
farther the earth is from the sun1 the  
less rapidly does its shadow taper1 and 
therefore the greater its diameter at   
the distance of the moon2 and1 the      
nearer the moon to the earth1 the       
greater the diameter of the earth's     
shadow at the distance of the moon4 ,of 
course1 the greater the diameter of the 
earth's shadow1 the greater the ecliptic
 limits3 hence the lunar ecliptic limits

 vary somewhat from time to time1       
according to the distance from the earth
 to the sun and from the earth to the   
moon4 ,the limits within which an       
eclipse is inevitable under all         
circumstances are called the minor      
ecliptic limits2 and those within which 
an eclipse is possible under some       
circumstances1 the major ecliptic       
limits4  ,illustration3 ,fig4 #bcg4     
#bjh4 ,lunar ,eclipses4--,fig4 #bcg     
shows the path of the moon through the  
earth's shadow in the case of a partial 
eclipse4 ,the magnitude of such an      
eclipse depends upon the nearness of the
 moon to her nodes4 ,the magnitude of an
 eclipse is usually denoted in digits1 a
 digit being one-twelfth of the diameter
 of the moon4  ,illustration3 ,fig4     
#bch4  ,fig4 #bch shows the path of the 
moon through the earth's shadow in the  
case of a total eclipse4 ,it will be    
seen from the figure that it is not     
necessary for the moon to pass through  
the centre of the earth's shadow in     

order to have a total eclipse4 ,when the
 moon passes through the centre of the  
earth's shadow1 the eclipse is both     
total and central4  ,at the time of a   
total eclipse1 the moon is not entirely 
invisible1 but shines with a faint      
copper-colored light4 ,this light is    
refracted into the shadow by the earth's
 atmosphere1 and its amount varies with 
the quantity of clouds and vapor in that
 portion of the atmosphere which the    
sunlight must graze in order to reach   
the moon4  ,the duration of an eclipse  
varies between very wide limits1 being1 
of course1 greatest when the eclipse is 
central4 ,a total eclipse of the moon   
may last nearly two hours1 or1 including
 the partial portions of the eclipse1   
three or four hours4  ,every eclipse of 
the moon1 whether total or partial1 is  
visible at the same time to the whole   
hemisphere of the earth which is turned 
towards the moon2 and the eclipse will  
have exactly the same magnitude at every
 point of observation4  #bji4 ,when     

there will be an ,eclipse of the        
,sun4--,there will be an eclipse of the 
sun whenever any portion of the moon's  
shadow is thrown on the earth4 ,it will 
be seen from ,fig4 #bce that this can   
occur only when the moon is in          
conjunction1 or at new4 ,it does not    
occur every month1 because1 owing to the
 inclination of the moon's orbit to the 
ecliptic1 the moon's shadow is usually  
thrown either above or below the earth  
at the time of new moon4 ,there can be  
an eclipse of the sun only when new moon
 occurs at or near one of the nodes of  
her orbit4  #baj4 ,solar ,ecliptic      
,limits4--,the distances from the moon's
 node within which a new moon would     
throw some portion of its shadow on the 
earth so as to produce an eclipse of the
 sun are called the solar ecliptic      
limits4 ,as in the case of the moon1    
there are major and minor ecliptic      
limits2 the former being the limits     
within which an eclipse of the sun is   
possible under some circumstances1 and  

the latter those under which an eclipse 
is inevitable under all circumstances4  
,the limits within which a solar eclipse
 may occur are greater than those within
 which a lunar eclipse may occur4 ,this 
will be evident from an examination of  
,fig4 #bce4 ,were the moon in that      
figure just outside of the lines ,,ab   
and ,,cd1 it will be seen that the      
penumbra of her shadow would just graze 
the earth3 hence the moon must be       
somewhere within the space bounded by   
these lines in order to cause an eclipse
 of the sun4 ,now1 these lines mark the 
prolongation to the sun of the cone of  
the umbra of the earth's shadow3 hence1 
in order to produce an eclipse of the   
sun1 new moon must occur somewhere      
within this prolongation of the umbra of
 the earth's shadow4 ,now1 it is evident
 that the diameter of this cone is      
greater on the side of the earth toward 
the sun than on the opposite side3 hence
 the solar ecliptic limits are greater  
than the lunar ecliptic limits4  #baa4  

,solar ,eclipses4--,an observer in the  
umbra of the moon's shadow would see a  
total eclipse of the sun1 while one in  
the penumbra would see only a partial   
eclipse4 ,the magnitude of this partial 
eclipse would depend upon the distance  
of the observer from the umbra of the   
moon's shadow4  ,illustration3 ,fig4    
#bci4  ,illustration3 ,fig4 #bdj4  ,the 
umbra of the moon's shadow is just about
 long enough to reach the earth4        
,sometimes the point of this shadow     
falls short of the earth's surface1 as  
shown in ,fig4 #bci1 and sometimes it   
falls upon the earth1 as shown in ,fig4 
#bdj1 according to the varying distance 
of the sun and moon from the earth4 ,the
 diameter of the umbra at the surface of
 the earth is seldom more than a hundred
 miles3 hence the belt of a total       
eclipse is1 on the average1 not more    
than a hundred miles wide2 and a total  
eclipse seldom lasts more than five or  
six minutes1 and sometimes only a few   
seconds4 ,owing1 however1 to the        

rotation of the earth1 the umbra of the 
moon's shadow may pass over a long reach
 of the earth's surface4 ,fig4 #bda     
shows the track of the umbra of the     
moon's shadow over the earth in the     
total eclipse of #ahfj4  ,illustration3 
,fig4 #bda4  ,illustration3 ,fig4 #bdb4 
,fig4 #bdb shows the track of the total 
eclipse of #ahga across ,india and the  
adjacent seas4  ,illustration3 ,fig4    
#bdc4  ,illustration3 ,fig4 #bdd4  ,in a
 partial eclipse of the sun1 more or    
less of one side of the sun's disk is   
usually concealed1 as shown in ,fig4    
#bdc4 ,occasionally1 however1 the centre
 of the sun's disk is covered1 leaving a
 bright ring around the margin1 as shown
 in ,fig4 #bdd4 ,such an eclipse is     
called an annular eclipse4 ,an eclipse  
can be annular only when the cone of the
 moon's shadow is too short to reach the
 earth1 and then only to observers who  
are in the central portion of the       
penumbra4  #bab4 ,comparative ,frequency
 of ,solar and ,lunar ,eclipses4--,there

 are more eclipses of the sun in the    
year than of the moon2 and yet1 at any  
one place1 eclipses of the moon are more
 frequent than those of the sun4  ,there
 are more lunar than solar eclipses1    
because1 as we have seen1 the limits    
within which a solar eclipse may occur  
are greater than those within which a   
lunar eclipse may occur4 ,there are more
 eclipses of the moon visible at any one
 place than of the sun2 because1 as we  
have seen1 an eclipse of the moon1      
whenever it does occur1 is visible to a 
whole hemisphere at a time1 while an    
eclipse of the sun is visible to only a 
portion of a hemisphere1 and a total    
eclipse to only a very small portion of 
a hemisphere4 ,a total eclipse of the   
sun is1 therefore1 a very rare          
occurrence at any one place4  ,the      
greatest number of eclipses that can    
occur in a year is seven1 and the least 
number1 two4 ,in the former case1 five  
may be of the sun and two of the moon1  
or four of the sun and three of the     

moon4 ,in the latter case1 both must be 
of the sun4                       ,,vi4 
,,the ,,three ,,groups ,,of ,,planets4  
,i4 ,,general ,,characteristics ,,of    
,,the ,,groups4   #bac4 ,the ,inner     
,group4--,the inner group of planets is 
composed of ,mercury1 ,venus1 the       
,earth1 and ,mars2 that is1 of all the  
planets which lie between the asteroids 
and the sun4 ,the planets of this group 
are comparatively small and dense4 ,so  
far as known1 they rotate on their axes 
in about twenty-four hours1 and they are
 either entirely without moons1 or are  
attended by comparatively few4  ,the    
comparative sizes and eccentricities of 
the orbits of this group are shown in   
,fig4 #bde4 ,the dots round the orbits  
show the position of the planets at     
intervals of ten days4  ,illustration3  
,fig4 #bde4  #bad4 ,the ,outer          
,group4--,the outer group of planets is 
composed of ,jupiter1 ,saturn1 ,uranus1 
and ,neptune4 ,these planets are all    
very large and of slight density4 ,so   

far as known1 they rotate on their axes 
in about ten hours1 and are accompanied 
with complicated systems of moons4 ,fig4
 #bdf1 which represents the comparative 
sizes of the planets1 shows at a glance 
the immense difference between those of 
the inner and outer group4 ,fig4 #bdg   
shows the comparative sizes and         
eccentricities of the orbits of the     
outer planets4 ,the dots round the      
orbits show the position of the planets 
at intervals of a thousand days4        
,illustration3 ,fig4 #bdf4              
,illustration3 ,fig4 #bdg4  #bae4 ,the  
,asteroids4--,between the inner and     
outer groups of planets there is a great
 number of very small planets known as  
the minor planets1 or asteroids4 ,over  
two hundred planets belonging to this   
group have already been discovered4     
,their orbits are shown by the dotted   
lines in ,fig4 #bdg4 ,the sizes of the  
four largest of these planets1 compared 
with the earth1 are shown in ,fig4 #bdh4
  ,illustration3 ,fig4 #bdh4  ,the      

asteroids of this group are             
distinguished from the other planets1   
not only by their small size1 but by the
 great eccentricities and inclinations  
of their orbits4 ,if we except ,mercury1
 none of the larger planets has an      
eccentricity amounting to one-tenth the 
diameter of its orbit 7#dc71 nor is any 
orbit inclined more than two or three   
degrees to the ecliptic2 but the        
inclinations of many of the minor       
planets exceed ten degrees1 and the     
eccentricities frequently amount to an  
eighth of the orbital diameter4 ,the    
orbit of ,pallas is inclined thirty-four
 degrees to the ecliptic1 while there   
are some planets of this group whose    
orbits nearly coincide with the plane of
 the ecliptic4  ,illustration3 ,fig4    
#bdi4  ,fig4 #bdi shows one of the most 
and one of the least eccentric of the   
orbits of this group as compared with   
that of the earth4  ,illustration3 ,fig4
 #bej4  ,the intricate complexity of the
 orbits of the asteroids is shown in    

,fig4 #bej4                       ,,ii4 
,,the ,,inner ,,group ,,of ,,planets4   
,mercury4   #baf4 ,the ,orbit of        
,mercury4--,the orbit of ,mercury is    
more eccentric than that of any of the  
larger planets1 and it has also a       
greater inclination to the ecliptic4    
,its eccentricity 7#dc7 is a little over
 a fifth1 and its inclination to the    
ecliptic somewhat over seven degrees4   
,the mean distance of ,mercury from the 
sun is about thirty-five million miles2 
but1 owing to the great eccentricity of 
its orbit1 its distance from the sun    
varies from about forty-three million   
miles at aphelion to about twenty-eight 
million at perihelion4  ,illustration3  
,fig4 #bea4  #bag4 ,distance of ,mercury
 from the ,earth4--,it is evident1 from 
,fig4 #bea1 that an inferior planet1    
like ,mercury1 is the whole diameter of 
its orbit nearer the earth at inferior  
conjunction than at superior            
conjunction3 hence ,mercury's distance  
from the earth varies considerably4     

,owing to the great eccentricity of its 
orbit1 its distance from the earth at   
inferior conjunction also varies        
considerably4 ,mercury is nearest to the
 earth when its inferior conjunction    
occurs at its own aphelion and at the   
earth's perihelion4  ,illustration3     
,fig4 #beb4  #bah4 ,apparent ,size of   
,mercury4--,since ,mercury's distance   
from the earth is variable1 the apparent
 size of the planet is also variable4   
,fig4 #beb shows its apparent size at   
its extreme and mean distances from the 
earth4 ,its apparent diameter varies    
from five seconds to twelve seconds4    
,illustration3 ,fig4 #bec4  #bai4       
,volume and ,density of ,mercury4--,the 
real diameter of ,mercury is about three
 thousand miles4 ,its size1 compared    
with that of the earth1 is shown in     
,fig4 #bec4 ,the earth is about sixteen 
times as large as ,mercury2 but ,mercury
 is about one-fifth more dense than the 
earth4  #bbj4 ,greatest ,elongation of  
,mercury4--,mercury1 being an inferior  

planet 7or one within the orbit of the  
earth71 appears to oscillate to and fro 
across the sun4 ,its greatest apparent  
distance from the sun1 or its greatest  
elongation1 varies considerably4 ,the   
farther ,mercury is from the sun1 and   
the nearer the earth is to ,mercury1 the
 greater is its angular distance from   
the sun at the time of its greatest     
elongation4 ,under the most favorable   
circumstances1 the greatest elongation  
amounts to about twenty-eight degrees1  
and under the least favorable to only   
sixteen or seventeen degrees4  #bba4    
,sidereal and ,synodical ,periods of    
,mercury4--,mercury accomplishes a      
complete revolution around the sun in   
about eighty-eight days2 but it takes it
 a hundred and sixteen days to pass from
 its greatest elongation east to the    
same elongation again4 ,the orbital     
motion of this planet is at the rate of 
nearly thirty miles a second4  ,in ,fig4
 #bea1 ,p''' represents elongation east 
of the sun1 and ,p' elongation west4 ,it

 will be seen that it is much farther   
from ,p' around to ,p''' than from ,p'''
 on to ,p'4 ,mercury is only about      
forty-eight days in passing from        
greatest elongation east to greatest    
elongation west1 while it is about      
sixty-eight days in passing back again4 
#bbb4 ,visibility of ,mercury4--,mercury
 is too close to the sun for favorable  
observation4 ,it is never seen long     
after sunset1 or long before sunrise1   
and never far from the horizon4 ,when   
visible at all1 it must be sought for   
low down in the west shortly after      
sunset1 or low in the east shortly      
before sunrise1 according as the planet 
is at its east or west elongation4 ,it  
is often visible to the naked eye in our
 latitude2 but the illumination of the  
twilight sky1 and the excess of vapor in
 our atmosphere near the horizon1       
combine to make the telescopic study of 
the planet difficult and unsatisfactory4
  ,illustration3 ,fig4 #bed4  #bbc4 ,the
 ,atmosphere and ,surface of            

,mercury4--,mercury seems to be         
surrounded by a dense atmosphere4 ,one  
proof of the existence of such an       
atmosphere is furnished at the time of  
the planet's transit across the disk of 
the sun1 which occasionally happens4    
,the planet is then seen surrounded by a
 border1 as shown in ,fig4 #bed4 ,a     
bright spot has also been observed on   
the dark disk of the planet during a    
transit1 as shown in ,fig4 #bee4 ,the   
border around the planet seems to be due
 to the action of the planet's          
atmosphere2 but it is difficult to      
account for the bright spot4            
,illustration3 ,fig4 #bee4              
,illustration3 ,fig4 #bef4  ,schrter1 a 
celebrated ,german astronomer1 at about 
the beginning of the present century1   
thought that he detected spots and      
shadings on the disk of the planet1     
which indicated both the presence of an 
atmosphere and of elevations4 ,the      
shading along the terminator1 which     
seemed to indicate the presence of a    

twilight1 and therefore of an           
atmosphere1 are shown in ,fig4 #bef4 ,it
 also shows the blunted aspect of one of
 the cusps1 which ,schrter noticed at   
times1 and which he attributed to the   
shadow of a mountain1 estimated to be   
ten or twelve miles high4 ,fig4 #beg    
shows this mountain near the upper cusp1
 as ,schrter believed he saw it in the  
year #ahjj4 ,by watching certain marks  
upon the disk of ,mercury1 ,schrter came
 to the conclusion that the planet      
rotates on its axis in about twenty-four
 hours4 ,modern observers1 with more    
powerful telescopes1 have failed to     
verify ,schrter's observations as to the
 indications of an atmosphere and of    
elevations4 ,nothing is known with      
certainty about the rotation of the     
planet4  ,illustration3 ,fig4 #beg4     
,the border around ,mercury1 and the    
bright spot on its disk at the time of  
the transit of the planet across the    
sun1 have been seen since ,schrter's    
time1 and the existence of these        

phenomena is now well established2 but  
astronomers are far from being agreed as
 to their cause4  #bbd4                 
,intra-,mercurial ,planets4--,it has for
 some time been thought probable that   
there is a group of small planets       
between ,mercury and the sun2 and at    
various times the discovery of such     
bodies has been announced4 ,in #ahei a  
,french observer believed that he had   
detected an intra-,mercurial planet1 to 
which the name of ,vulcan was given1 and
 for which careful search has since been
 made1 but without success4 ,during the 
total eclipse of #ahgh ,professor       
,watson observed two objects near the   
sun1 which he thought to be planets2 but
 this is still matter of controversy4   
,venus4   #bbe4 ,the ,orbit of          
,venus4--,the orbit of ,venus has but   
slight eccentricity1 differing less from
 a circle than that of any other large  
planet4 ,it is inclined to the ecliptic 
somewhat more than three degrees4 ,the  
mean distance of the planet from the sun

 is about sixty-seven million miles4    
#bbf4 ,distance of ,venus from the      
,earth4--,the distance of ,venus from   
the earth varies within much wider      
limits than that of ,mercury4 ,when     
,venus is at inferior conjunction1 her  
distance from the earth is ninety-two   
million miles minus sixty-seven million 
miles1 or twenty-five million miles2 and
 when at superior conjunction it is     
ninety-two million miles plus           
sixty-seven million miles1 or a hundred 
and fifty-nine million miles4 ,venus is 
considerably more than six times as far 
off at superior conjunction as at       
inferior conjunction4  ,illustration3   
,fig4 #beh4  #bbg4 ,apparent ,size of   
,venus4--,owing to the great variation  
in the distance of ,venus from the      
earth1 her apparent diameter varies from
 about ten seconds to about sixty-six   
seconds4 ,fig4 #beh shows the apparent  
size of ,venus at her extreme and mean  
distances from the earth4  #bbh4 ,volume
 and ,density of ,venus4--,the real size

 of ,venus is about the same as that of 
the earth1 her diameter being only about
 three hundred miles less4 ,the         
comparative sizes of the two planets are
 shown in ,fig4 #bei4 ,the density of   
,venus is a little less than that of the
 earth4  ,illustration3 ,fig4 #bei4     
#bbi4 ,the ,greatest ,elongation of     
,venus4--,venus1 like ,mercury1 appears 
to oscillate to and fro across the sun4 
,the angular value of the greatest      
elongation of ,venus varies but         
slightly1 its greatest value being about
 forty-seven degrees4  #bcj4 ,sidereal  
and ,synodical ,periods of ,venus4--,the
 sidereal period of ,venus1 or that of a
 complete revolution around the sun1 is 
about two hundred and twenty-five days2 
her orbital motion being at the rate of 
nearly twenty-two miles a second4 ,her  
synodical period1 or the time it takes  
her to pass around from her greatest    
eastern elongation to the same          
elongation again1 is about five hundred 
and eighty-four days1 or eighteen       

months4 ,venus is a hundred and         
forty-six days1 or nearly five months1  
in passing from her greatest elongation 
east through inferior conjunction to her
 greatest elongation west4  #bca4 ,venus
 as a ,morning and an ,evening          
,star4--,for a period of about nine     
months1 while ,venus is passing from    
superior conjunction to her greatest    
eastern elongation1 she will be east of 
the sun1 and will therefore set after   
the sun4 ,during this period she is the 
evening star1 the ,hesperus of the      
ancients4 ,while passing from inferior  
conjunction to superior conjunction1    
,venus is west of the sun1 and therefore
 rises before the sun4 ,during this     
period of nine months she is the morning
 star1 the ,phosphorus1 or ,lucifer1 of 
the ancients4  #bcb4 ,brilliancy of     
,venus4--,next to the sun and moon1     
,venus is at times the most brilliant   
object in the heavens1 being bright     
enough to be seen in daylight1 and to   
cast a distinct shadow at night4 ,her   

brightness1 however1 varies             
considerably1 owing to her phases and to
 her varying distance from the earth4   
,she does not appear brightest when at  
full1 for she is then farthest from the 
earth1 at superior conjunction2 nor does
 she appear brightest when nearest the  
earth1 at inferior conjunction1 for her 
phase is then a thin crescent 7see ,fig4
 #beh74 ,she is most conspicuous while  
passing from her greatest eastern       
elongation to her greatest western      
elongation4 ,after she has passed her   
eastern elongation1 she becomes brighter
 and brighter till she is within about  
forty degrees of the sun4 ,her phase at 
this point in her orbit is shown in     
,fig4 #bfj4 ,her brilliancy then begins 
to wane1 until she comes too near the   
sun to be visible4 ,when she re-appears 
on the west of the sun1 she again       
becomes more brilliant2 and her         
brilliancy increases till she is about  
forty degrees from the sun1 when she is 
again at her brightest4 ,venus passes   

from her greatest brilliancy as an      
evening star to her greatest brilliancy 
as a morning star in about seventy-three
 days4 ,she has the same phase1 and is  
at the same distance from the earth1 in 
both cases of maximum brilliancy4 ,of   
course1 the brilliancy of ,venus when at
 the maximum varies somewhat from time  
to time1 owing to the eccentricities of 
the orbits of the earth and of ,venus1  
which cause her distance from the earth1
 at her phase of greatest brilliancy1 to
 vary4 ,she is most brilliant when the  
phase of her greatest brilliancy occurs 
when she is at her aphelion and the     
earth at its perihelion4  ,illustration3
 ,fig4 #bfj4  #bcc4 ,the ,atmosphere and
 ,surface of ,venus4--,schrter believed 
that he saw shadings and markings on    
,venus similar to those on ,mercury1    
indicating the presence of an atmosphere
 and of elevations on the surface of the
 planet4 ,fig4 #bfa represents the      
surface of ,venus as it appeared to this
 astronomer4 ,by watching certain       

markings on the disk of ,venus1 ,schrter
 came to the conclusion that ,venus     
rotates on her axis in about twenty-four
 hours4  ,illustration3 ,fig4 #bfa4  ,it
 is now generally conceded that ,venus  
has a dense atmosphere2 but ,schrter's  
observations of the spots on her disk   
have not been verified by modern        
astronomers1 and we really know nothing 
certainly of her rotation4  #bcd4       
,transits of ,venus4--,when ,venus      
happens to be near one of the nodes of  
her orbit when she is in inferior       
conjunction1 she makes a transit across 
the sun's disk4 ,these transits occur in
 pairs1 separated by an interval of over
 a hundred years4 ,the two transits of  
each pair are separated by an interval  
of eight years1 the dates of the most   
recent being #ahgd and #ahhb4 ,venus1   
like ,mercury1 appears surrounded with a
 border on passing across the sun's     
disk1 as shown in ,fig4 #bfb4           
,illustration3 ,fig4 #bfb4              
,mars4   #bce4 ,the ,orbit of           

,mars4--,the orbit of ,mars is more     
eccentric than that of any of the larger
 planets1 except ,mercury2 its          
eccentricity being about one-eleventh4  
,the inclination of the orbit to the    
ecliptic is somewhat under two degrees4 
,the mean distance of ,mars from the sun
 is about a hundred and forty million   
miles2 but1 owing to the eccentricity of
 his orbit1 the distance varies from a  
hundred and fifty-three million miles to
 a hundred and twenty-seven million     
miles4  ,illustration3 ,fig4 #bfc4      
#bcf4 ,distance of ,mars from the       
,earth4--,it will be seen1 from ,fig4   
#bfc1 that a superior planet 7or one    
outside the orbit of the earth71 like   
,mars1 is nearer the earth1 by the whole
 diameter of the earth's orbit1 when in 
opposition than when in conjunction4    
,the mean distance of ,mars from the    
earth1 at the time of opposition1 is a  
hundred and forty million miles minus   
ninety-two million miles1 or forty-eight
 million miles4 ,owing to the           

eccentricity of the orbit of the earth  
and of ,mars1 the distance of this      
planet when in opposition varies        
considerably4 ,when the earth is in     
aphelion1 and ,mars in perihelion1 at   
the time of opposition1 the distance of 
the planet from the earth is only about 
thirty-three million miles4 ,on the     
other hand1 when the earth is in        
perihelion1 and ,mars in aphelion1 at   
the time of opposition1 the distance of 
the planet is over sixty-two million    
miles4  ,the mean distance of ,mars from
 the earth when in conjunction is a     
hundred and forty million miles plus    
ninety-two million miles1 or two hundred
 and thirty-two million miles4 ,it will 
therefore be seen that ,mars is nearly  
five times as far off at conjunction as 
at opposition4  ,illustration3 ,fig4    
#bfd4  #bcg4 ,the ,apparent ,size of    
,mars4--,owing to the varying distance  
of ,mars from the earth1 the apparent   
size of the planet varies almost as much
 as that of ,venus4 ,fig4 #bfd shows the

 apparent size of ,mars at its extreme  
and mean distances from the earth4 ,the 
apparent diameter varies from about four
 seconds to about thirty seconds4       
,illustration3 ,fig4 #bfe4  #bch4 ,the  
,volume and ,density of ,mars4--,among  
the larger planets ,mars is next in size
 to ,mercury4 ,its real diameter is     
somewhat more than four thousand miles1 
and its bulk is about one-seventh of    
that of the earth4 ,its size1 compared  
with that of the earth1 is shown in     
,fig4 #bfe4  ,illustration3 ,plate #d4  
,the density of ,mars is only about     
three-fourths of that of the earth4     
#bci4 ,sidereal and ,synodical ,periods 
of ,mars4--,the sidereal period of      
,mars1 or the time in which he makes a  
complete revolution around the sun1 is  
about six hundred and eighty-seven days1
 or nearly twenty-three months2 but he  
is about seven hundred and eighty days  
in passing from opposition to opposition
 again1 or in performing a synodical    
revolution4 ,mars moves in his orbit at 

the rate of about fifteen miles a       
second4  #bdj4 ,brilliancy of           
,mars4--,when near his opposition1 ,mars
 is easily recognized with the naked eye
 by his fiery-red light4 ,he is much    
more brilliant at some oppositions than 
at others1 for reasons already explained
 7#bcf71 but always shines brighter than
 an ordinary star of the first          
magnitude4  #bda4 ,telescopic           
,appearance of ,mars4--,when viewed with
 a good telescope 7see ,plate ,,iv471   
,mars is seen to be covered with dusky1 
dull-red patches1 which are supposed to 
be continents1 like those of our own    
globe4 ,other portions1 of a greenish   
hue1 are believed to be tracts of water4
 ,the ruddy color1 which overpowers the 
green1 and makes the whole planet seem  
red to the naked eye1 was believed by   
,sir ,j4 ,herschel to be due to an      
ochrey tinge in the general soil1 like  
that of the red sandstone districts on  
the earth4 ,in a telescope1 ,mars       
appears less red1 and the higher the    

power the less the intensity of the     
color4 ,the disk1 when well seen1 is    
mapped out in a way which gives at once 
the impression of land and water4 ,the  
bright part is red inclining to orange1 
sometimes dotted with brown and greenish
 points4 ,the darker spaces1 which vary 
greatly in depth of tone1 are of a dull 
gray-green1 having the aspect of a fluid
 which absorbs the solar rays4 ,the     
proportion of land to water on the earth
 appears to be reversed on ,mars4 ,on   
the earth every continent is an island2 
on ,mars all seas are lakes4 ,long1     
narrow straits are more common than on  
the earth2 and wide expanses of water1  
like our ,atlantic ,ocean1 are rare4    
7,see ,fig4 #bff47  ,illustration3 ,fig4
 #bff4  ,illustration3 ,fig4 #bfg4      
,fig4 #bfg represents a chart of the    
surface of ,mars1 which has been        
constructed from careful telescopic     
observation4 ,the outlines1 as seen in  
the telescope1 are1 however1 much less  
distinct than they are represented here2

 and it is by no means certain that the 
light and dark portions are bodies of   
land and water4  ,in the vicinity of the
 poles brilliant white spots may be     
noticed1 which are considered by many   
astronomers to be masses of snow4 ,this 
conjecture is favored by the fact that  
they appear to diminish under the sun's 
influence at the beginning of the       
,martial summer1 and to increase again  
on the approach of winter4  #bdb4       
,rotation of ,mars4--,on watching ,mars 
with a telescope1 the spots on the disk 
are found to move 7as shown in ,fig4    
#bfh7 in a manner which indicates that  
the planet rotates in about twenty-four 
hours on an axis inclined about         
twenty-eight degrees from a             
perpendicular to the plane of its orbit4
 ,the inclination of the axis is shown  
in ,fig4 #bfi4 ,it is evident from the  
figure that the variation in the length 
of day and night1 and the change of     
seasons1 are about the same on ,mars as 
on the earth4 ,the changes will1 of     

course1 be somewhat greater1 and the    
seasons will be about twice as long4    
,illustration3 ,fig4 #bfh4              
,illustration3 ,fig4 #bfi4              
,illustration3 ,fig4 #bgj4  #bdc4 ,the  
,satellites of ,mars4--,in #ahgg        
,professor ,hall of the ,washington     
,observatory discovered that ,mars is   
accompanied by two small moons1 whose   
orbits are shown in ,fig4 #bgj4 ,the    
inner satellite has been named ,phobos1 
and the outer one ,deimos4 ,it is       
estimated that the diameter of the outer
 moon is from five to ten miles1 and    
that of the inner one from ten to forty 
miles4  ,phobos is remarkable for its   
nearness to the planet and the rapidity 
of its revolution1 which is performed in
 seven hours thirty-eight minutes4 ,its 
distance from the centre of the planet  
is about six thousand miles1 and from   
the surface less than four thousand4    
,astronomers on ,mars1 with telescopes  
and eyes like ours1 could readily find  
out whether this satellite is inhabited1

 the distance being less than           
one-sixtieth of that of our moon4  ,it  
will be seen that ,phobos makes about   
three revolutions to one rotation of the
 planet4 ,it will1 of course1 rise in   
the west2 though the sun1 the stars1 and
 the other satellite rise in the east4  
,deimos makes a complete revolution in  
about thirty hours4                     
,,iii4 ,,the ,,asteroids4   #bdd4       
,bode's ,law of ,planetary              
,distances4--,there is a very remarkable
 law connecting the distances of the    
planets from the sun1 which is generally
 known by the name of ,bode's ,law4     
,attention was drawn to it in #aggh by  
the astronomer ,bode1 but he was not    
really its author4  ,to express this law
 we write the following series of       
numbers3--      #j1 #c1 #f1 #ab1 #bd1   
#dh1 #if2  each number1 with the        
exception of the first1 being double the
 one which precedes it4 ,if we add #d to
 each of these numbers1 the series      
becomes--      #d1 #g1 #aj1 #af1 #bh1   

#eb1 #ajj2  which series was known to   
,kepler4 ,these numbers1 with the       
exception of #bh1 are sensibly          
proportional to the distances of the    
principal planets from the sun1 the     
actual distances being as follows3--    
,mercury4 ,venus4  ,earth4 ,mars4 ----  
,jupiter4 ,saturn4            #ci       
#gb     #aj     #aeb       #ebi     #ied
  #bde4 ,the ,first ,discovery of the   
,asteroids4--,the great gap between     
,mars and ,jupiter led astronomers1 from
 the time of ,kepler1 to suspect the    
existence of an unknown planet in this  
region2 but no such planet was          
discovered till the beginning of the    
present century4 ,ceres was discovered  
,jan4 #a1 #ahja1 ,pallas in #ahjb1 ,juno
 in #ahjd1 and ,vesta in #ahjg4 ,then   
followed a long interval of thirty-eight
 years before ,astra1 the fifth of these
 minor planets1 was discovered in #ahde4
  #bdf4 ,olbers's ,hypothesis4--,after  
the discovery of ,pallas1 ,olbers       
suggested his celebrated hypothesis1    

that the two bodies might be fragments  
of a single planet which had been       
shattered by some explosion4 ,if such   
were the case1 the orbits of all the    
fragments would at first intersect each 
other at the point where the explosion  
occurred4 ,he therefore thought it      
likely that other fragments would be    
found1 especially if a search were kept 
up near the intersection of the orbits  
of ,ceres and ,pallas4      ,professor  
,newcomb makes the following            
observations concerning this            
hypothesis3--      ,the question whether
 these bodies could ever have formed a  
single     one has now become one of    
cosmogony rather than of astronomy4 ,if 
a     planet were shattered1 the orbit  
of each fragment would at first     pass
 through the point at which the         
explosion occurred1 however     widely  
they might be separated through the rest
 of their course2     but1 owing to the 
secular changes produced by the         
attractions of the     other planets1   

this coincidence would not continue4    
,the orbits would     slowly move away1 
and after the lapse of a few thousand   
years no     trace of a common          
intersection would be seen4 ,it is      
therefore     curious that ,olbers and  
his contemporaries should have expected 
to     find such a region of            
intersection1 as it implied that the    
explosion     had occurred within a few 
thousand years4 ,the fact that the      
required     conditions were not        
fulfilled was no argument against the   
hypothesis1 because the explosion might 
have occurred millions of     years ago2
 and in the mean time the perihelion and
 node of each     orbit would have made 
many entire revolutions1 so that the    
orbits     would have been completely   
mixed up4444 ,a different explanation of
     the group is given by the nebular  
hypothesis2 so that ,olbers's           
hypothesis is no longer considered by   
astronomers4  #bdg4 ,later ,discoveries 
of ,asteroids4--,since #ahde over two   

hundred asteroids have been discovered4 
,all these are so small1 that it        
requires a very good telescope to see   
them2 and even in very powerful         
telescopes they appear as mere points of
 light1 which can be distinguished from 
the stars only by their motions4  ,to   
facilitate the discovery of these       
bodies1 very accurate maps have been    
constructed1 including all the stars    
down to the thirteenth magnitude in the 
neighborhood of the ecliptic4 ,a reduced
 copy of one of these maps is shown in  
,fig4 #bga4  ,illustration3 ,fig4 #bga4 
,furnished with a map of this kind1 and 
with a telescope powerful enough to show
 all the stars marked on it1 the        
observer who is searching for these     
small planets will place in the field of
 view of his telescope six spider-lines 
at right angles to each other1 and at   
equal distances apart1 in such a manner 
that several small squares will be      
formed1 embracing just as much of the   
heavens as do those shown in the map4   

,he will then direct his telescope to   
the region of the sky he wishes to      
examine1 represented by the map1 so as  
to be able to compare successively each 
square with the corresponding portion of
 the sky4 ,fig4 #bgb shows at the right 
hand the squares in the telescopic field
 of view1 and at the left hand the      
corresponding squares of the map4       
,illustration3 ,fig4 #bgb4  ,he can then
 assure himself if the numbers and      
positions of the stars mapped1 and of   
the stars observed1 are identical4 ,if  
he observes in the field of view a      
luminous point which is not marked in   
the map1 it is evident that either the  
new body is a star of variable          
brightness which was not visible at the 
time the map was made1 or it is a       
planet1 or perhaps a comet4 ,if the new 
body remains fixed at the same point1 it
 is the former2 but1 if it changes its  
position with regard to the neighboring 
stars1 it is the latter4 ,the motion is 
generally so sensible1 that in the      

course of one evening the change of     
position may be detected2 and it can    
soon be determined1 by the direction and
 rate of the motion1 whether the body is
 a planet or a comet4                   
,,iv4 ,,outer ,,group ,,of ,,planets4   
,jupiter4   #bdh4 ,orbit of             
,jupiter4--,the orbit of ,jupiter is    
inclined only a little over one degree  
to the ecliptic2 and its eccentricity is
 only about half of that of ,mars1 being
 less than one-twentieth4 ,the mean     
distance of ,jupiter from the sun is    
about four hundred and eighty million   
miles2 but1 owing to the eccentricity of
 his orbit1 his actual distance from the
 sun ranges from four hundred and       
fifty-seven to five hundred and three   
million miles4  #bdi4 ,distance of      
,jupiter from the ,earth4--,when        
,jupiter is in opposition1 his mean     
distance from the earth is four hundred 
and eighty million miles minus          
ninety-two million miles1 or three      
hundred and eighty-eight million miles1 

and1 when he is in conjunction1 four    
hundred and eighty million miles plus   
ninety-two million miles1 or five       
hundred and seventy-two million miles4  
,it will be seen that he is less than   
twice as far off in conjunction as in   
opposition1 and that the ratio of his   
greatest to his least distance is very  
much less than in the case of ,venus and
 ,mars4 ,this is owing to his very much 
greater distance from the sun4 ,owing to
 the eccentricities of the orbits of the
 earth and of ,jupiter1 the greatest and
 least distances of ,jupiter from the   
earth vary somewhat from year to year4  
,illustration3 ,fig4 #bgc4  #bej4 ,the  
,brightness and ,apparent ,size of      
,jupiter4--,the apparent diameter of    
,jupiter varies from about fifty seconds
 to about thirty seconds4 ,his apparent 
size at his extreme and mean distances  
from the earth is shown in ,fig4 #bgc4  
,jupiter shines with a brilliant white  
light1 which exceeds that of every other
 planet except ,venus4 ,the planet is1  

of course1 brightest when near          
opposition4  #bea4 ,the ,volume and     
,density of ,jupiter4--,jupiter is the  
giant planet of our system1 his mass    
largely exceeding that of all the other 
planets combined4 ,his mean diameter is 
about eighty-five thousand miles2 but   
the equatorial exceeds the polar        
diameter by five thousand miles4 ,in    
volume he exceeds our earth about       
thirteen hundred times1 but in mass only
 about two hundred and thirteen times4  
,his specific gravity is1 therefore1 far
 less than that of the earth1 and even  
less than that of water4 ,the           
comparative size of ,jupiter and the    
earth is shown in ,fig4 #bgd4           
,illustration3 ,fig4 #bgd4  #beb4 ,the  
,sidereal and ,synodical ,periods of    
,jupiter4--,it takes ,jupiter nearly    
twelve years to make a sidereal         
revolution1 or a complete revolution    
around the sun1 his orbital motion being
 at the rate of about eight miles a     
second4 ,his synodical period1 or the   

time of his passage from opposition to  
opposition again1 is three hundred and  
ninety-eight days4  #bec4 ,the          
,telescopic ,aspect of ,jupiter4--,there
 are no really permanent markings on the
 disk of ,jupiter2 but his surface      
presents a very diversified appearance4 
,the earlier telescopic observers       
descried dark belts across it1 one north
 of the equator1 and the other south of 
it4 ,with the increase of telescopic    
power1 it was seen that these bands were
 of a more complex structure than had   
been supposed1 and consisted of         
stratified1 cloud-like appearances1     
varying greatly in form and number4     
,these change so rapidly1 that the face 
of the planet rarely presents the same  
appearance on two successive nights4    
,they are most strongly marked at some  
distance on each side of the planet's   
equator1 and thus appear as two belts   
under a low magnifying power4  ,both the
 outlines of the belts1 and the color of
 portions of the planet1 are subject to 

considerable changes4 ,the equatorial   
regions1 and the spaces between the     
belts generally1 are often of a rosy    
tinge4 ,this color is sometimes strongly
 marked1 while at other times hardly a  
trace of it can be seen4 ,a general     
telescopic view of ,jupiter is given in 
,plate ,v4  ,illustration3 ,plate #e4   
#bed4 ,the ,physical ,constitution of   
,jupiter4--,from the changeability of   
the belts1 and of nearly all the visible
 features of ,jupiter1 it is clear that 
what we see on that planet is not the   
solid nucleus1 but cloud-like           
formations1 which cover the entire      
surface to a great depth4 ,the planet   
appears to be covered with a deep and   
dense atmosphere1 filled with thick     
masses of clouds and vapor4 ,until      
recently this cloud-laden atmosphere was
 supposed to be somewhat like that of   
our globe2 but at present the physical  
constitution of ,jupiter is believed to 
resemble that of the sun rather than    
that of the earth4 ,like the sun1 he is 

brighter in the centre than near the    
edges1 as is shown in the transits of   
the satellites over his disk4 ,when the 
satellite first enters on the disk1 it  
commonly seems like a bright spot on a  
dark background2 but1 as it approaches  
the centre1 it appears like a dark spot 
on the bright surface of the planet4    
,the centre is probably two or three    
times brighter than the edges4 ,this may
 be1 as in the case of the sun1 because 
the light near the edge passes through a
 greater depth of atmosphere1 and is    
diminished by absorption4  ,it has also 
been suspected that ,jupiter shines     
partly by his own light1 and not wholly 
by reflected sunlight4 ,the planet      
cannot1 however1 emit any great amount  
of light2 for1 if it did1 the satellites
 would shine by this light when they are
 in the shadow of the planet1 whereas   
they totally disappear4 ,it is possible 
that the brighter portions of the       
surface are from time to time slightly  
self-luminous4  ,illustration3 ,fig4    

#bge4  ,again3 the interior of ,jupiter 
seems to be the seat of an activity so  
enormous that it can be ascribed only to
 intense heat4 ,rapid movements are     
always occurring on his surface1 often  
changing its aspect in a few hours4 ,it 
is therefore probable that ,jupiter is  
not yet covered by a solid crust1 and   
that the fiery interior1 whether liquid 
or gaseous1 is surrounded by the dense  
vapors which cease to be luminous on    
rising into the higher and cooler       
regions of the atmosphere4 ,figs4 #bge  
and #bgf show the disk of ,jupiter as it
 appeared in ,december1 #ahha4          
,illustration3 ,fig4 #bgf4  #bee4       
,rotation of ,jupiter4--,spots are      
sometimes visible which are much more   
permanent than the ordinary markings on 
the belts4 ,the most remarkable of these
 is the great red spot1 which was first 
observed in ,july1 #ahgh1 and is still  
to be seen in ,february1 #ahhb4 ,it is  
shown just above the centre of the disk 
in ,fig4 #bge4 ,by watching these spots 

from day to day1 the time of ,jupiter's 
axial rotation has been found to be     
about nine hours and fifty minutes4     
,the axis of ,jupiter deviates but      
slightly from a perpendicular to the    
plane of its orbit1 as is shown in ,fig4
 #bgg4  ,illustration3 ,fig4 #bgg4      
,,the ,,satellites ,,of ,,jupiter4      
,illustration3 ,fig4 #bgh4  #bef4       
,jupiter's ,four ,moons4--,jupiter is   
accompanied by four moons1 as shown in  
,fig4 #bgh4 ,the diameters of these     
moons range from about twenty-two       
hundred to thirty-seven hundred miles4  
,the second from the planet is the      
smallest1 and the third the largest4    
,the smallest is about the size of our  
moon2 the largest considerably exceeds  
,mercury1 and almost rivals ,mars1 in   
bulk4 ,the sizes of these moons1        
compared with those of the earth and its
 moon1 are shown in ,fig4 #bgi4         
,illustration3 ,fig4 #bgi4  ,the names  
of these satellites1 in the order of    
their distance from the planet1 are ,io1

 ,europa1 ,ganymede1 and ,callisto4     
,their times of revolution range from   
about a day and three-fourths up to     
about sixteen days and a half4 ,their   
orbits are shown in ,fig4 #bhj4         
,illustration3 ,fig4 #bhj4  #beg4 ,the  
,variability of ,jupiter's              
,satellites4--,remarkable variations in 
the light of these moons have led to the
 supposition that violent changes are   
taking place on their surfaces4 ,it was 
formerly believed1 that1 like our moon1 
they always present the same face to the
 planet1 and that the changes in their  
brilliancy are due to differences in the
 luminosity of parts of their surface   
which are successively turned towards us
 during a revolution2 but careful       
measurements of their light show that   
this hypothesis does not account for the
 changes1 which are sometimes very      
sudden4 ,the satellites are too distant 
for examination of their surfaces with  
the telescope3 hence it is impossible to
 give any certain explanation of these  

phenomena4  ,illustration3 ,fig4 #bha4  
#beh4 ,eclipses of ,jupiter's           
,satellites4--,jupiter1 like the earth1 
casts a shadow away from the sun1 as    
shown in ,fig4 #bha2 and1 whenever one  
of his moons passes into this shadow1 it
 becomes eclipsed4 ,on the other hand1  
whenever one of the moons throws its    
shadow on ,jupiter1 the sun is eclipsed 
to that part of the planet which lies   
within the shadow4  ,to the inhabitants 
of ,jupiter 7if there are any1 and if   
they can see through the clouds7 these  
eclipses must be very familiar affairs2 
for in consequence of the small         
inclinations of the orbits of the       
satellites to the planet's equator1 and 
the small inclination of the latter to  
the plane of ,jupiter's orbit1 all the  
satellites1 except the most distant one1
 are eclipsed in every revolution4 ,a   
spectator on ,jupiter might therefore   
witness during the planetary year       
forty-five hundred eclipses of the      
moons1 and about the same number of the 

sun4  ,illustration3 ,fig4 #bhb4  #bei4 
,transits of ,jupiter's                 
,satellites4--,whenever one of          
,jupiter's moons passes in front of the 
planet1 it is said to make a transit    
across his disk4 ,when a moon is making 
a transit1 it presents its bright       
hemisphere towards the earth1 as will be
 seen from ,fig4 #bhb3 hence it is      
usually seen as a bright spot on the    
planet's disk2 though sometimes1 on the 
brighter central portions of the disk1  
it appears dark4  ,illustration3 ,fig4  
#bhc4  ,it will be seen from ,fig4 #bhb 
that the shadow of a moon does not fall 
upon the part of the planet's disk that 
is covered by the moon3 hence we may    
observe the transit of both the moon and
 its shadow4 ,the shadow appears as a   
small black spot1 which will precede or 
follow the moon according to the        
position of the earth in its orbit4     
,fig4 #bhc shows two moons of ,jupiter  
in transit4  #bfj4 ,occultations of     
,jupiter's ,satellites4--,the eclipse of

 a moon of ,jupiter must be carefully   
distinguished from the occultation of a 
moon by the planet4 ,in the case of an  
eclipse1 the moon ceases to be visible1 
because the mass of ,jupiter is         
interposed between the sun and the moon1
 which ceases to be luminous1 because   
the sun's light is cut off2 but1 in the 
case of an occupation1 the moon gets    
into such a position that the body of   
,jupiter is interposed between it and   
the earth1 thus rendering the moon      
invisible to us4 ,the third satellite1  
m'' 7,fig4 #bhb71 is invisible from the 
earth ,e1 having become occulted when it
 passed behind the planet's disk2 but it
 will not be eclipsed until it passes   
into the shadow of ,jupiter4  #bfa4     
,jupiter without ,satellites4--,it      
occasionally happens that every one of  
,jupiter's satellites will disappear at 
the same time1 either by being eclipsed 
or occulted1 or by being in transit4 ,in
 this event1 ,jupiter will appear       
without satellites4 ,this occurred on   

the #bast of ,august1 #ahfg4 ,the       
position of ,jupiter's satellites at    
this time is shown in ,fig4 #bhd4       
,illustration3 ,fig4 #bhd4              
,saturn4                                
,,the ,,planet ,,and ,,his ,,moons4     
#bfb4 ,the ,orbit of ,saturn4--,the     
orbit of ,saturn is rather more         
eccentric than that of ,jupiter1 its    
eccentricity being somewhat more than   
one-twentieth4 ,its inclination to the  
ecliptic is about two degrees and a     
half4 ,the mean distance of ,saturn from
 the sun is about eight hundred and     
eighty million miles4 ,it is about a    
hundred million miles nearer the sun at 
perihelion than at aphelion4  #bfc4     
,distance of ,saturn from the           
,earth4--,the mean distance of ,saturn  
from the earth at opposition is eight   
hundred and eighty million miles minus  
ninety-two million miles1 or seven      
hundred and eighty-eight million2 and at
 conjunction1 eight hundred and eighty  
million miles plus ninety-two million1  

or nine hundred and seventy-two million4
 ,owing to the eccentricity of the orbit
 of ,saturn1 his distance from the earth
 at opposition and at conjunction varies
 by about a hundred million miles at    
different times2 but he is so immensely 
far away1 that this is only a small     
fraction of his mean distance4  #bfd4   
,apparent ,size and ,brightness of      
,saturn4--,the apparent diameter of     
,saturn varies from about twenty seconds
 to about fourteen seconds4 ,his        
apparent size at his extreme and mean   
distances from the earth is shown in    
,fig4 #bhe4  ,illustration3 ,fig4 #bhe4 
,the planet generally shines with the   
brilliancy of a moderate first-magnitude
 star1 and with a dingy1 reddish light1 
as if seen through a smoky atmosphere4  
#bfe4 ,volume and ,density of           
,saturn4--,the real diameter of ,saturn 
is about seventy thousand miles1 and its
 volume over seven hundred times that of
 the earth4 ,the comparative size of the
 earth and ,saturn is shown in ,fig4    

#bhf4 ,this planet is a little more than
 half as dense as ,jupiter4             
,illustration3 ,fig4 #bhf4  #bff4 ,the  
,sidereal and ,synodical ,periods of    
,saturn4--,saturn makes a complete      
revolution round the sun in a period of 
about twenty-nine years and a half1     
moving in his orbit at the rate of about
 six miles a second4 ,the planet passes 
from opposition to opposition again in a
 period of three hundred and            
seventy-eight days1 or thirteen days    
over a year4  #bfg4 ,physical           
,constitution of ,saturn4--,the physical
 constitution of ,saturn seems to       
resemble that of ,jupiter2 but1 being   
twice as far away1 the planet cannot be 
so well studied4 ,the farther an object 
is from the sun1 the less it is         
illuminated2 and1 the farther it is from
 the earth1 the smaller it appears3     
hence there is a double difficulty in   
examining the more distant planets4     
,under favorable circumstances1 the     
surface of ,saturn is seen to be        

diversified with very faint markings2   
and1 with high telescopic powers1 two or
 more very faint streaks1 or belts1 may 
be discerned parallel to its equator4   
,these belts1 like those of ,jupiter1   
change their aspect from time to time2  
but they are so faint that the changes  
cannot be easily followed4 ,it is only  
on rare occasions that the time of      
rotation can be determined from a study 
of the markings4  #bfh4 ,rotation of    
,saturn4--,on the evening of ,dec4 #g1  
#ahgf1 ,professor ,hall1 who had been   
observing the satellites of ,saturn with
 the great ,washington telescope 7#ah71 
saw a brilliant white spot near the     
equator of the planet4 ,it seemed as if 
an immense eruption of incandescent     
matter had suddenly burst up from the   
interior4 ,the spot gradually spread    
itself out into a long light streak1 of 
which the brightest point was near the  
western end4 ,it remained visible until 
,january1 when it became faint and      
ill-defined1 and the planet was lost in 

the rays of the sun4  ,from all the     
observations on this spot1 ,professor   
,hall found the period of ,saturn to be 
ten hours fourteen minutes1 reckoning by
 the brightest part of the streak4 ,had 
the middle of the streak been taken1 the
 time would have been less1 because the 
bright matter seemed to be carried along
 in the direction of the planet's       
rotation4 ,if this motion was due to a  
wind1 the velocity of the current must  
have been between fifty and a hundred   
miles an hour4 ,the axis of ,saturn is  
inclined twenty-seven degrees from the  
perpendicular to its orbit4             
,illustration3 ,fig4 #bhg4  #bfi4 ,the  
,satellites of ,saturn4--,saturn is     
accompanied by eight moons4 ,seven of   
these are shown in ,fig4 #bhg4 ,the     
names of these satellites1 in the order 
of their distances from the planet1 are 
given in the accompanying table3--      
,number4 ,name4      ,distance          
,sidereal              ,discoverer4     
from    ,period4                        

,planet            #a ,mimas            
#abj1hjj    #j #bb #cg         #j4id    
,herschel            #b ,enceladus      
#aee1jjj    #a  #h #ec         #a4cg    
,herschel            #c ,tethys         
#aia1ijj    #a #ba #ah         #a4hh    
,cassini            #d ,dione           
#bde1hjj    #b #ag #da         #b4gc    
,cassini            #e ,rhea            
#cdc1djj    #d #ab #be         #d4ea    
,cassini            #f ,titan           
#gif1ajj   #ae #bb #da        #ae4id    
,huyghens            #g ,hyperion       
#ifc1cjj   #ba  #g  #g        #ba4bi    
,bond            #h ,japetus            
#b1cac1hjj   #gi  #g #ec        #gi4cc  
,cassini  ,the apparent brightness or   
visibility of these satellites follows  
the order of their discovery4 ,the      
smallest telescope will show ,titan1 and
 one of very moderate size will show    
,japetus in the western part of its     
orbit4 ,an instrument of four or five   
inches aperture will show ,rhea1 and    
perhaps ,tethys and ,dione2 while seven 

or eight inches are required for        
,enceladus1 even at its greatest        
elongation from the planet4 ,mimas can  
rarely be seen except at its greatest   
elongation1 and then only with an       
aperture of twelve inches or more4      
,hyperion can be detected only with the 
most powerful telescopes1 on account of 
its faintness and the difficulty of     
distinguishing it from minute stars4    
,japetus1 the outermost satellite1 is   
remarkable for the fact1 that while1 in 
one part of its orbit1 it is the        
brightest of the satellites except      
,titan1 in the opposite part it is      
almost as faint as ,hyperion1 and can be
 seen only in large telescopes4 ,when   
west of the planet1 it is bright2 when  
east of it1 faint4 ,this peculiarity has
 been accounted for by supposing that   
the satellite1 like our moon1 always    
presents the same face to the planet1   
and that one side of it is white and the
 other intensely black2 but it is       
doubtful whether any known substance is 

so black as one side of the satellite   
must be to account for such             
extraordinary changes of brilliancy4    
,illustration3 ,fig4 #bhh4  ,titan1 the 
largest of these satellites1 is about   
the size of the largest satellite of    
,jupiter4 ,the relative sizes of the    
satellites are shown in ,fig4 #bhh1 and 
their orbits in ,fig4 #bhi4             
,illustration3 ,fig4 #bhi4              
,illustration3 ,fig4 #bij4  ,fig4 #bij  
shows the transit of one of the         
satellites1 and of its shadow1 across   
the disk of the planet4                 
,,the ,,rings ,,of ,,saturn4   #bgj4    
,general ,appearance of the             
,rings4--,saturn is surrounded by a thin
 flat ring lying in the plane of its    
equator4 ,this ring is probably less    
than a hundred miles thick4 ,the part of
 it nearest ,saturn reflects little     
sunlight to us2 so that it has a dusky  
appearance1 and is not easily seen1     
although it is not quite so dark as the 
sky seen between it and the planet4 ,the

 outer edge of this dusky portion of the
 ring is at a distance from ,saturn of  
between two and three times the earth's 
diameter4 ,outside of this dusky part of
 the ring is a much brighter portion1   
and outside of this another1 which is   
somewhat fainter1 but still so much     
brighter than the dusky part as to be   
easily seen4 ,the width of the brighter 
parts of the ring is over three times   
the earth's diameter4 ,to distinguish   
the parts1 the outer one is called ring 
,a1 the middle one ring ,b1 and the     
dusky one ring ,c4 ,between ,a and ,b is
 an apparently open space1 nearly two   
thousand miles wide1 which looks like a 
black line on the ring4 ,other divisions
 in the ring have been noticed at times2
 but this is the only one always seen   
with good telescopes at times when      
either side of the ring is in view from 
the earth4 ,the general telescopic      
appearance of the ring is shown in ,fig4
 #bia4  ,illustration3 ,fig4 #bia4      
,illustration3 ,fig4 #bib4  ,fig4 #bib  

shows the divisions of the rings as they
 were seen by ,bond4  #bga4 ,phases of  
,saturn's ,ring4--,the ring is inclined 
to the plane of the planet's orbit by an
 angle of twenty-seven degrees4 ,the    
general aspect from the earth is nearly 
the same as from the sun4 ,as the planet
 revolves around the sun1 the axis and  
plane of the ring keep the same         
direction in space1 just as the axis of 
the earth and the plane of the equator  
do4  ,when the planet is in one part of 
its orbit1 we see the upper or northern 
side of the ring at an inclination of   
twenty-seven degrees1 the greatest angle
 at which the ring can ever be seen4    
,this phase of the ring is shown in     
,fig4 #bic4  ,illustration3 ,fig4 #bic4 
,when the planet has moved through a    
quarter of a revolution1 the edge of the
 ring is turned towards the sun and the 
earth2 and1 owing to its extreme        
thinness1 it is visible only in the most
 powerful telescopes as a fine line of  
light1 stretching out on each side of   

the planet4 ,this phase of the ring is  
shown in ,fig4 #bid4  ,illustration3    
,fig4 #bid4  ,all the satellites1 except
 ,japetus1 revolve very nearly in the   
plane of the ring3 consequently1 when   
the edge of the ring is turned towards  
the earth1 the satellites seem to swing 
from one side of the planet to the other
 in a straight line1 running along the  
thin edge of the ring like beads on a   
string4 ,this phase affords the best    
opportunity of seeing the inner         
satellites1 ,mimas and ,enceladus1 which
 at other times are obscured by the     
brilliancy of the ring4  ,illustration3 
,fig4 #bie4  ,fig4 #bie shows a phase of
 the ring intermediate between the last 
two4  ,when the planet has moved ninety 
degrees farther1 we again see the ring  
at an angle of twenty-seven degrees2 but
 now it is the lower or southern side   
which is visible4 ,when it has moved    
ninety degrees farther1 the edge of the 
ring is again turned towards the earth  
and sun4  ,illustration3 ,fig4 #bie4    

,the successive phases of ,saturn's ring
 during a complete revolution are shown 
in ,fig4 #bif4  ,it will be seen that   
there are two opposite points of        
,saturn's orbit in which the rings are  
turned edgewise to us1 and two points   
half-way between the former in which the
 ring is seen at its maximum inclination
 of about twenty-seven degrees4 ,since  
the planet performs a revolution in     
twenty-nine years and a half1 these     
phases occur at average intervals of    
about seven years and four months4      
,illustration3 ,fig4 #big4              
,illustration3 ,fig4 #bih4  #bgb4       
,disappearance of ,saturn's ,ring4--,it 
will be seen from ,fig4 #big that the   
plane of the ring may not be turned     
towards the sun and the earth at exactly
 the same time1 and also that the earth 
may sometimes come on one side of the   
plane of the ring while the sun is      
shining on the other4 ,in the figure1   
,e1 ,e'1 ,e''1 and ,e''' is the orbit of
 the earth4 ,when ,saturn is at ,s'1 or 

opposite1 at ,f1 the plane of the ring  
will pass through the sun1 and then only
 the edge of the ring will be illumined4
 ,were ,saturn at ,s1 and the earth at  
,e'1 the plane of the ring would pass   
through the earth4 ,this would also be  
the case were the earth at ,e'''1 and   
,saturn at ,s''4 ,were ,saturn at ,s or 
at ,s''1 and the earth farther to the   
left or to the right1 the sun would be  
shining on one side of the ring while we
 should be looking on the other4 ,in all
 these cases the ring will disappear    
entirely in a telescope of ordinary     
power4 ,with very powerful telescopes   
the ring will appear1 in the first two  
cases1 as a thin line of light 7,fig4   
#bih74 ,it will be seen that all these  
cases of disappearance must take place  
when ,saturn is in the parts of his     
orbit intercepted between the parallel  
lines ,,ac and ,,bd4 ,these lines are   
tangent to the earth's orbit1 which they
 enclose1 and are parallel to the plane 
of ,saturn's ring4 ,as ,saturn passes   

away from these two lines on either     
side1 the rings appear more and more    
open4 ,when the dark side of the ring is
 in view1 it appears as a black line    
crossing the planet2 and on such        
occasions the sunlight reflected from   
the outer and inner edges of the rings  
,a and ,b enables us to see traces of   
the ring on each side of ,saturn1 at    
least in places where two such          
reflections come nearly together4 ,fig4 
#bii illustrates this reflection from   
the edges at the divisions of the rings4
  ,illustration3 ,fig4 #bii4  #bgc4     
,changes in ,saturn's ,ring4--,the      
question whether changes are going on in
 the rings of ,saturn is still          
unsettled4 ,some observers have believed
 that they saw additional divisions in  
the rings from time to time2 but these  
may have been errors of vision1 due     
partly to the shading which is known to 
exist on portions of the ring4          
,professor ,newcomb says1 ,as seen with 
the great ,washington equatorial in the 

autumn of #ahgd1 there was no great or  
sudden contrast between the inner or    
dark edge of the bright ring and the    
outer edge of the dusky ring4 ,there was
 some suspicion that the one shaded into
 the other by insensible gradations4 ,no
 one could for a moment suppose1 as some
 observers have1 that there was a       
separation between these two rings4 ,all
 these considerations give rise to the  
question whether the dusky ring may not 
be growing at the expense of the inner  
bright ring4  ,struve1 in #ahea1        
advanced the startling theory that the  
inner edge of the ring was gradually    
approaching the planet1 the whole ring  
spreading inwards1 and making the       
central opening smaller4 ,the theory was
 based upon the descriptions and        
drawings of the rings by the astronomers
 of the seventeenth century1 especially 
,huyghens1 and the measures made by     
later astronomers up to #ahea4 ,this    
supposed change in the dimension of the 
ring is shown in ,fig4 #cjj4            

,illustration3 ,fig4 #cjj4  #bgd4       
,constitution of ,saturn's ,ring4--,the 
theory now generally held by astronomers
 is1 that the ring is composed of a     
cloud of satellites too small to be     
separately seen in the telescope1 and   
too close together to admit of visible  
intervals between them4 ,the ring looks 
solid1 because its parts are too small  
and too numerous to be seen singly4     
,they are like the minute drops of water
 that make up clouds and fogs1 which to 
our eyes seem like solid masses4 ,in the
 dusky ring the particles may be so     
scattered that we can see through the   
cloud1 the duskiness being due to the   
blending of light and darkness4 ,some   
believe1 however1 that the duskiness is 
caused by the darker color of the       
particles rather than by their being    
farther apart4                          
,uranus4   #bge4 ,orbit and ,dimensions 
of ,uranus4--,uranus1 the smallest of   
the outer group of planets1 has a       
diameter of nearly thirty-two thousand  

miles4 ,it is a little less dense than  
,jupiter1 and its mean distance from the
 sun is about seventeen hundred and     
seventy millions of miles4 ,its orbit   
has about the same eccentricity as that 
of ,jupiter1 and is inclined less than a
 degree to the ecliptic4 ,uranus makes a
 revolution around the sun in           
eighty-four years1 moving at the rate of
 a little over four miles a second4 ,it 
is visible to the naked eye as a star of
 the sixth magnitude4  ,as seen in a    
large telescope1 the planet has a       
decidedly sea-green color2 but no       
markings have with certainty been       
detected on its disk1 so that nothing is
 really known with regard to its        
rotation4 ,fig4 #cja shows the          
comparative size of ,uranus and the     
earth4  ,illustration3 ,fig4 #cja4      
#bgf4 ,discovery of ,uranus4--,this     
planet was discovered by ,sir ,william  
,herschel in ,march1 #agha4 ,he was     
engaged at the time in examining the    
small stars of the constellation        

,gemini1 or the ,twins4 ,he noticed that
 this object which had attracted his    
attention had an appreciable disk1 and  
therefore could not be a star4 ,he also 
perceived by its motion that it could   
not be a nebula2 he therefore concluded 
that it was a comet1 and announced his  
discovery as such4 ,on attempting to    
compute its orbit1 it was soon found    
that its motions could be accounted for 
only on the supposition that it was     
moving in a circular orbit at about     
twice the distance of ,saturn from the  
sun4 ,it was therefore recognized as a  
new planet1 whose discovery nearly      
doubled the dimensions of the solar     
system as it was then known4  #bgg4 ,the
 ,name of the ,planet4--,herschel1 out  
of compliment to his patron1 ,george    
,,iii41 proposed to call the new planet 
,georgium ,sidus 7the ,georgian ,star72 
but this name found little favor4 ,the  
name of ,herschel was proposed1 and     
continued in use in ,england for a time1
 but did not meet with general approval4

 ,various other names were suggested1   
and finally that of ,uranus was adopted4
  ,illustration3 ,fig4 #cjb4  #bgh4 ,the
 ,satellites of ,uranus4--,uranus is    
accompanied by four satellites1 whose   
orbits are shown in ,fig4 #cjb4 ,these  
satellites are remarkable for the great 
inclination of their orbits to the plane
 of the planet's orbit1 amounting to    
about eighty degrees1 and for their     
retrograde motion2 that is1 they move   
from east to west1 instead of from west 
to east1 as in the case of all the      
planets and of all the satellites       
previously discovered4                  
,neptune4   #bgi4 ,orbit and ,dimensions
 of ,neptune4--,so far as known1        
,neptune is the most remote member of   
the solar system1 its mean distance from
 the sun being twenty-seven hundred and 
seventy-five million miles4 ,this       
distance is considerably less than twice
 that of ,uranus4 ,neptune revolves     
around the sun in a period of a little  
less than a hundred and sixty-five      

years4 ,its orbit has but slight        
eccentricity1 and is inclined less than 
two degrees to the ecliptic4 ,this      
planet is considerably larger than      
,uranus1 its diameter being nearly      
thirty-five thousand miles4 ,it is      
somewhat less dense than ,uranus4       
,neptune is invisible to the naked eye1 
and no telescope has revealed any       
markings on its disk3 hence nothing is  
certainly known as to its rotation4     
,fig4 #cjc shows the comparative size of
 ,neptune and the earth4  ,illustration3
 ,fig4 #cjc4  #bhj4 ,the ,discovery of  
,neptune4--,the discovery of ,neptune   
was made in #ahdf1 and is justly        
regarded as one of the grandest triumphs
 of astronomy4  ,soon after ,uranus was 
discovered1 certain irregularities in   
its motion were observed1 which could   
not be explained4 ,it is well known that
 the planets are all the while          
disturbing each other's motions1 so that
 none of them describe perfect ellipses4
 ,these mutual disturbances are called  

perturbations4 ,in the case of ,uranus  
it was found1 that1 after making due    
allowance for the action of all the     
known planets1 there were still certain 
perturbations in its course which had   
not been accounted for4 ,this led       
astronomers to the suspicion that these 
might be caused by an unknown planet4   
,leverrier in ,france1 and ,adams in    
,england1 independently of each other1  
set themselves the difficult problem of 
computing the position and magnitude of 
a planet which would produce these      
perturbations4 ,both1 by a most         
laborious computation1 showed that the  
perturbations were such as would be     
produced by a planet revolving about the
 sun at about twice the distance of     
,uranus1 and having a mass somewhat     
greater than that of this planet2 and   
both pointed out the same part of the   
heavens as that in which the planet     
ought to be found at that time4 ,almost 
immediately after they had announced the
 conclusion to which they had arrived1  

the planet was found with the telescope4
 ,the astronomer who was searching for  
the planet at the suggestion of         
,leverrier was the first to recognize   
it3 hence ,leverrier has obtained the   
chief credit of the discovery4  ,the    
observed planet is proved to be nearer  
than the one predicted by ,leverrier and
 ,adams1 and therefore of smaller       
magnitude4  #bha4 ,the ,observed ,planet
 not the ,predicted ,one4--,professor   
,peirce always maintained that the      
planet found by observation was not the 
one whose existence had been predicted  
by ,leverrier and ,adams1 though its    
action would completely explain all the 
irregularities in the motion of ,uranus4
 ,his last statement on this point is as
 follows3 ,my position is1 that there   
were two possible planets1 either of    
which might have caused the observed    
irregular motions of ,uranus4 ,each     
planet excluded the other2 so that1 if  
one was1 the other was not4 ,they       
coincided in direction from the earth at

 certain epochs1 once in six hundred and
 fifty years4 ,it was at one of these   
epochs that the prediction was made1 and
 at no other time for six centuries     
could the prediction of the one planet  
have revealed the other4 ,the observed  
planet was not the predicted one4  #bhb4
 ,bode's ,law ,disproved4--,the         
following table gives the distances of  
the planets according to ,bode's law1   
their actual distances1 and the error of
 the law in each case3--                
,planet4      ,numbers of     ,actual   
,errors4                                
,bode4 ,distances4                      
,mercury     #j  #d    #d        #c4i   
#j4a                 ,venus       #c  #d
    #g        #g4b     #j4b             
,earth       #f  #d   #aj       #aj4j   
#j4j                 ,mars       #ab  #d
   #af       #ae4b     #j4h             
,minor      #bd  #d   #bh   #bj to #ce  
planets                 ,jupiter    #dh 
#d   #eb       #eb4j     #j4j           
,saturn     #if  #d  #ajj       #ie4d   

#d4f                 ,uranus    #aib  #d
  #aif      #aia4i     #d4a             
,neptune   #chd  #d  #chh      #cjj4f   
#hg4d  ,it will be seen1 that1 before   
the discovery of ,neptune1 the agreement
 was so close as to indicate that this  
was an actual law of the distances2 but 
the discovery of this planet completely 
disproved its existence4  ,illustration3
 ,fig4 #cjd4  #bhc4 ,the ,satellite of  
,neptune4--,neptune is accompanied by at
 least one moon1 whose orbit is shown in
 ,fig4 #cjd4 ,the orbit of this         
satellite is inclined about thirty      
degrees to the plane of the ecliptic1   
and the motion of the satellite is      
retrograde1 or from east to west4       
,,vii4 ,,comets ,,and ,,meteors4        
,i4 ,,comets4                           
,general ,phenomena of ,comets4  #bhd4  
,general ,appearance of a ,bright       
,comet4--,comets bright enough to be    
seen with the naked eye are composed of 
three parts1 which run into each other  
by insensible gradations4 ,these are the

 nucleus1 the coma1 and the tail4  ,the 
nucleus is the bright centre of the     
comet1 and appears to the eye as a star 
or planet4  ,the coma is a nebulous mass
 surrounding the nucleus on all sides4  
,close to the nucleus it is almost as   
bright as the nucleus itself2 but it    
gradually shades off in every direction4
 ,the nucleus and coma combined appear  
like a star shining through a small     
patch of fog2 and these two together    
form what is called the head of the     
comet4  ,the tail is a continuation of  
the coma1 and consists of a stream of   
milky light1 growing wider and fainter  
as it recedes from the head1 till the   
eye is unable to trace it4              
,illustration3 ,fig4 #cje4  ,the general
 appearance of one of the smaller of the
 brilliant comets is shown in ,fig4     
#cje4  ,illustration3 ,fig4 #cjf4       
,illustration3 ,fig4 #cjg4  #bhe4       
,general ,appearance of a ,telescopic   
,comet4--,the great majority of comets  
are too faint to be visible with the    

naked eye1 and are called telescopic    
comets4 ,in these comets there seems to 
be a development of coma at the expense 
of nucleus and tail4 ,in some cases the 
telescope fails to reveal any nucleus at
 all in one of these comets2 at other   
times the nucleus is so faint and       
ill-defined as to be barely             
distinguishable4 ,fig4 #cjf shows a     
telescopic comet without any nucleus at 
all1 and another with a slight          
condensation at the centre4 ,in these   
comets it is generally impossible to    
distinguish the coma from the tail1 the 
latter being either entirely invisible1 
as in ,fig4 #cjf1 or else only an       
elongation of the coma1 as shown in     
,fig4 #cjg4 ,many comets appear simply  
as patches of foggy light of more or    
less irregular form4  ,illustration3    
,fig4 #cjh4  #bhf4 ,the ,development of 
,telescopic ,comets on their ,approach  
to the ,sun4--,as a rule1 all comets    
look nearly alike when they first come  
within the reach of the telescope4 ,they

 appear at first as little foggy        
patches1 without any tail1 and often    
without any visible nucleus4 ,as they   
approach the sun their peculiarities are
 rapidly developed4 ,fig4 #cjh shows    
such a comet as first seen1 and the     
gradual development of its nucleus1     
head1 and tail1 as it approaches the    
sun4  ,illustration3 ,fig4 #cji4        
,illustration3 ,fig4 #caj4              
,illustration3 ,fig4 #caa4  ,if the     
comet is only a small one1 the tail     
developed is small2 but these small     
appendages have a great variety of form 
in different comets4 ,fig4 #cji shows   
the singular form into which ,encke's   
comet was developed in #ahga4 ,figs4    
#caj and #caa show other peculiar       
developments of telescopic comets4      
#bhg4 ,development of ,brilliant ,comets
 on their ,approach to the              
,sun4--,brilliant comets1 as well as    
telescopic comets1 appear nearly alike  
when they come into the view of the     
telescope2 and it is only on their      

approach to the sun that their          
distinctive features are developed4 ,not
 only do these comets1 when they first  
come into view1 resemble each other1 but
 they also bear a close resemblance to  
telescopic comets4  ,as the comet       
approaches the sun1 bright vaporous     
jets1 two or three in number1 are       
emitted from the nucleus on the side of 
the sun and in the direction of the sun4
 ,these jets1 though directed towards   
the sun1 are soon more or less carried  
backward1 as if repelled by the sun4    
,fig4 #cab shows a succession of views  
of these jets as they were developed in 
the case of ,halley's comet in #ahce4   
,illustration3 ,fig4 #cab4  ,the jets in
 this case seemed to have an oscillatory
 motion4 ,at #a and #b they seemed to be
 attracted towards the sun1 and in #c to
 be repelled by him4 ,in #d and #e they 
seemed to be again attracted1 and in #f 
to be repelled1 but in a reverse        
direction to that in #c4 ,in #g they    
appeared to be again attracted4 ,bessel 

likened this oscillation of the jets to 
the vibration of a magnetic needle when 
presented to the pole of a magnet4  ,in 
the case of larger comets these luminous
 jets are surrounded by one or more     
envelops1 which are thrown off in       
succession as the comet approaches the  
sun4 ,the formation of these envelops   
was a conspicuous feature of ,donati's  
comet of #aheh4 ,a rough view of the    
jets and the surrounding envelops is    
given in ,fig4 #cac4 ,fig4 #cad gives a 
view of the envelops without the jets4  
,illustration3 ,fig4 #cac4              
,illustration3 ,fig4 #cad4  #bhh4 ,the  
,tails of ,comets4--,the tails of       
brilliant comets are rapidly formed as  
the comet approaches the sun1 their     
increase in length often being at the   
rate of several million miles a day4    
,these appendages seem to be formed     
entirely out of the matter which is     
emitted from the nucleus in the luminous
 jets which are at first directed       
towards the sun4 ,the tails of comets   

are1 however1 always directed away from 
the sun1 as shown in ,fig4 #cae4        
,illustration3 ,fig4 #cae4  ,it will be 
seen that the comet1 as it approaches   
the sun1 travels head foremost2 but as  
it leaves the sun it goes tail foremost4
  ,the apparent length of the tail of a 
comet depends partly upon its real      
length1 partly upon the distance of the 
comet1 and partly upon the direction of 
the axis of the tail with reference to  
the line of vision4 ,the longer the     
tail1 the nearer the comet2 and the more
 nearly at right angles to the line of  
vision is the axis of the tail1 the     
greater is the apparent length of the   
tail4 ,in the majority of cases the     
tails of comets measure only a few      
degrees2 but1 in the case of many comets
 recorded in history1 the tail has      
extended half way across the heavens4   
,the tail of a comet1 when seen at all1 
is usually several million miles in     
length2 and in some instances the tail  
is long enough to reach across the orbit

 of the earth1 or twice as far as from  
the earth to the sun4  ,the tails of    
comets are apparently hollow1 and are   
sometimes a million of miles in         
diameter4 ,so great1 however1 is the    
tenuity of the matter in them1 that the 
faintest stars are seen through it      
without any apparent obscuration4 ,see  
,fig4 #caf1 which is a view of the great
 comet of #abfd4  ,illustration3 ,fig4  
#caf4  ,illustration3 ,fig4 #cag4       
,illustration3 ,fig4 #cah4              
,illustration3 ,fig4 #cai4              
,illustration3 ,fig4 #cbj4  ,the tails  
of comets are sometimes straight1 as in 
,fig4 #caf1 but usually more or less    
curved1 as in ,fig4 #cag1 which is a    
view of ,donati's comet as it appeared  
at one time4 ,the tail of a comet is    
occasionally divided into a number of   
streamers1 as in ,figs4 #cah and #cai4  
,fig4 #cah is a view of the great comet 
of #agdd1 and ,fig4 #cai of the great   
comet of #ahfa4 ,no4 #a1 in ,fig4 #cbj1 
is a view of the comet of #aegg2 ,no4   

#b1 of the comet of #afhj2 and ,no4 #c1 
of the comet of #agfi4  ,illustration3  
,fig4 #cba4  ,fig4 #cba shows some of   
the forms which the imagination of a    
superstitious age saw depicted in       
comets1 when these heavenly visitants   
were thought to be the forerunners of   
wars1 pestilence1 famine1 and other dire
 calamities4  #bhi4 ,visibility of      
,comets4--,even the brightest comets are
 visible only a short time near their   
perihelion passage4 ,when near the sun1 
they sometimes become very brilliant1   
and on rare occasions have been visible 
even at mid-day4 ,it is seldom that a   
comet can be seen1 even with a powerful 
telescope1 during its perihelion        
passage1 unless its perihelion is either
 inside of the earth's orbit1 or but    
little outside of it4                   
,motion and ,origin of ,comets4   #bij4 
,recognition of a ,telescopic           
,comet4--,it is impossible to           
distinguish telescopic comets by their  
appearance from another class of        

heavenly bodies known as nebul4 ,such   
comets can be recognized only by their  
motion4 ,thus1 in ,fig4 #cbb1 the upper 
and lower bodies look exactly alike2 but
 the upper one is found to remain       
stationary1 while the lower one moves   
across the field of view4 ,the upper one
 is thus shown to be a nebula1 and the  
lower one a comet4  ,illustration3 ,fig4
 #cbb4  #bia4 ,orbits of ,comets4--,all 
comets are found to move in very        
eccentric ellipses1 in parabolas1 or in 
hyperbolas4  ,since an ellipse is a     
closed curve 7#dh71 all comets that move
 in ellipses1 no matter how eccentric1  
are permanent members of the solar      
system1 and will return to the sun at   
intervals of greater or less length1    
according to the size of the ellipses   
and the rate of the comet's motion4     
,parabolas and hyperbolas being open    
curves 7#dh71 comets that move in either
 of these orbits are only temporary     
members of our solar system4 ,after     
passing the sun1 they move off into     

space1 never to return1 unless deflected
 hither by the action of some heavenly  
body which they pass in their journey4  
,illustration3 ,fig4 #cbc4      ,since a
 comet is visible only while it is near 
the sun1 it is     impossible to tell1  
by the form of the portion of the orbit 
which it     describes during the period
 of its visibility1 whether it is a part
     of a very elongated ellipse1 a     
parabola1 or a hyperbola4 ,thus in      
,fig4 #cbc are shown two orbits1 one of 
which is a very elongated     ellipse1  
and the other a parabola4 ,the part ab1 
in each case1 is     the portion of the 
orbit described by the comet during its 
visibility4 ,while describing the dotted
 portions of the orbit1 the     comet is
 invisible4 ,now it is impossible to    
distinguish the form of     the visible 
portion in the two orbits4 ,the same    
would be true were     one of the orbits
 a hyperbola4      ,whether a comet will
 describe an ellipse1 a parabola1 or a  
hyperbola1 can be determined only by its

 velocity1 taken in     connection with 
its distance from the sun4 ,were a comet
 ninety-two     and a half million miles
 from the sun1 moving away from the sun 
at     the rate of twenty-six miles a   
second1 it would have just the          
velocity necessary to describe a        
parabola4 ,were it moving with a        
greater velocity1 it would necessarily  
describe a hyperbola1 and1     with a   
less velocity1 an ellipse4 ,so1 at any  
distance from the     sun1 there is a   
certain velocity which would cause a    
comet to     describe a parabola2 while 
a greater velocity would cause it to    
describe a hyperbola1 and a less        
velocity to describe an ellipse4 ,if    
the comet is moving in an ellipse1 the  
less its velocity1 the less     the     
eccentricity of its orbit3 hence1 in    
order to determine the form     of the  
orbit of any comet1 it is only necessary
 to ascertain its     distance from the 
sun1 and its velocity at any given time4
      ,comets move in every direction in

 their orbits1 and these orbits     have
 every conceivable inclination to the   
ecliptic4  #bib4 ,periodic              
,comets4--,there are quite a number of  
comets which are known to be periodic1  
returning to the sun at regular         
intervals in elliptic orbits4 ,some of  
these have been observed at several     
returns1 so that their period has been  
determined with great certainty4 ,in the
 case of others the periodicity is      
inferred from the fact that the velocity
 fell so far short of the parabolic     
limit that the comet must move in an    
ellipse4 ,the number of known periodic  
comets is increasing every year1 three  
having been added to the list in #ahha4 
,the velocity of most comets is so near 
the parabolic limit that it is not      
possible to decide1 from observations1  
whether it falls short of it1 or exceeds
 it4 ,in the case of a few comets the   
observations indicate a minute excess of
 velocity2 but this cannot be           
confidently asserted4 ,it is not1       

therefore1 absolutely certain that any  
known comet revolves in a hyperbolic    
orbit2 and thus it is possible that all 
comets belong to our system1 and will   
ultimately return to it4 ,it is1        
however1 certain1 that1 in the majority 
of cases1 the return will be delayed for
 many centuries1 and perhaps for many   
thousand years4  #bic4 ,origin of       
,comets4--,it is now generally believed 
that the original home of the comets is 
in the stellar spaces outside of our    
solar system1 and that they are drawn   
towards the sun1 one by one1 in the long
 lapse of ages4 ,were the sun           
unaccompanied by planets1 or were the   
planets immovable1 a comet thus drawn in
 would whirl around the sun in a        
parabolic orbit1 and leave it again     
never to return1 unless its path were   
again deflected by its approach to some 
star4 ,but1 when a comet is moving in a 
parabola1 the slightest retardation     
would change its orbit to an ellipse1   
and the slightest acceleration into a   

hyperbola4 ,owing to the motion of the  
several planets in their orbits1 the    
velocity of a comet would be changed on 
passing each of them4 ,whether its      
velocity would be accelerated or        
retarded1 would depend upon the way in  
which it passed4 ,were the comet        
accelerated by the action of the        
planets1 on its passage through our     
system1 more than it was retarded by    
them1 it would leave the system with a  
more than parabolic orbit1 and would    
therefore move in a hyperbola4 ,were it1
 on the contrary1 retarded more than    
accelerated by the action of the        
planets1 its velocity would be reduced1 
so that the comet would move in a more  
or less elongated ellipse1 and thus     
become a permanent member of the solar  
system4  ,in the majority of cases the  
retardation would be so slight that it  
could not be detected by the most       
delicate observation1 and the comet     
would return to the sun only after the  
expiration of tens or hundreds of       

thousands of years2 but1 were the comet 
to pass very near one of the larger     
planets1 the retardation might be       
sufficient to cause the comet to revolve
 in an elliptical orbit of quite a short
 period4 ,the orbit of a comet thus     
captured by a planet would have its     
aphelion point near the orbit of the    
planet which captured it4 ,now1 it      
happens that each of the larger planets 
has a family of comets whose aphelia are
 about its own distance from the sun4   
,it is therefore probable that these    
comets have been captured by the action 
of these planets4 ,as might be expected 
from the gigantic size of ,jupiter1 the 
,jovian family of comets is the largest4
 ,the orbits of several of the comets of
 this group are shown in ,fig4 #cbd4    
,illustration3 ,fig4 #cbd4  #bid4       
,number of ,comets4--,the number of     
comets recorded as visible to the naked 
eye since the birth of ,christ is about 
five hundred1 while about two hundred   
telescopic comets have been observed    

since the invention of the telescope4   
,the total number of comets observed    
since the ,christian era is therefore   
about seven hundred4 ,it is certain1    
however1 that only an insignificant     
fraction of all existing comets have    
ever been observed4 ,since they can be  
seen only when near their perihelion1   
and since it is probable that the period
 of most of those which have been       
observed is reckoned by thousands of    
years 7if1 indeed1 they ever return at  
all71 our observations must be continued
 for many thousand years before we have 
seen all which come within range of our 
telescopes4 ,besides1 as already stated 
7#bhi71 a comet can seldom be seen      
unless its perihelion is either inside  
the orbit of the earth1 or but little   
outside of it2 and it is probable that  
the perihelia of the great majority of  
comets are beyond this limit of         
visibility4                             
,remarkable ,comets4   #bie4 ,the ,comet
 of #afhj4--,the great comet of #afhj1  

shown in ,fig4 #cbj1 is one of the most 
celebrated on record4 ,it was by his    
study of its motions that ,newton proved
 the orbit of a comet to be one of the  
conic sections1 and therefore that these
 bodies move under the influence of     
gravity4 ,this comet descended almost in
 a direct line to the sun1 passing      
nearer to that luminary than any comet  
before known4 ,newton estimated1 that1  
at its perihelion point1 it was exposed 
to a temperature two thousand times that
 of red-hot iron4 ,during its perihelion
 passage it was exceedingly brilliant4  
,halley suspected that this comet had a 
period of five hundred and seventy-five 
years1 and that its first recorded      
appearance was in #dc ,b4,c41 its third 
in #aajf1 and its fourth in #afhj4 ,if  
this is its real period1 it will return 
in #bbee4 ,the comet of #dc ,b4,c4 made 
its appearance just after the           
assassination of ,julius ,csar4 ,the    
,romans called it the ,julian ,star1 and
 regarded it as a celestial chariot sent

 to convey the soul of ,csar to the     
skies4 ,it was seen two or three hours  
before sunset1 and continued visible for
 eight successive days4 ,the great comet
 of #aajf was described as an object of 
terrific splendor1 and was visible in   
close proximity to the sun4 ,the comet  
of #afhj has become celebrated1 not only
 on account of its great brilliance1 and
 on account of ,newton's investigation  
of its orbit1 but also on account of the
 speculation of the theologian ,whiston 
in regard to it4 ,he accepted five      
hundred and seventy-five years as its   
period1 and calculated that one of its  
earlier apparitions must have occurred  
at the date of the flood1 which he      
supposed to have been caused by its near
 approach to the earth2 and he imagined 
that the earth is doomed to be destroyed
 by fire on some future encounter with  
this comet4  ,illustration3 ,fig4 #cbe4 
#bif4 ,the ,comet of #ahaa4--,the great 
comet of #ahaa1 a view of which is given
 in ,fig4 #cbe1 is1 perhaps1 the most   

remarkable comet on record4 ,it was     
visible for nearly seventeen months1 and
 was very brilliant1 although at its    
perihelion passage it was over a hundred
 million miles from the sun4 ,its tail  
was a hundred and twenty million miles  
in length1 and several million miles    
through4 ,it has been calculated that   
its aphelion point is about two hundred 
times as far from the sun as its        
perihelion point1 or some seven times   
the distance of ,neptune from the sun4  
,its period is estimated at about three 
thousand years4 ,it was an object of    
superstitious terror1 especially in the 
,east4 ,the ,russians regarded it as    
presaging ,napoleon's great and fatal   
war with ,russia4  ,illustration3 ,fig4 
#cbf4  ,illustration3 ,fig4 #cbg4  #big4
 ,halley's ,comet4--,halley's comet has 
become one of the most celebrated of    
modern times4 ,it is the first comet    
whose return was both predicted and     
observed4 ,it made its appearance in    
#afhb4 ,halley computed its orbit1 and  

compared it with those of previous      
comets1 whose orbits he also computed   
from recorded observations4 ,he found   
that it coincided so exactly with that  
of the comet observed by ,kepler in     
#afjg1 that there could be no doubt of  
the identity of the two orbits4 ,so     
close were they together1 that1 were    
they both drawn in the heavens1 the     
naked eye would almost see them joined  
into one line4 ,there could therefore be
 no doubt that the comet of #afhb was   
the same that had appeared in #afjg1 and
 that it moved in an elliptic orbit1    
with a period of about seventy-five     
years4 ,he found that this comet had    
previously appeared in #aeca and in     
#adef2 and he predicted that it would   
return about #ageh4 ,its actual return  
was retarded somewhat by the action of  
the planets on it in its passage through
 the solar system4 ,it1 however1        
appeared again in #agei1 and a third    
time in #ahce4 ,its next appearance will
 be about #aiaa4 ,the orbit of this     

comet is shown in ,fig4 #cbf4 ,fig4 #cbg
 shows the comet as it appeared to the  
naked eye1 and in a telescope of        
moderate power1 in #ahce4 ,this comet   
appears to be growing less brilliant4   
,in #adef it appeared as a comet of     
great splendor2 and coming as it did in 
a very superstitious age1 soon after the
 fall of ,constantinople1 and during the
 threatened invasion of ,europe by the  
,turks1 it caused great alarm4 ,fig4    
#cbh shows the changes undergone by the 
nucleus of this comet during its        
perihelion passage in #ahce4            
,illustration3 ,fig4 #cbh4              
,illustration3 ,fig4 #cbi4              
,illustration3 ,fig4 #ccj4  #bih4       
,encke's ,comet4--,this telescopic      
comet1 two views of which are given in  
,figs4 #cbi and #ccj1 appeared in #ahah4
 ,encke computed its orbit1 and found it
 to lie wholly within the orbit of      
,jupiter 7,fig4 #cbd71 and the period to
 be about three years and a third4 ,by  
comparing the intervals between the     

successive returns of this comet1 it has
 been ascertained that its orbit is     
continually growing smaller and smaller4
 ,to account for the retardation of this
 comet1 ,olbers announced his celebrated
 hypothesis1 that the celestial spaces  
are filled with a subtile resisting     
medium4 ,this hypothesis was adopted by 
,encke1 and has been accepted by certain
 other astronomers2 but it has by no    
means gained universal assent4  #bii4   
,biela's ,comet4--,this comet appeared  
in #ahbf1 and was found to have a period
 of about six years and two thirds4 ,on 
its return in #ahde1 it met with a      
singular1 and as yet unexplained1       
accident1 which has rendered the        
otherwise rather insignificant comet    
famous4 ,in ,november and ,december of  
that year it was observed as usual1     
without any thing remarkable about it2  
but1 in ,january of the following year1 
it was found to have been divided into  
two distinct parts1 so as to appear as  
two comets instead of one4 ,the two     

parts were at first of very unequal     
brightness2 but1 during the following   
month1 the smaller of the two increased 
in brilliancy until it equalled its     
companion2 it then grew fainter till it 
entirely disappeared1 a month before its
 companion4 ,the two parts were about   
two hundred thousand miles apart4 ,fig4 
#cca shows these two parts as they      
appeared on the #aith of ,february1 and 
,fig4 #ccb as they appeared on the #bast
 of ,february4 ,on its return in #aheb1 
the comets were found still to be       
double2 but the two components were now 
about a million and a half miles apart4 
,they are shown in ,fig4 #ccc as they   
appeared at this time4 ,sometimes one of
 the parts appeared the brighter1 and   
sometimes the other2 so that it was     
impossible to decide which was really   
the principal comet4 ,the two portions  
passed out of view in ,september1 and   
have not been seen since2 although in   
#ahgb the position of the comet would   
have been especially favorable for      

observation4 ,the comet appears to have 
become completely broken up4            
,illustration3 ,fig4 #cca4              
,illustration3 ,fig4 #ccb4              
,illustration3 ,fig4 #ccc4              
,illustration3 ,fig4 #ccd4  #cjj4 ,the  
,comet of #ahdc4--,the great comet of   
#ahdc1 a view of which is given in ,fig4
 #ccd1 was favorably situated for       
observation only in southern latitudes4 
,it was exceedingly brilliant1 and was  
easily seen in full daylight1 in close  
proximity to the sun4 ,the apparent     
length of its tail was sixty-five       
degrees1 and its real length a hundred  
and fifty million miles1 or nearly twice
 the distance from the earth to the sun4
 ,this comet is especially remarkable on
 account of its near approach to the    
sun4 ,at the time of its perihelion     
passage the distance of the comet from  
the photosphere of the sun was less than
 one-fourteenth of the diameter of the  
sun4 ,this distance was only one-half   
that of the comet of #afhj when at its  

perihelion4 ,when at perihelion1 this   
comet was plunging through the sun's    
outer atmosphere at the rate of one     
million1 two hundred and eighty thousand
 miles an hour4 ,it passed half way     
round the sun in the space of two hours1
 and its tail was whirled round through 
a hundred and eighty degrees in that    
brief time4 ,as the tail extended almost
 double the earth's distance from the   
sun1 the end of the tail must have      
traversed in two hours a space nearly   
equal to the circumference of the       
earth's orbit1--a distance which the    
earth1 moving at the rate of about      
twenty miles a second1 is a whole year  
in passing4 ,it is almost impossible to 
suppose that the matter forming this    
tail remained the same throughout this  
tremendous sweep4  #cja4 ,donati's      
,comet4--,the great comet of #aheh1     
known as ,donati's comet1 was one of the
 most magnificent of modern times4 ,when
 at its brightest it was only about     
fifty million miles from the earth4 ,its

 tail was then more than fifty million  
miles long4 ,had the comet at this time 
been directly between the earth and sun1
 the earth must have passed through its 
tail2 but this did not occur4 ,the orbit
 of this comet was found to be decidedly
 elliptic1 with a period of about two   
thousand years4 ,this comet is          
especially celebrated on account of the 
careful telescopic observations of its  
nucleus and coma at the time of its     
perihelion passage4 ,attention has      
already been called 7#bhg7 to the       
changes it underwent at that time4 ,its 
tail was curved1 and of a curious       
feather-like form1 as shown in ,fig4    
#cce4 ,at times it developed lateral    
streamers1 as shown in ,fig4 #ccf4 ,fig4
 #ccg shows the head of the comet as it 
was seen by ,bond of the ,harvard       
,observatory1 whose delineations of this
 comet have been justly celebrated4     
,illustration3 ,fig4 #cce4              
,illustration3 ,fig4 #ccf4              
,illustration3 ,fig4 #ccg4  #cjb4 ,the  

,comet of #ahfa4--,the great comet of   
#ahfa is remarkable for its great       
brilliancy1 for its peculiar fan-shaped 
tail1 and for the probable passage of   
the earth through its tail4 ,sir ,john  
,herschel declared that it far exceeded 
in brilliancy any comet he had ever     
seen1 not excepting those of #ahaa and  
#aheh4 ,secchi found its tail to be a   
hundred and eighteen degrees in length1 
the largest but one on record4 ,fig4    
#cch shows this comet as it appeared at 
one time4 ,fig4 #cci shows the position 
of the earth at ,e1 in the tail of this 
comet1 on the #cjth of ,june1 #ahfa4    
,fig4 #cdj shows the probable passage of
 the earth through the tail of the comet
 on that date4 ,as the tail of a comet  
doubtless consists of something much    
less dense than our atmosphere1 it is   
not surprising that no noticeable effect
 was produced upon us by the encounter1 
if it occurred4  ,illustration3 ,fig4   
#cch4  ,illustration3 ,fig4 #cci4       
,illustration3 ,fig4 #cdj4  #cjc4       

,coggia's ,comet4--,this comet1 which   
appeared in #ahgd1 looked very large1   
because it came very near the earth4 ,it
 was not at all brilliant4 ,its nucleus 
was carefully studied1 and was found to 
develop a series of envelops similar to 
those of ,donati's comet4 ,figs4 #cda   
and #cdb are two views of the head of   
this comet4 ,fig4 #cdc shows the system 
of envelops that were developed during  
its perihelion passage4  ,illustration3 
,fig4 #cda4  ,illustration3 ,fig4 #cdb4 
,illustration3 ,fig4 #cdc4  #cjd4 ,the  
,comet of ,june1 #ahha4--,this comet1   
though far from being one of the largest
 of modern times1 was still very        
brilliant4 ,it will ever be memorable as
 the first brilliant comet which has    
admitted of careful examination with the
 spectroscope4                          
,connection between ,meteors and        
,comets4   #cje4 ,shooting-,stars4--,on 
watching the heavens any clear night1 we
 frequently see an appearance as of a   
star shooting rapidly through a short   

space in the sky1 and then suddenly     
disappearing4 ,three or four such       
shooting-stars may1 on the average1 be  
observed in the course of an hour4 ,they
 are usually seen only a second or two2 
but they sometimes move slowly1 and are 
visible much longer4 ,these stars begin 
to be visible at an average height of   
about seventy-five miles1 and they      
disappear at an average height of about 
fifty miles4 ,they are occasionally seen
 as high as a hundred and fifty miles1  
and continue to be visible till within  
thirty miles of the earth4 ,their       
visible paths vary from ten to a hundred
 miles in length1 though they are       
occasionally two hundred or three       
hundred miles long4 ,their average      
velocity1 relatively to the earth's     
surface1 varies from ten to forty-five  
miles a second4  ,the average number of 
shooting-stars visible to the naked eye 
at any one place is estimated at about a
 thousand an hour2 and the average      
number large enough to be visible to the

 naked eye1 that traverse the atmosphere
 daily1 is estimated at over eight      
millions4 ,the number of telescopic     
shooting-stars would of course be much  
greater4  ,occasionally1 shooting-stars 
leave behind them a trail of light which
 lasts for several seconds4 ,these      
trails are sometimes straight1 as shown 
in ,fig4 #cdd1 and sometimes curved1 as 
in ,figs4 #cde and #cdf4 ,they often    
disappear like trails of smoke1 as shown
 in ,fig4 #cdg4  ,illustration3 ,fig4   
#cdd4  ,illustration3 ,fig4 #cde4       
,illustration3 ,fig4 #cdf4              
,illustration3 ,fig4 #cdg4              
,shooting-stars are seen to move in all 
directions through the heavens4 ,their  
apparent paths are1 however1 generally  
inclined downward1 though sometimes     
upward2 and after midnight they come in 
the greatest numbers from that quarter  
of the heavens toward which the earth is
 moving in its journey around the sun4  
#cjf4 ,meteors4--,occasionally these    
bodies are brilliant enough to          

illuminate the whole heavens4 ,they are 
then called meteors1 although this term 
is equally applicable to ordinary       
shooting-stars4 ,such a meteor is shown 
in ,fig4 #cdh4  ,illustration3 ,fig4    
#cdh4  ,sometimes these brilliant       
meteors are seen to explode1 as shown in
 ,fig4 #cdi2 and the explosion is       
accompanied with a loud detonation1 like
 the discharge of cannon4               
,illustration3 ,fig4 #cdi4  ,ordinary   
shooting-stars are not accompanied by   
any audible sound1 though they are      
sometimes seen to break in pieces4      
,meteors which explode with an audible  
sound are called detonating meteors4    
#cjg4 ,aerolites4--,there is no certain 
evidence that any deposit from ordinary 
shooting-stars ever reaches the surface 
of the earth2 though a peculiar dust has
 been found in certain localities1 which
 has been supposed to be of meteoric    
origin1 and which has been called       
meteoric dust4 ,but solid bodies        
occasionally descend to the earth from  

beyond our atmosphere4 ,these generally 
penetrate a foot or more into the earth1
 and1 if picked up soon after their     
fall1 are found to be warm1 and         
sometimes even hot4 ,these bodies are   
called aerolites4 ,when they have a     
stony appearance1 and contain but little
 iron1 they are called meteoric stones2 
when they have a metallic appearance1   
and are composed largely of iron1 they  
are called meteoric iron4  ,there are   
eighteen well-authenticated cases in    
which aerolites have fallen in the      
,united ,states during the last sixty   
years1 and their aggregate weight is    
twelve hundred and fifty pounds4 ,the   
entire number of known aerolites the    
date of whose fall is well determined is
 two hundred and sixty-one4 ,there are  
also on record seventy-four cases of    
which the date is more or less          
uncertain4 ,there have also been found  
eighty-six masses1 which1 from their    
peculiar composition1 are believed to be
 aerolites1 though their fall was not   

seen4 ,the weight of these masses varies
 from a few pounds to several tons4 ,the
 entire number of aerolites of which we 
have any knowledge is therefore about   
four hundred and twenty4  ,aerolites are
 composed of the same elementary        
substances as occur in terrestrial      
minerals1 not a single new element      
having been found in their analysis4 ,of
 the sixty or more elements now         
recognized by chemists1 about twenty    
have been found in aerolites4  ,while   
aerolites contain no new elements1 their
 appearance is quite peculiar2 and the  
compounds found in them are so peculiar 
as to enable us by chemical analysis to 
distinguish an aerolite from any        
terrestrial substance4  ,iron ores are  
very abundant in nature1 but iron in the
 metallic state is exceedingly rare4    
,now1 aerolites invariably contain      
metallic iron1 sometimes from ninety to 
ninety-six per cent4 ,this iron is      
malleable1 and may be readily worked    
into cutting instruments4 ,it always    

contains eight or ten per cent of       
nickel1 together with small quantities  
of cobalt1 copper1 tin1 and chromium4   
,this composition has never been found  
in any terrestrial mineral4 ,aerolites  
also contain1 usually in small amount1 a
 compound of iron1 nickel1 and          
phosphorus1 which has never been found  
elsewhere4  ,meteorites often present   
the appearance of having been fused on  
the surface to a slight depth1 and      
meteoric iron is found to have a        
peculiar crystalline structure4 ,the    
external appearance of a piece of       
meteoric iron found near ,lockport1     
,n4,y41 is shown in ,fig4 #cej4 ,fig4   
#cea shows the peculiar internal        
structure of meteoric iron4             
,illustration3 ,fig4 #cej4              
,illustration3 ,fig4 #cea4  #cjh4       
,meteoroids4--,astronomers now          
universally hold that shooting-stars1   
meteors1 and aerolites are all minute   
bodies1 revolving1 like the comets1     
about the sun4 ,they are moving in every

 possible direction through the         
celestial spaces4 ,they may not average 
more than one in a million of cubic     
miles1 and yet their total number       
exceeds all calculation4 ,of the nature 
of the minuter bodies of this class     
nothing is certainly known4 ,the earth  
is continually encountering them in its 
journey around the sun4 ,they are burned
 by passing through the upper regions of
 our atmosphere1 and the shooting-star  
is simply the light of that burning4    
,these bodies1 which are invisible till 
they plunge into the earth's atmosphere1
 are called meteoroids4  #cji4 ,origin  
of the ,light of ,meteors4--,when one of
 these meteoroids enters our atmosphere1
 the resistance of the air arrests its  
motion to some extent1 and so converts a
 portion of its energy of motion into   
that of heat4 ,the heat thus developed  
is sufficient to raise the meteoroid and
 the air around it to incandescence1 and
 in most cases either to cause the      
meteoroid to burn up1 or to dissipate it

 as vapor4 ,the luminous vapor thus     
formed constitutes the luminous train   
which occasionally accompanies a meteor1
 and often disappears as a puff of      
smoke4 ,when a meteoroid is large enough
 and refractory enough to resist the    
heat to which it is exposed1 its motion 
is sufficiently arrested1 on entering   
the lower layers of our atmosphere1 to  
cause it to fall to the earth4 ,we then 
have an aerolite4 ,a brilliant meteor   
differs from a shooting-star simply in  
magnitude4  #caj4 ,the ,intensity of the
 ,heat to which a ,meteoroid is         
,exposed4--,it has been ascertained by  
experiment that a body moving through   
the atmosphere at the rate of a hundred 
and twenty-five feet a second raises the
 temperature of the air immediately in  
front of it one degree1 and that the    
temperature increases as the square of  
the velocity of the moving body2 that is
 to say1 that1 with a velocity of two   
hundred and fifty feet1 the temperature 
in front of the body would be raised    

four degrees2 with a velocity of five   
hundred feet1 sixteen degrees2 and so   
on4 ,to find1 therefore1 the temperature
 to which a meteoroid would be exposed  
in passing through our atmosphere1 we   
have merely to divide its velocity in   
feet per second by a hundred and        
twenty-five1 and square the quotient4   
,with a velocity of forty-four miles a  
second in our atmosphere1 a meteoroid   
would therefore be exposed to a         
temperature of between three and four   
million degrees4 ,the air acts upon the 
body as if it were raised to this       
intense heat4 ,at such a temperature    
small masses of the most refractory or  
incombustible substances known to us    
would flash into vapor with the         
evolution of intense light and heat4    
,if one of these meteoric bodies is     
large enough to pass through the        
atmosphere and reach the earth1 without 
being volatilized by the heat1 we have  
an aerolite4 ,as it is only a few       
seconds in making the passage1 the heat 

has not time to penetrate far into its  
interior1 but is expended in melting and
 vaporizing the outer portions4 ,the    
resistance of the denser strata of the  
atmosphere to the motion of the aerolite
 sometimes becomes so enormous that the 
body is suddenly rent to pieces with a  
loud detonation4 ,it seems like an      
explosion produced by some disruptive   
action within the mass2 but there can be
 little doubt that it is due to the     
velocity--perhaps ten1 twenty1 or thirty
 miles a second--with which the body    
strikes the air4  ,if1 on the other     
hand1 the meteoroid is so small as to be
 burned up or volatilized in the upper  
regions of the atmosphere1 we have a    
common shooting-star1 or a meteor of    
greater or less brilliancy4             
,illustration3 ,fig4 #ceb4  #caa4       
,meteoric ,showers4--,on ordinary nights
 only four or five shooting-stars are   
seen in an hour1 and these move in every
 direction4 ,their orbits lie in all    
possible positions1 and are seemingly   

scattered at random4 ,such meteors are  
called sporadic meteors4 ,on occasional 
nights1 shooting-stars are more         
numerous1 and all move in a common      
direction4 ,such a display is called a  
meteoric shower4 ,these showers differ  
greatly in brilliancy2 but during any   
one shower the meteors all appear to    
radiate from some one point in the      
heavens4 ,if we mark on a celestial     
globe the apparent paths of the meteors 
which fall during a shower1 or if we    
trace them back on the celestial sphere1
 we shall find that they all meet in the
 same point1 as shown in ,fig4 #ceb4    
,this point is called the radiant point4
 ,it always appears in the same         
position1 wherever the observer is      
situated1 and does not partake of the   
diurnal motion of the earth4 ,as the    
stars move towards the west1 the radiant
 point moves with them4 ,the point in   
question is purely an effect of         
perspective1 being the vanishing point  
of the parallel lines in which the      

meteors are actually moving4 ,these     
lines are seen1 not in their real       
direction in space1 but as projected on 
the celestial sphere4 ,if we look       
upwards1 and watch snow falling through 
a calm atmosphere1 the flakes which fall
 directly towards us do not seem to move
 at all1 while the surrounding flakes   
seem to diverge from them on all sides4 
,so1 in a meteoric shower1 a meteor     
coming directly towards the observer    
does not seem to move at all1 and marks 
the point from which all the others seem
 to radiate4  #cab4 ,the ,august        
,meteors4--,a meteoric shower of no     
great brilliancy occurs annually about  
the #ajth of ,august4 ,the radiant point
 of this shower is in the constellation 
,perseus1 and hence these meteors are   
often called the ,perseids4 ,the orbit  
of these meteoroids has been pretty     
accurately determined1 and is shown in  
,fig4 #cec4  ,illustration3 ,fig4 #cec4 
,it will be seen that the perihelion    
point of this orbit is at about the     

distance of the earth from the sun2 so  
that the earth encounters the meteors   
once a year1 and this takes place in the
 month of ,august4 ,the orbit is a very 
eccentric ellipse1 reaching far beyond  
,neptune4 ,as the meteoric display is   
about equally brilliant every year1 it  
seems probable that the meteoroids form 
a stream quite uniformly distributed    
throughout the whole orbit4 ,it probably
 takes one of the meteoroids about a    
hundred and twenty-four years to pass   
around this orbit4  ,illustration3 ,fig4
 #ced4  #cac4 ,the ,november            
,meteors4--,a somewhat brilliant        
meteoric shower also occurs annually1   
about the #acth of ,november4 ,the      
radiant point of these meteors is in the
 constellation ,leo1 and hence they are 
often called the ,leonids4 ,their orbit 
has been determined with great accuracy1
 and is shown in ,fig4 #ced4 ,while the 
,november meteors are not usually very  
numerous or bright1 a remarkably        
brilliant display of them has been seen 

once in about thirty-three or           
thirty-four years3 hence we infer1 that1
 while there are some meteoroids        
scattered throughout the whole extent of
 the orbit1 the great majority are      
massed in a group which traverses the   
orbit in a little over thirty-three     
years4 ,a conjectural form of this      
condensed group is shown in ,fig4 #cee4 
,the group is so large that it takes it 
two or three years to pass the          
perihelion point3 hence there may be a  
brilliant meteoric display two or three 
years in succession4  ,illustration3    
,fig4 #cee4  ,the last brilliant display
 of these meteors was in the years #ahff
 and #ahfg4 ,the display was visible in 
this country only a short time before   
sunrise1 and therefore did not attract  
general attention4 ,the display of #ahcc
 was remarkably brilliant in this       
country1 and caused great consternation 
among the ignorant and superstitious4   
,illustration3 ,fig4 #cef4  #cad4       
,connection between ,meteors and        

,comets4--,it has been found that a     
comet which appeared in #ahff1 and which
 is designated as #ahff1 ,i41 has       
exactly the same orbit and period as the
 ,november meteors1 and that another    
comet1 known as the #ahfb1 ,,iii41 has  
the same orbit as the ,august meteors4  
,it has also been ascertained that a    
third comet1 #ahfa1 ,i41 has the same   
orbit as a stream of meteors which the  
earth encounters in ,april4             
,furthermore1 it was found1 in #ahgb1   
that there was a small stream of meteors
 following in the train of the lost     
comet of ,biela4 ,these various orbits  
of comets and meteoric streams are shown
 in ,fig4 #cef4 ,the coincidence of the 
orbits of comets and of meteoric streams
 indicates that these two classes of    
bodies are very closely related4 ,they  
undoubtedly have a common origin4 ,the  
fact that there is a stream of meteors  
in the train of ,biela's comet has led  
to the supposition that comets may      
become gradually disintegrated into     

meteoroids4                ,physical and
 ,chemical ,constitution of ,comets4    
#cae4 ,physical ,constitution of        
,telescopic ,comets4--,we have no       
certain knowledge of the physical       
constitution of telescopic comets4 ,they
 are usually tens of thousands of miles 
in diameter1 and yet of such tenuity    
that the smallest stars can readily be  
seen through them4 ,it would seem that  
they must shine in part by reflected    
light2 yet the spectroscope shows that  
their spectrum is composed of bright    
bands1 which would indicate that they   
are composed1 in part at least1 of      
incandescent gases4 ,it is1 however1    
difficult to conceive how these gases   
become sufficiently heated to be        
luminous2 and at the same time such     
gases would reflect no sunlight4  ,it   
seems probable that these comets are    
really made up of a combination of      
small1 solid particles in the form of   
minute meteoroids1 and of gases which   
are1 perhaps1 rendered luminous by      

electric discharges of slight intensity4
  #caf4 ,physical ,constitution of      
,large ,comets4--,in the case of large  
comets the nucleus is either a dense    
mass of solid matter several hundred    
miles in diameter1 or a dense group of  
meteoroids4 ,professor ,peirce estimated
 that the density of the nucleus is at  
least equal to that of iron4 ,as such a 
comet approaches the sun1 the nucleus   
is1 to a slight extent1 vaporized1 and  
out of this vapor is formed the coma and
 the tail4  ,that some evaporating      
process is going on from the nucleus of 
the comet is proved by the movements of 
the tail4 ,it is evident that the tail  
cannot be an appendage carried along    
with the comet1 as it seems to be4 ,it  
is impossible that there should be any  
cohesion in matter of such tenuity that 
the smallest stars could be seen through
 a million of miles of it1 and which is1
 moreover1 continually changing its     
form4 ,then1 again1 as a comet is       
passing its perihelion1 the tail appears

 to be whirled from one side of the sun 
to another with a rapidity which would  
tear it to pieces if the movement were  
real4 ,the tail seems to be1 not        
something attached to the comet1 and    
carried along with it1 but a stream of  
vapor issuing from it1 like smoke from a
 chimney4 ,the matter of which it is    
composed is continually streaming       
outwards1 and continually being replaced
 by fresh vapor from the nucleus4  ,the 
vapor1 as it emanates from the nucleus1 
is repelled by the sun with a force     
often two or three times as great as the
 ordinary solar attraction4 ,the most   
probable explanation of this phenomenon 
is1 that it is a case of electrical     
repulsion1 the sun and the particles of 
the cometary mist being similarly       
electrified4 ,with reference to this    
electrical theory of the formation of   
comets' tails1 ,professor ,peirce makes 
the following observation3 ,in its      
approach to the sun1 the surface of the 
nucleus is rapidly heated3 it is melted 

and vaporized1 and subjected to frequent
 explosions4 ,the vapor rises in its    
atmosphere with a well-defined upper    
surface1 which is known to observers as 
an envelop4444 ,the electrification of  
the cometary mist is analogous to that  
of our own thunder-clouds4 ,any portion 
of the coma which has received the      
opposite kind of electricity to the sun 
and to the repelled tail will be        
attracted4 ,this gives a simple         
explanation of the negative tails which 
have been sometimes seen directed       
towards the sun4 ,in cases of violent   
explosion1 the whole nucleus might be   
broken to pieces1 and the coma dashed   
around so as to give varieties of tail1 
and even multiple tails4 ,there seems1  
indeed1 to be no observed phenomenon of 
the tail or the coma which is not       
consistent with a reasonable            
modification of the theory4 ,professor  
,peirce regarded comets simply as the   
largest of the meteoroids4 ,they appear 
to shine partly by reflected sunlight1  

and partly by their own proper light1   
which seems to be that of vapor rendered
 luminous by an electric discharge of   
slight intensity4  ,illustration3 ,fig4 
#ceg4  #cag4 ,collision of a ,comet and 
the ,earth4--,it sometimes happens that 
the orbit of a comet intersects that of 
the earth1 as is shown in ,fig4 #ceg1   
which shows a portion of the orbit of   
,biela's comet1 with the positions of   
the comet and of the earth in #ahcb4 ,of
 course1 were a comet and the earth both
 to reach the intersection of their     
orbits at the same time1 a collision of 
the two bodies would be inevitable4     
,with reference to the probable effect  
of such a collision1 ,professor ,newcomb
 remarks1--  ,the question is frequently
 asked1 ,what would be the effect if a  
comet should strike the earth8 ,this    
would depend upon what sort of a comet  
it was1 and what part of the comet came 
in contact with our planet4 ,the latter 
might pass through the tail of the      
largest comet without the slightest     

effect being produced2 the tail being so
 thin and airy that a million miles     
thickness of it looks only like gauze in
 the sunlight4 ,it is not at all        
unlikely that such a thing may have     
happened without ever being noticed4 ,a 
passage through a telescopic comet would
 be accompanied by a brilliant meteoric 
shower1 probably a far more brilliant   
one than has ever been recorded4 ,no    
more serious danger would be encountered
 than that arising from a possible fall 
of meteorites2 but a collision between  
the nucleus of a large comet and the    
earth might be a serious matter4 ,if1 as
 ,professor ,peirce supposes1 the       
nucleus is a solid body of metallic     
density1 many miles in diameter1 the    
effect where the comet struck would be  
terrific beyond conception4 ,at the     
first contact in the upper regions of   
the atmosphere1 the whole heavens would 
be illuminated with a resplendence      
beyond that of a thousand suns1 the sky 
radiating a light which would blind     

every eye that beheld it1 and a heat    
which would melt the hardest rocks4 ,a  
few seconds of this1 while the huge body
 was passing through the atmosphere1 and
 the collision at the earth's surface   
would in an instant reduce everything   
there existing to fiery vapor1 and bury 
it miles deep in the solid earth4       
,happily1 the chances of such a calamity
 are so minute that they need not cause 
the slightest uneasiness4 ,there is     
hardly a possible form of death which is
 not a thousand times more probable than
 this4 ,so small is the earth in        
comparison with the celestial spaces1   
that if one should shut his eyes1 and   
fire a gun at random in the air1 the    
chance of bringing down a bird would be 
better than that of a comet of any kind 
striking the earth4  ,illustration3     
,fig4 #ceh4  ,illustration3 ,fig4 #cei4 
#cah4 ,the ,chemical ,constitution of   
,comets4--,fig4 #ceh shows the bands of 
the spectrum of a telescopic comet of   
#ahgc1 as seen by two different         

observers4 ,fig4 #cei shows the spectrum
 of the coma and tail of the comet of   
#ahgd2 and the spectrum of the bright   
comet of #ahha showed the same three    
bands for the coma and tail4 ,now1 these
 three bands are those of certain       
hydrocarbon vapors3 hence it would seem 
that the coma and tails of comets are   
composed chiefly of such vapors 7#cae74 
,,ii4 ,,the ,,zodiacal ,,light4   #cai4 
,the ,general ,appearance of the        
,zodiacal ,light4--,the phenomenon known
 as the zodiacal light consists of a    
very faint luminosity1 which may be seen
 rising from the western horizon after  
twilight on any clear winter or spring  
evening1 also from the eastern horizon  
just before daybreak in the summer or   
autumn4 ,it extends out on each side of 
the sun1 and lies nearly in the plane of
 the ecliptic4 ,it grows fainter the    
farther it is from the sun1 and can     
generally be traced to about ninety     
degrees from that luminary1 when it     
gradually fades away4 ,in a very clear1 

tropical atmosphere1 it has been traced 
all the way across the heavens from east
 to west1 thus forming a complete ring4 
,the general appearance of this column  
of light1 as seen in the morning1 in the
 latitude of ,europe1 is shown in ,fig4 
#cfj4  ,illustration3 ,fig4 #cfj4       
,taking all these appearances together1 
they indicate that it is due to a       
lens-shaped appendage surrounding the   
sun1 and extending a little beyond the  
earth's orbit4 ,it lies nearly in the   
plane of the ecliptic2 but its exact    
position is not easily determined4 ,fig4
 #cfa shows the general form and        
position of this solar appendage1 as    
seen in the west4  ,illustration3 ,fig4 
#cfa4  #cbj4 ,the ,visibility of the    
,zodiacal ,light4--,the reason why the  
zodiacal light is more favorably seen in
 the evening during the winter and      
spring than in the summer and fall is   
evident from ,fig4 #cfb1 which shows the
 position of the ecliptic and the       
zodiacal light with reference to the    

western horizon at the time of sunset in
 ,march and in ,september4 ,it will be  
seen that in ,september the axis of the 
light forms a small angle with the      
horizon1 so that the phenomenon is      
visible only a short time after sunset  
and low down where it is difficult to   
distinguish it from the glimmer of the  
twilight2 while in ,march1 its axis     
being nearly perpendicular to the       
horizon1 the light may be observed for  
some hours after sunset and well up in  
the sky4 ,fig4 #cfc gives the position  
of the ecliptic and of the zodiacal     
light with reference to the eastern     
horizon at the time of sunrise1 and     
shows why the zodiacal light is seen to 
better advantage in the morning during  
the summer and fall than during the     
winter and spring4 ,it will be observed 
that here the angle made by the axis of 
the light with the horizon is small in  
,march1 while it is large in ,september2
 the conditions represented in the      
preceding figure being thus reversed4   

,illustration3 ,fig4 #cfb4              
,illustration3 ,fig4 #cfc4  #cba4       
,nature of the ,zodiacal                
,light4--,various attempts have been    
made to explain the phenomena of the    
zodiacal light2 but the most probable   
theory is1 that it is due to an immense 
number of meteors which are revolving   
around the sun1 and which lie mostly    
within the earth's orbit4 ,each of these
 meteors reflects a sensible portion of 
sunlight1 but is far too small to be    
separately visible4 ,all of these       
meteors together would1 by their        
combined reflection1 produce a kind of  
pale1 diffused light4                   
,,iii4 ,,the ,,stellar ,,universe4      
,i4 ,,general ,,aspect ,,of ,,the       
,,heavens4   #cbb4 ,the ,magnitude of   
the ,stars4--,the stars that are visible
 to the naked eye are divided into six  
classes1 according to their brightness4 
,the brightest stars are called stars of
 the first magnitude2 the next          
brightest1 those of the second          

magnitude2 and so on to the sixth       
magnitude4 ,the last magnitude includes 
the faintest stars that are visible to  
the naked eye on the most favorable     
night4 ,stars which are fainter than    
those of the sixth magnitude can be seen
 only with the telescope1 and are called
 telescopic stars4 ,telescopic stars are
 also divided into magnitudes2 the      
division extending to the sixteenth     
magnitude1 or the faintest stars that   
can be seen with the most powerful      
telescopes4  ,the classification of     
stars according to magnitudes has       
reference only to their brightness1 and 
not at all to their actual size4 ,a     
sixth magnitude star may actually be    
larger than a first magnitude star2 its 
want of brilliancy being due to its     
greater distance1 or to its inferior    
luminosity1 or to both of these causes4 
,none of the stars present any sensible 
disk1 even in the most powerful         
telescope3 they all appear as mere      
points of light4 ,the larger the        

telescope1 the greater is its power of  
revealing faint stars2 not because it   
makes these stars appear larger1 but    
because of its greater light-gathering  
power4 ,this power increases with the   
size of the object-glass of the         
telescope1 which plays the part of a    
gigantic pupil of the eye4  ,the        
classification of the stars into        
magnitudes is not made in accordance    
with any very accurate estimate of their
 brightness4 ,the stars which are       
classed together in the same magnitude  
are far from being equally bright4  ,the
 stars of each lower magnitude are about
 two-fifths as bright as those of the   
magnitude above4 ,the ratio of          
diminution is about a third from the    
higher magnitude down to the fifth4     
,were the ratio two-fifths exact1 it    
would take about              #b-a_/b   
stars of the #b;d magnitude to make one 
of the #ast4             #f     stars of
 the #c;d magnitude to make one of the  
#ast4            #af     stars of the   

#dth magnitude to make one of the #ast4 
#dj     stars of the #eth magnitude to  
make one of the #ast4           #ajj    
stars of the #fth magnitude to make one 
of the #ast4        #aj1jjj     stars of
 the #aath magnitude to make one of the 
#ast4     #a1jjj1jjj     stars of the   
#afth magnitude to make one of the #ast4
  #cbc4 ,the ,number of the             
,stars4--,the total number of stars in  
the celestial sphere visible to the     
average naked eye is estimated1 in round
 numbers1 at five thousand2 but the     
number varies much with the perfection  
and the training of the eye and with the
 atmospheric conditions4 ,for every star
 visible to the naked eye1 there are    
thousands too minute to be seen without 
telescopic aid4 ,fig4 #cfd shows a      
portion of the constellation of the     
,twins as seen with the naked eye2 and  
,fig4 #cfe shows the same region as seen
 in a powerful telescope4               
,illustration3 ,fig4 #cfd4              
,illustration3 ,fig4 #cfe4  ,struve has 

estimated that the total number of stars
 visible with ,herschel's twenty-foot   
telescope was about twenty million4 ,the
 number that can be seen with the great 
telescopes of modern times has not been 
carefully estimated1 but is probably    
somewhere between thirty million and    
fifty million4  ,the number of stars    
between the north pole and the circle   
thirty-five degrees south of the equator
 is about as follows3--      ,of the    
#ast magnitude about        #ad stars4  
,of the #b;d magnitude about         #dh
 stars4     ,of the #c;d magnitude about
        #aeb stars4     ,of the #dth    
magnitude about       #cac stars4       
,of the #eth magnitude about       #hed 
stars4     ,of the #fth magnitude about 
#bjaj stars4                            
----     ,total visible to naked eye    
#ccia stars4  ,the number of stars of   
the several magnitudes is approximately 
in inverse proportion to that of their  
brightness1 the ratio being a little    
greater in the higher magnitudes1 and   

probably a little less in the lower     
ones4  #cbd4 ,the ,division of the      
,stars into ,constellations4--,a glance 
at the heavens is sufficient to show    
that the stars are not distributed      
uniformly over the sky4 ,the larger ones
 especially are collected into more or  
less irregular groups4 ,the larger      
groups are called constellations4 ,at a 
very early period a mythological figure 
was allotted to each constellation2 and 
these figures were drawn in such a way  
as to include the principal stars of    
each constellation4 ,the heavens thus   
became covered1 as it were1 with immense
 hieroglyphics4  ,there is no historic  
record of the time when these figures   
were formed1 or of the principle in     
accordance with which they were         
constructed4 ,it is probable that the   
imagination of the earlier peoples may1 
in many instances1 have discovered some 
fanciful resemblance in the             
configuration of the stars to the forms 
depicted4 ,the names are still retained1

 although the figures no longer serve   
any astronomical purpose4 ,the          
constellation ,hercules1 for instance1  
no longer represents the figure of a man
 among the stars1 but a certain portion 
of the heavens within which the ancients
 placed that figure4 ,in star-maps      
intended for school and popular use it  
is still customary to give these        
figures2 but they are not generally     
found on maps designed for astronomers4 
#cbe4 ,the ,naming of the ,stars4--,the 
brighter stars have all proper names1 as
 ,sirius1 ,procyon1 ,arcturus1 ,capella1
 ,aldebaran1 etc4 ,this method of       
designating the stars was adopted by the
 ,arabs4 ,most of these names have      
dropped entirely out of astronomical    
use1 though many are popularly retained4
 ,the brighter stars are now generally  
designated by the letters of the ,greek 
alphabet1--alpha1 beta1 gamma1 etc41--to
 which is appended the genitive of the  
name of the constellation1 the first    
letter of the alphabet being used for   

the brightest star1 the second for the  
next brightest1 and so on4 ,thus        
,aldebaran would be designated as ,alpha
 ,tauri4 ,in speaking of the stars of   
any one constellation1 we simply        
designate them by the letters of the    
,greek alphabet1 without the addition of
 the name of the constellation1 which   
answers to a person's surname1 while the
 ,greek letter answers to his ,christian
 name4 ,the names of the seven stars of 
the ,dipper are given in ,fig4 #cff4    
,when the letters of the ,greek alphabet
 are exhausted1 those of the ,roman     
alphabet are employed4 ,the fainter     
stars in a constellation are usually    
designated by some system of numbers4   
,illustration3 ,fig4 #cff4  #cbf4 ,the  
,milky-,way1 or ,galaxy4--,the          
,milky-,way is a faint luminous band1 of
 irregular outline1 which surrounds the 
heavens with a great circle1 as shown in
 ,fig4 #cfg4 ,through a considerable    
portion of its course it is divided into
 two branches1 and there are various    

vacant spaces at different points in    
this band2 but at only one point in the 
southern hemisphere is it entirely      
interrupted4  ,illustration3 ,fig4 #cfg4
  ,the telescope shows that the ,galaxy 
arises from the light of countless stars
 too minute to be separately visible    
with the naked eye4 ,the telescopic     
stars1 instead of being uniformly       
distributed over the celestial sphere1  
are mostly condensed in the region of   
the ,galaxy4 ,they are fewest in the    
regions most distant from this belt1 and
 become thicker as we approach it4 ,the 
greater the telescopic power1 the more  
marked is the condensation4 ,with the   
naked eye the condensation is hardly    
noticeable2 but with the aid of a very  
small telescope1 we see a decided       
thickening of the stars in and near the 
,galaxy1 while the most powerful        
telescopes show that a large majority of
 the stars lie actually in the ,galaxy4 
,if all the stars visible with a        
twelve-inch telescope were blotted out1 

we should find that the greater part of 
those remaining were in the ,galaxy4    
,illustration3 ,fig4 #cfh4  ,the        
increase in the number of the stars of  
all magnitudes as we approach the plane 
of the ,milky-,way is shown in ,fig4    
#cfh4 ,the curve acb shows by its height
 the distribution of the stars above the
 ninth magnitude1 and the curve ,,acb   
those of all magnitudes4  #cbg4         
,star-,clusters4--,besides this gradual 
and regular condensation towards the    
,galaxy1 occasional aggregations of     
stars into clusters may be seen4 ,some  
of these are visible to the naked eye1  
sometimes as separate stars1 like the   
,seven ,stars1 or ,pleiades1 but more   
commonly as patches of diffused light1  
the stars being too small to be seen    
separately4 ,the number visible in      
powerful telescopes is1 however1 much   
greater4 ,sometimes hundreds or even    
thousands of stars are visible in the   
field of view at once1 and sometimes the
 number is so great that they cannot be 

counted4  #cbh4 ,nebul4--,another class 
of objects which are found in the       
celestial spaces are irregular masses of
 soft1 cloudy light1 known as nebul4    
,many objects which look like nebul in  
small telescopes are shown by more      
powerful instruments to be really       
star-clusters4 ,but many of these       
objects are not composed of stars at    
all1 being immense masses of gaseous    
matter4  ,illustration3 ,fig4 #cfi4     
,the general distribution of nebul is   
the reverse of that of the stars4 ,nebul
 are thickest where stars are thinnest4 
,while stars are most numerous in the   
region of the ,milky-,way1 nebul are    
most abundant about the poles of the    
,milky-,way4 ,this condensation of nebul
 about the poles of the ,milky-,way is  
shown in ,figs4 #cfg and #cfi1 in which 
the points represent1 not stars1 but    
nebul4                                  
,,ii4 ,,the ,,stars4                    
,the ,constellations4   ,illustration3  
,fig4 #cgj4  ,illustration3 ,fig4 #cga4 

#cbi4 ,the ,great ,bear4--,the ,great   
,bear1 or ,ursa ,major1 is one of the   
circumpolar constellations 7#d71 and    
contains one of the most familiar       
asterisms1 or groups of stars1 in our   
sky2 namely1 the ,great ,dipper1 or     
,charles's ,wain4 ,the positions and    
names of the seven prominent stars in it
 are shown in ,fig4 #cgj4 ,the two stars
 ,alpha and ,beta are called the        
,pointers4 ,this asterism is sometimes  
called the ,butcher's ,cleaver4 ,the    
whole constellation is shown in ,fig4   
#cga4 ,a rather faint star marks the    
nose of the bear1 and three equidistant 
pairs of faint stars mark his feet4     
#ccj4 ,the ,little ,bear1 ,draco1 and   
,cassiopeia4--,these are all circumpolar
 constellations4 ,the most important    
star of the ,little ,bear1 or ,ursa     
,minor1 is ,polaris1 or the ,pole ,star4
 ,this star may be found by drawing a   
line from ,beta to ,alpha of the        
,dipper1 and prolonging it as shown in  
,fig4 #cgb4 ,this explains why these    

stars are called the ,pointers4 ,the    
,pole ,star1 with the six other chief   
stars of the ,little ,bear1 form an     
asterism called the ,little ,dipper4    
,these six stars are joined with        
,polaris by a dotted line in ,fig4 #cgb4
  ,illustration3 ,fig4 #cgb4  ,the stars
 in a serpentine line between the two   
,dippers are the chief stars of ,draco1 
or the ,dragon2 the trapezium marking   
its head4 ,fig4 #cgc shows the          
constellations of ,ursa ,minor and      
,draco as usually figured4              
,illustration3 ,fig4 #cgc4  ,to find    
,cassiopeia1 draw a line from ,delta of 
the ,dipper to ,polaris1 and prolong it 
about an equal distance beyond1 as shown
 in ,fig4 #cgb4 ,this line will pass    
near ,alpha of ,cassiopeia4 ,the five   
principal stars of this constellation   
form an irregular ,w1 opening towards   
the pole4 ,between ,cassiopeia and      
,draco are five rather faint stars1     
which form an irregular ,k4 ,these are  
the principal stars of the constellation

 ,cepheus4 ,these two constellations are
 shown in ,fig4 #cgd4  ,illustration3   
,fig4 #cgd4  ,illustration3 ,fig4 #cge4 
#cca4 ,the ,lion1 ,berenice's ,hair1 and
 the ,hunting-,dogs4--,a line drawn from
 ,alpha to ,beta of the ,dipper1 and    
prolonged as shown in ,fig4 #cge1 will  
pass between the two stars ,denebola and
 ,regulus of ,leo1 or the ,lion4        
,regulus forms a sickle with several    
other faint stars1 and marks the heart  
of the lion4 ,denebola is at the apex of
 a right-angled triangle1 which it forms
 with two other stars1 and marks the end
 of the lion's tail4 ,this constellation
 is visible in the evening from         
,february to ,july1 and is figured in   
,fig4 #cgf4  ,illustration3 ,fig4 #cgf4 
,in a straight line between ,denebola   
and ,eta1 at the end of the ,great      
,bear's tail1 are1 at about equal       
distances1 the two small constellations 
of ,coma ,berenices1 or ,berenice's     
,hair1 and ,canes ,venatici1 or the     
,hunting-,dogs4 ,these are shown in     

,fig4 #cgg4 ,the dogs are represented as
 pursuing the bear1 urged on by the     
huntsman ,botes4  ,illustration3 ,fig4  
#cgg4  #ccb4 ,botes1 ,hercules1 and the 
,northern ,crown4--,arcturus1 the       
principal star of ,botes1 may be found  
by drawing a line from ,zeta to ,eta of 
the ,dipper1 and then prolonging it with
 a slight bend1 as shown in ,fig4 #cgh4 
,arcturus and ,polaris form a large     
isosceles triangle with a               
first-magnitude star called ,vega4 ,this
 triangle encloses at one corner the    
principal stars of ,botes1 and the head 
of the ,dragon near the opposite side4  
,the side running from ,arcturus to     
,vega passes through ,corona ,borealis1 
or the ,northern ,crown1 and the body of
 ,hercules1 which is marked by a        
quadrilateral of four stars4            
,illustration3 ,fig4 #cgh4  ,botes1 who 
is often represented as a husbandman1   
,corona ,borealis1 and ,hercules1 are   
delineated in ,fig4 #cgi4 ,these        
constellations are visible in the       

evening from ,may to ,september4        
,illustration3 ,fig4 #cgi4              
,illustration3 ,fig4 #chj4  #ccc4 ,the  
,lyre1 the ,swan1 the ,eagle1 and the   
,dolphin4--,altair1 the principal star  
of ,aquila1 or the ,eagle1 lies on the  
opposite side of the ,milky-,way from   
,vega4 ,altair is a first-magnitude     
star1 and has a faint star on each side 
of it1 as shown in ,fig4 #chj4 ,vega1   
also of the first magnitude1 is the     
principal star of ,lyra1 or the ,lyre4  
,between these two stars1 and a little  
farther to the north1 are several stars 
arranged in the form of an immense      
cross4 ,the bright star at the head of  
this cross is called ,deneb4 ,the cross 
lies in the ,milky-,way1 and contains   
the chief stars of the constellation    
,cygnus1 or the ,swan4 ,a little to the 
north of ,altair are four stars in the  
form of a diamond4 ,this asterism is    
popularly known as ,job's ,coffin4      
,these four stars are the chief stars of
 ,delphinus1 or the ,dolphin4 ,these    

four constellations are shown together  
in ,fig4 #cha4 ,the ,swan is visible    
from ,june to ,december1 in the evening4
  ,illustration3 ,fig4 #cha4  #ccd4     
,virgo4--,a line drawn from ,alpha to   
,gamma of the ,dipper1 and prolonged    
with a slight bend at ,gamma1 will reach
 to a first-magnitude star called ,spica
 7,fig4 #chb74 ,this is the chief star  
of the constellation ,virgo1 or the     
,virgin1 and forms a large isosceles    
triangle with ,arcturus and ,denebola4  
,illustration3 ,fig4 #chb4  ,virgo is   
represented in ,fig4 #chc4 ,to the right
 of this constellation1 as shown in the 
figure1 there are four stars which form 
a trapezium1 and mark the constellation 
,corvus1 or the ,crow4 ,this bird is    
represented as standing on the body of  
,hydra1 or the ,water-,snake4 ,virgo is 
visible in the evening1 from ,april to  
,august4  ,illustration3 ,fig4 #chc4    
,illustration3 ,fig4 #chd4              
,illustration3 ,fig4 #che4  #cce4 ,the  
,twins4--,a line drawn from ,delta to   

,beta of the ,dipper1 and prolonged as  
shown in ,fig4 #chd1 passes between two 
bright stars called ,castor and ,pollux4
 ,the latter of these is usually        
reckoned as a first-magnitude star4     
,these are the principal stars of the   
constellation ,gemini1 or the ,twins1   
which is shown in ,fig4 #che4 ,the      
constellation ,canis ,minor1 or the     
,little ,dog1 is shown in the lower part
 of the figure4 ,there are two          
conspicuous stars in this constellation1
 the brightest of which is of the first 
magnitude1 and called ,procyon4  ,the   
region to which we have now been brought
 is the richest of the northern sky1    
containing no less than seven           
first-magnitude stars4 ,these are       
,sirius1 ,procyon1 ,pollux1 ,capella1   
,aldebaran1 ,betelgeuse1 and ,rigel4    
,they are shown in ,fig4 #chf4          
,illustration3 ,fig4 #chf4  ,betelgeuse 
and ,rigel are in the constellation     
,orion1 being about equally distant to  
the north and south from the three stars

 forming the belt of ,orion4 ,betelgeuse
 is a red star4 ,sirius is the brightest
 star in the heavens1 and belongs to the
 constellation ,canis ,major1 or the    
,great ,dog4 ,it lies to the east of the
 belt of ,orion4 ,aldebaran lies at     
about the same distance to the west of  
the belt4 ,it is a red star1 and belongs
 to the constellation ,taurus1 or the   
,bull4 ,capella is in the constellation 
,auriga1 or the ,wagoner4 ,these stars  
are visible in the evening1 from about  
,december to ,april4  #ccf4 ,orion and  
his ,dogs1 and ,taurus4--,orion and his 
,dogs are shown in ,fig4 #chg1 and      
,orion and ,taurus in ,fig4 #chh4       
,aldebaran marks one of the eyes of the 
bull1 and is often called the ,bull's   
,eye4 ,the irregular ,v in the face of  
the bull is called the ,hyades1 and the 
cluster on the shoulder the ,pleiades4  
,illustration3 ,fig4 #chg4              
,illustration3 ,fig4 #chh4              
,illustration3 ,fig4 #chi4  #ccg4 ,the  
,wagoner4--,the constellation ,auriga1  

or the ,wagoner 7sometimes called the   
,charioteer71 is shown in ,fig4 #chi4   
,capella marks the ,goat1 which he is   
represented as carrying on his back1 and
 the little right-angled triangle of    
stars near it the ,kids4 ,the five chief
 stars of this constellation form a     
large1 irregular pentagon4 ,gamma of    
,auriga is also ,beta of ,taurus1 and   
marks one of the horns of the ,bull4    
,illustration3 ,fig4 #cij4  #cch4       
,pegasus1 ,andromeda1 and ,perseus4--,a 
line drawn from ,polaris near to ,beta  
of ,cassiopeia will lead to a bright    
second-magnitude star at one corner of a
 large square 7,fig4 #cij74 ,alpha      
belongs both to the ,square of ,pegasus 
and to ,andromeda4 ,beta and ,gamma1    
which are connected with ,alpha in the  
figure by a dotted line1 also belong to 
,andromeda4 ,algol1 which forms1 with   
the last-named stars and with the       
,square of ,pegasus1 an asterism similar
 in configuration to the ,great ,dipper1
 belongs to ,perseus4 ,algenib1 which is

 reached by bending the line at ,gamma  
in the opposite direction1 is the       
principal star of ,perseus4             
,illustration3 ,fig4 #cia4              
,illustration3 ,fig4 #cib4              
,illustration3 ,fig4 #cic4  ,pegasus is 
shown in ,fig4 #cia1 and ,andromeda in  
,fig4 #cib4 ,cetus1 the ,whale1 or the  
,sea ,monster1 shown in ,fig4 #cic1     
belongs to the same mythological group  
of constellations4  ,illustration3 ,fig4
 #cid4  #cci4 ,scorpio1 ,sagittarius1   
and ,ophiuchus4--,during the summer     
months a brilliant constellation is     
visible1 called ,scorpio1 or the        
,scorpion4 ,the configuration of the    
chief stars of this constellation is    
shown in ,fig4 #cid4 ,they bear some    
resemblance to a boy's kite4 ,the       
brightest star is of the first          
magnitude1 and called ,antares 7from    
anti1 instead of1 and ,ares1 the ,greek 
name of ,mars71 because it rivals ,mars 
in redness4 ,the stars in the tail of   
the ,scorpion are visible in our        

latitude only under very favorable      
circumstances4 ,this constellation is   
shown in ,fig4 #cie1 together with      
,sagittarius and ,ophiuchus4            
,sagittarius1 or the ,archer1 is to the 
east of ,scorpio4 ,it contains no bright
 stars1 but is easily recognized from   
the fact that five of its principal     
stars form the outline of an inverted   
dipper1 which1 from the fact of its     
being partly in the ,milky-,way1 is     
often called the ,milk ,dipper4         
,illustration3 ,fig4 #cie4  ,ophiuchus1 
or the ,serpent-,bearer1 is a large     
constellation1 filling all the space    
between the head of ,hercules and       
,scorpio4 ,it is difficult to trace1    
since it contains no very brilliant     
stars4 ,this constellation and ,libra1  
or the ,balances1 which is the zodiacal 
constellation to the west of ,scorpio1  
are shown in ,fig4 #cif4  ,illustration3
 ,fig4 #cif4  ,illustration3 ,fig4 #cig4
  #cdj4 ,capricornus1 ,aquarius1 and the
 ,southern ,fish4--,the two zodiacal    

constellations to the east of           
,sagittarius are ,capricornus and       
,aquarius4 ,capricornus contains three  
pairs of small stars1 which mark the    
head1 the tail1 and the knees of the    
animal4  ,aquarius is marked by no      
conspicuous stars4 ,an irregular line of
 minute stars marks the course of the   
stream of water which flows from the    
,water-,bearer's ,urn into the mouth of 
the ,southern ,fish4 ,this mouth is     
marked by the first-magnitude star      
,fomalhaut4 ,these constellations are   
shown in ,fig4 #cig4  ,illustration3    
,fig4 #cih4  #cda4 ,pisces and          
,aries4--,the remaining zodiacal        
constellations are ,pisces1 or the      
,fishes1 ,aries1 or the ,ram 7,fig4     
#cih71 and ,cancer1 or the ,crab4  ,the 
,fishes lie under ,pegasus and          
,andromeda1 but contain no bright stars4
 ,aries 7between ,pisces and ,taurus7 is
 marked by a pair of stars on the       
head1--one of the second1 and one of the
 third magnitude4 ,cancer 7between ,leo 

and ,gemini7 has no bright stars1 but   
contains a remarkable cluster of small  
stars called ,prsepe1 or the ,beehive4  
,clusters4   #cdb4 ,the ,hyades4--,the  
,hyades are a very open cluster in the  
face of ,taurus 7#ccd74 ,the three      
brightest stars of this cluster form a  
letter ,v1 the point of the ,v being on 
the nose1 and the open ends at the eyes4
 ,this cluster is shown in ,fig4 #cii4  
,the name1 according to the most        
probable etymology1 means rainy2 and    
they are said to have been so called    
because their rising was associated with
 wet weather4 ,they were usually        
considered the daughters of ,atlas1 and 
sisters of the ,pleiades1 though        
sometimes referred to as the nurses of  
,bacchus4  ,illustration3 ,fig4 #cii4   
#cdc4 ,the ,pleiades4--,the ,pleiades   
constitute a celebrated group of stars1 
or a miniature constellation1 on the    
shoulder of ,taurus4 ,hesiod mentions   
them as the seven virgins of ,atlas     
born1 and ,milton calls them the seven  

,atlantic sisters4 ,they are referred to
 in the ,book of ,job4 ,the ,spaniards  
term them the little nanny-goats2 and   
they are sometimes called the hen and   
chickens4  ,illustration3 ,fig4 #djj4   
,illustration3 ,fig4 #dja4  ,usually    
only six stars in this cluster can be   
seen with the naked eye1 and this fact  
has given rise to the legend of the lost
 ,pleiad4 ,on a clear1 moonless night1  
however1 a good eye can discern seven or
 eight stars1 and some observers have   
distinguished as many as eleven4 ,fig4  
#djj shows the ,pleiades as they appear 
to the naked eye under the most         
favorable circumstances4 ,fig4 #dja     
shows this cluster as it appears in a   
powerful telescope4 ,with such an       
instrument more than five hundred stars 
are visible4  #cdd4 ,cluster in the     
,sword-handle of ,perseus4--,this is a  
somewhat dense double cluster4 ,it is   
visible to the naked eye1 appearing as a
 hazy star4 ,a line drawn from ,algenib1
 or ,alpha of ,perseus 7#cch71 to ,delta

 of ,cassiopeia 7#ccj71 will pass       
through this cluster at about two-thirds
 the distance from the former4 ,this    
double cluster is one of the most       
brilliant objects in the heavens1 with a
 telescope of moderate power4           
,illustration3 ,fig4 #djb4  #cde4       
,cluster of ,hercules4--,the celebrated 
globular cluster of ,hercules can be    
seen only with a telescope of           
considerable power1 and to resolve it   
into distinct stars 7as shown in ,fig4  
#djb7 requires an instrument of the very
 highest class4  ,illustration3 ,fig4   
#djc4  #cdf4 ,other ,clusters4--,fig4   
#djc shows a magnificent globular       
cluster in the constellation ,aquarius4 
,herschel describes it as appearing like
 a heap of sand1 being composed of      
thousands of stars of the fifteenth     
magnitude4  ,illustration3 ,fig4 #djd4  
,fig4 #djd shows a cluster in the       
constellation ,toucan1 which ,sir ,john 
,herschel describes as a most glorious  
globular cluster1 the stars of the      

fourteenth magnitude being immensely    
numerous4 ,there is a marked            
condensation of light at the centre4    
,illustration3 ,fig4 #dje4              
,illustration3 ,fig4 #djf4  ,fig4 #dje  
shows a cluster in the ,centaur1 which1 
according to the same astronomer1 is    
beyond comparison the richest and       
largest object of the kind in the       
heavens1 the stars in it being literally
 innumerable4 ,fig4 #djf shows a cluster
 in ,scorpio1 remarkable for the        
peculiar arrangement of its component   
stars4  ,star clusters are especially   
abundant in the region of the           
,milky-,way1 the law of their           
distribution being the reverse of that  
of the nebul4                           
,double and ,multiple ,stars4   #cdg4   
,double ,stars4--,the telescope shows   
that many stars which appear single to  
the naked eye are really double1 or     
composed of a pair of stars lying side  
by side4 ,there are several pairs of    
stars in the heavens which lie so near  

together that they almost seem to touch 
when seen with the naked eye4           
,illustration3 ,fig4 #djg4              
,illustration3 ,fig4 #djh4  ,pairs of   
stars are not considered double unless  
the components are so near together that
 they both appear in the field of view  
when examined with a telescope4 ,in the 
majority of the pairs classed as double 
stars the distance between the          
components ranges from half a second to 
fifteen seconds4  ,illustration3 ,fig4  
#dji4  ,epsilon ,lyr is a good example  
of a pair of stars that can barely be   
separated with a good eye4 ,figs4 #djg  
and #djh show this pair as it appears in
 telescopes magnifying respectively four
 and fifteen times2 and ,fig4 #dji shows
 it as seen in a more powerful          
telescope1 in which each of the two     
components of the pair is seen to be a  
truly double star4  ,illustration3 ,fig4
 #daj4  ,illustration3 ,fig4 #daa4      
#cdh4 ,multiple ,stars4--,when a star is
 resolved into more than two components 

by a telescope1 it is called a multiple 
star4 ,fig4 #daj shows a triple star in 
,pegasus4 ,fig4 #daa shows a quadruple  
star in ,taurus4 ,fig4 #dab shows a     
sextuple star1 and ,fig4 #dac a septuple
 star4 ,fig4 #dad shows the celebrated  
septuple star in ,orion1 called ,theta  
,orionis1 or the trapezium of ,orion4   
#cdi4 ,optically ,double and ,multiple  
,stars4--,two or more stars which are   
really very distant from each other1 and
 which have no physical connection      
whatever1 may appear to be near         
together1 because they happen to lie in 
the same direction1 one behind the      
other4 ,such accidental combinations are
 called optically double or multiple    
stars4  ,illustration3 ,fig4 #dab4      
,illustration3 ,fig4 #dac4  #cej4       
,physically ,double and ,multiple       
,stars4--,in the majority of cases the  
components of double and multiple stars 
are in reality comparatively near       
together1 and are bound together by     
gravity into a physical system4 ,such   

combinations are called physically      
double and multiple stars4 ,the         
components of these systems all revolve 
around their common centre of gravity4  
,in many instances their orbits and     
periods of revolution have been         
ascertained by observation and          
calculation4 ,fig4 #dae shows the orbit 
of one of the components of a double    
star in the constellation ,hercules4    
,illustration3 ,fig4 #dad4  #cea4       
,colors of ,double and ,multiple        
,stars4--,the components of double and  
multiple stars are often highly colored1
 and frequently the components of the   
same system are of different colors4    
,sometimes one star of a binary system  
is white1 and the other red2 and        
sometimes a white star is combined with 
a blue one4 ,other colors found in      
combination in these systems are red and
 blue1 orange and green1 blue and green1
 yellow and blue1 yellow and red1 etc4  
,illustration3 ,fig4 #dae4  ,if these   
double and multiple stars are           

accompanied by planets1 these planets   
will sometimes have two or more suns in 
the sky at once4 ,on alternate days they
 may have suns of different colors1 and 
perhaps on the same day two suns of     
different colors4 ,the effect of these  
changing colored lights on the landscape
 must be very remarkable4               
,new and ,variable ,stars4   #ceb4      
,variable ,stars4--,there are many stars
 which undergo changes of brilliancy1   
sometimes slight1 but occasionally very 
marked4 ,these changes are in some cases
 apparently irregular1 and in others    
periodic4 ,all such stars are said to be
 variable1 though the term is applied   
especially to those stars whose         
variability is periodic4  ,illustration3
 ,fig4 #daf4  #cec4 ,algol4--,algol1 a  
star of ,perseus1 whose position is     
shown in ,fig4 #daf1 is a remarkable    
variable star of a short period4        
,usually it shines as a faint           
second-magnitude star2 but at intervals 
of a little less than three days it     

fades to the fourth magnitude for a few 
hours1 and then regains its former      
brightness4 ,these changes were first   
noticed some two centuries ago1 but it  
was not till #aghb that they were       
accurately observed4 ,the period is now 
known to be two days1 twenty hours1     
forty-nine minutes4 ,it takes about four
 hours and a half to fade away1 and four
 hours more to recover its brilliancy4  
,near the beginning and end of the      
variations1 the change is very slow1 so 
that there are not more than five or six
 hours during which an ordinary observer
 would see that the star was less bright
 than usual4  ,this variation of light  
was at first explained by supposing that
 a large dark planet was revolving round
 ,algol1 and passed over its face at    
every revolution1 thus cutting off a    
portion of its light2 but there are     
small irregularities in the variation1  
which this theory does not account for4 
#ced4 ,mira4--,another remarkable       
variable star is ,omicron ,ceti1 or     

,mira 7that is1 the wonderful star74 ,it
 is generally invisible to the naked    
eye2 but at intervals of about eleven   
months it shines forth as a star of the 
second or third magnitude4 ,it is about 
forty days from the time it becomes     
visible until it attains its greatest   
brightness1 and is then about two months
 in fading to invisibility2 so that its 
increase of brilliancy is more rapid    
than its waning4 ,its period is quite   
irregular1 ranging from ten to twelve   
months2 so that the times of its        
appearance cannot be predicted with     
certainty4 ,its maximum brightness is   
also variable1 being sometimes of the   
second magnitude1 and at others only of 
the third or fourth4  ,illustration3    
,fig4 #dag4  #cee4 ,eta                 
,argus4--,perhaps the most extraordinary
 variable star in the heavens is ,eta   
,argus1 in the constellation ,argo1 or  
the ,ship1 in the southern hemisphere   
7,fig4 #dag74 ,the first careful        
observations of its variability were    

made by ,sir ,john ,herschel while at   
the ,cape of ,good ,hope4 ,he says1 ,it 
was on the #afth of ,december1 #ahcg1   
that1 resuming the photometrical        
comparisons1 my astonishment was excited
 by the appearance of a new candidate   
for distinction among the very brightest
 stars of the first magnitude in a part 
of the heavens where1 being perfectly   
familiar with it1 ,i was certain that no
 such brilliant object had before been  
seen4 ,after a momentary hesitation1 the
 natural consequence of a phenomenon so 
utterly unexpected1 and referring to a  
map for its configuration with other    
conspicuous stars in the neighborhood1  
,i became satisfied of its identity with
 my old acquaintance1 ,eta ,argus4 ,its 
light was1 however1 nearly tripled4     
,while yet low1 it equalled ,rigel1 and1
 when it attained some altitude1 was    
decidedly greater4 ,it continued to     
increase until ,jan4 #b1 #ahch1 then    
faded a little till ,april following1   
though it was still as bright as        

,aldebaran4 ,in #ahdb and #ahdc it      
blazed up brighter than ever1 and in    
,march of the latter year was second    
only to ,sirius4 ,during the twenty-five
 years following it slowly but steadily 
diminished4 ,in #ahfg it was barely     
visible to the naked eye2 and the next  
year it vanished entirely from the      
unassisted view1 and has not yet begun  
to recover its brightness4 ,the curve in
 ,fig4 #dah shows the change in         
brightness of this remarkable star4 ,the
 numbers at the bottom show the years of
 the century1 and those at the side the 
brightness of the star4  ,illustration3 
,fig4 #dah4  #cef4 ,new ,stars4--,in    
several cases stars have suddenly       
appeared1 and even become very          
brilliant2 then1 after a longer or      
shorter time1 they have faded away and  
disappeared4 ,such stars are called new 
or temporary stars4 ,for a time it was  
supposed that such stars were actually  
new4 ,they are now1 however1 classified 
by astronomers among the variable stars1

 their changes being of a very irregular
 and fitful character4 ,there is        
scarcely a doubt that they were all in  
the heavens as very small stars before  
they blazed forth in so extraordinary a 
manner1 and that they are in the same   
places still4 ,there is a wide          
difference between these irregular      
variations1 or the breaking-forth of    
light on a single occasion in the course
 of centuries1 and the regular and      
periodic changes in the case of a star  
like ,algol2 but a long series of       
careful observation has resulted in the 
discovery of stars of nearly every      
degree of irregularity between these two
 extremes4 ,some of them change         
gradually from one magnitude to another1
 in the course of years1 without seeming
 to follow any law whatever2 while in   
others some slight tendency to          
regularity can be traced4 ,eta ,argus   
may be regarded as a connecting link    
between new and variable stars4  #ceg4  
,tycho ,brahe's ,star4--,an apparently  

new star suddenly appeared in           
,cassiopeia in #aegb4 ,it was first seen
 by ,tycho ,brahe1 and is therefore     
associated with his name4 ,its position 
in the constellation is shown in ,fig4  
#dai4 ,it was first seen on ,nov4 #aa1  
when it had already attained the first  
magnitude4 ,it became rapidly brighter1 
soon rivalling ,venus in splendor1 so   
that good eyes could discern it in full 
daylight4 ,in ,december it began to     
wane1 and gradually faded until the     
following ,may1 when it disappeared     
entirely4  ,illustration3 ,fig4 #dai4   
,a star showed itself in the same part  
of the heavens in #ide and in #abfd4 ,if
 these were three appearances of the    
same star1 it must be reckoned as a     
periodic star with a period of a little 
more than three hundred years4  #ceh4   
,kepler's ,star4--,in #afjd a new star  
was seen in the constellation           
,ophiuchus4 ,it was first noticed in    
,october of that year1 when it was of   
the first magnitude4 ,in the following  

winter it began to fade1 but remained   
visible during the whole year #afje4    
,early in #afjf it disappeared entirely4
 ,a very full history of this star was  
written by ,kepler4  ,one of the most   
remarkable things about this star was   
its brilliant scintillation4 ,according 
to ,kepler1 it displayed all the colors 
of the rainbow1 or of a diamond cut with
 multiple facets1 and exposed to the    
rays of the sun4 ,it is thought that    
this star also appeared in #cic1 #gih1  
and #abjc2 if so1 it is a variable star 
with a period of a little over four     
hundred years4  #cei4 ,new ,star of     
#ahff4--,the most striking case of this 
kind in recent times was in ,may1 #ahff1
 when a star of the second magnitude    
suddenly appeared in ,corona ,borealis4 
,on the #aath and #abth of that month it
 was observed independently by at least 
five observers in ,europe and ,america4 
,the fact that none of these new stars  
were noticed until they had nearly or   
quite attained their greatest brilliancy

 renders it probable that they all      
blazed up very suddenly4  #cfj4 ,cause  
of the ,variability of ,stars4--,the    
changes in the brightness of variable   
and temporary stars are probably due to 
operations similar to those which       
produce the spots and prominences in our
 sun4 ,we have seen 7#ahh7 that the     
frequency of solar spots shows a period 
of eleven years1 during one portion of  
which there are few or no spots to be   
seen1 while during another portion they 
are numerous4 ,if an observer so far    
away as to see our sun like a star could
 from time to time measure its light    
exactly1 he would find it to be a       
variable star with a period of eleven   
years1 the light being least when we see
 most spots1 and greatest when few are  
visible4 ,the variation would be slight1
 but it would nevertheless exist4 ,now1 
we have reason to believe that the      
physical constitution of the sun and the
 stars is of the same general nature4   
,it is therefore probable1 that1 if we  

could get a nearer view of the stars1 we
 should see spots on their disks as we  
do on the sun4 ,it is also likely that  
the varying physical constitution of the
 stars might give rise to great         
differences in the number and size of   
the spots2 so that the light of some of 
these suns might vary to a far greater  
degree than that of our own sun does4   
,if the variations had a regular period1
 as in the case of our sun1 the         
appearances to a distant observer would 
be precisely what we see in the case of 
a periodic variable star4  ,the spectrum
 of the new star of #ahff was found to  
be a continuous one1 crossed by bright  
lines1 which were apparently due to     
glowing hydrogen4 ,the continuous       
spectrum was also crossed by dark lines1
 indicating that the light had passed   
through an atmosphere of comparatively  
cool gas4 ,mr4 ,huggins infers from this
 that there was a sudden and            
extraordinary outburst of hydrogen gas  
from the star1 which by its own light1  

as well as by heating up the whole      
surface of the star1 caused the         
extraordinary increase of brilliancy4   
,now1 the spectroscope shows that the   
red flames of the solar chromosphere    
7#aig7 are largely composed of hydrogen2
 and it is not unlikely that the        
blazing-forth of this star arose from an
 action similar to that which produces  
these flames1 only on an immensely      
larger scale4                           
,distance of the ,stars4   #cfa4        
,parallax of the ,stars4--,such is the  
distance of the stars1 that only in a   
comparatively few instances has any     
displacement of these bodies been       
detected when viewed from opposite parts
 of the earth's orbit1 that is1 from    
points a hundred and eighty-five million
 miles apart2 and in no case can this   
displacement be detected except by the  
most careful and delicate measurement4  
,half of the above displacement1 or the 
displacement of the star as seen from   
the earth instead of the sun1 is called 

the parallax of the star4 ,in no case   
has a parallax of one second as yet been
 detected4  #cfb4 ,the ,distance of the 
,stars4--,the distance of a star whose  
parallax is one second would be #bjf1bfe
 times the distance of the earth from   
the sun1 or about nineteen million      
million miles4 ,it is quite certain that
 no star is nearer than this to the     
earth4 ,light has a velocity which would
 carry it seven times and a half around 
the earth in a second2 but it would take
 it more than three years to reach us   
from that distance4 ,were all the stars 
blotted out of existence to-night1 it   
would be at least three years before we 
should miss a single one4  ,alpha       
,centauri1 the brightest star in the    
constellation of the ,centaur1 is1 so   
far as we know1 the nearest of the fixed
 stars4 ,it is estimated that it would  
take its light about three years and a  
half to reach us4 ,it has also been     
estimated that it would take light over 
sixteen years to reach us from ,sirius1 

about eighteen years to reach us from   
,vega1 about twenty-five years from     
,arcturus1 and over forty years from the
 ,pole-,star4 ,in many instances it is  
believed that it would take the light of
 stars hundreds of years to make the    
journey to our earth1 and in some       
instances even thousands of years4      
,proper ,motion of the ,stars4   #cfc4  
,why the ,stars appear ,fixed4--,the    
stars seem to retain their relative     
positions in the heavens from year to   
year1 and from age to age2 and hence    
they have come universally to be        
denominated as fixed4 ,it is1 however1  
now well known that the stars1 instead  
of being really stationary1 are moving  
at the rate of many miles a second2 but 
their distance is so enormous1 that1 in 
the majority of cases1 it would be      
thousands of years before this rate of  
motion would produce a sufficient       
displacement to be noticeable to the    
unaided eye4  ,illustration3 ,fig4 #dbj4
  #cfd4 ,secular ,displacement of the   

,stars4--,though the proper motion of   
the stars is apparently slight1 it will1
 in the course of many ages1 produce a  
marked change in the configuration of   
the stars4 ,thus1 in ,fig4 #dbj1 the    
left-hand portion shows the present     
configuration of the stars of the ,great
 ,dipper4 ,the small arrows attached to 
the stars show the direction and        
comparative magnitudes of their motion4 
,the right-hand portion of the figure   
shows these stars as they will appear   
thirty-six thousand years from the      
present time4  ,illustration3 ,fig4     
#dba4  ,fig4 #dba shows in a similar way
 the present configuration and proper   
motion of the stars of ,cassiopeia1 and 
also these stars as they will appear    
thirty-six thousand years hence4        
,illustration3 ,fig4 #dbb4  ,fig4 #dbb  
shows the same for the constellation    
,orion4  #cfe4 ,the ,secular ,motion of 
the ,sun4--,the stars in all parts of   
the heavens are found to move in all    
directions and with all sorts of        

velocities4 ,when1 however1 the motions 
of the stars are averaged1 there is     
found to be an apparent proper motion   
common to all the stars4 ,the stars in  
the neighborhood of ,hercules appear to 
be approaching us1 and those in the     
opposite part of the heavens appear to  
be receding from us4 ,in other words1   
all the stars appear to be moving away  
from ,hercules1 and towards the opposite
 part of the heavens4  ,illustration3   
,fig4 #dbc4  ,this apparent motion      
common to all the stars is held by      
astronomers to be due to the real motion
 of the sun through space4 ,the point in
 the heavens towards which our sun is   
moving at the present time is indicated 
by the small circle in the constellation
 ,hercules in ,fig4 #dbc4 ,as the sun   
moves1 he carries the earth and all the 
planets along with him4 ,fig4 #dbd shows
 the direction of the sun's motion with 
reference to the ecliptic and to the    
axis of the earth4 ,fig4 #dbe shows the 
earth's path in space2 and ,fig4 #dbf   

shows the paths of the earth1 the moon1 
,mercury1 ,venus1 and ,mars in space4   
,illustration3 ,fig4 #dbd4              
,illustration3 ,fig4 #dbe4              
,illustration3 ,fig4 #dbf4  ,whether the
 sun is actually moving in a straight   
line1 or around some distant centre1 it 
is impossible to determine at the       
present time4 ,it is estimated that the 
sun is moving along his path at the rate
 of about a hundred and fifty million   
miles a year4 ,this is about five-sixths
 of the diameter of the earth's orbit4  
#cff4 ,star-,drift4--,in several        
instances1 groups of stars have a common
 proper motion entirely different from  
that of the stars around and among them4
 ,such groups probably form connected   
systems1 in the motion of which all the 
stars are carried along together without
 any great change in their relative     
positions4 ,the most remarkable case of 
this kind occurs in the constellation   
,taurus4 ,a large majority of the       
brighter stars in the region between    

,aldebaran and the ,pleiades have a     
common proper motion of about ten       
seconds per century towards the east4   
,proctor has shown that five out of the 
seven stars which form the ,great       
,dipper have a common proper motion1 as 
shown in ,fig4 #dbg 7see also ,fig4     
#dbj74 ,he proposes for this phenomenon 
the name of ,star-,drift4               
,illustration3 ,fig4 #dbg4  #cfg4       
,motion of ,stars along the ,line of    
,sight4--,a motion of a star in the     
direction of the line of sight would    
produce no displacement of the star that
 could be detected with the telescope2  
but it would cause a change in the      
brightness of the star1 which would     
become gradually fainter if moving from 
us1 and brighter if approaching us4     
,motion along the line of sight has1    
however1 been detected by the use of the
 tele-spectroscope 7#aeb71 owing to the 
fact that it causes a displacement of   
the spectral lines4 ,as has already been
 explained 7#afi71 a displacement of a  

spectral line towards the red end of the
 spectrum indicates a motion away from  
us1 and a displacement towards the      
violet end1 a motion towards us4        
99       99       99       99       99  
,by means of these displacements of the 
spectral lines1 ,huggins has detected   
motion in the case of a large number of 
stars1 and calculated its rate3--       
,,stars ,,receding ,,from ,,us4         
,sirius                  #bj miles per  
second4     ,betelgeuse              #bb
 miles per second4     ,rigel           
#ae miles per second4     ,castor       
#be miles per second4     ,regulus      
#ae miles per second4  ,,stars          
,,approaching ,,us4      ,arcturus      
#ee miles per second4     ,vega         
#ej miles per second4     ,deneb        
#ci miles per second4     ,pollux       
#di miles per second4     ,alpha ,urs   
,majoris      #df miles per second4     
,these results are confirmed by the fact
 that the amount of motion indicated is 
about what we should expect the stars to

 have1 from their observed proper       
motions1 combined with their probable   
distances4 ,again3 the stars in the     
neighborhood of ,hercules are mostly    
found to be approaching the earth1 and  
those which lie in the opposite         
direction to be receding from it2 which 
is exactly the effect which would result
 from the sun's motion through space4   
,the five stars in the ,dipper1 which   
have a common proper motion1 are also   
found to have a common motion in the    
line of sight4 ,but the displacement of 
the spectral lines is so slight1 and its
 measurement so difficult1 that the     
velocities in the above table are to be 
accepted as only an approximation to the
 true values4               ,chemical   
and ,physical ,constitution of the      
,stars4   #cfh4 ,the ,constitution of   
the ,stars ,similar to that of the      
,sun4--,the stellar spectra bear a      
general resemblance to that of the sun1 
with characteristic differences4 ,these 
spectra all show ,fraunhofer's lines1   

which indicate that their luminous      
surfaces are surrounded by atmospheres  
containing absorbent vapors1 as in the  
case of the sun4 ,the positions of these
 lines indicate that the stellar        
atmospheres contain elements which are  
also found in the sun's1 and on the     
earth4  ,illustration3 ,fig4 #dbh4      
#cfi4 ,four ,types of ,stellar          
,spectra4--,the spectra of the stars    
have been carefully observed by ,secchi 
and ,huggins4 ,they have found that     
stellar spectra may be reduced to four  
types1 which are shown in ,fig4 #dbh4   
,in the spectrum of ,sirius1 a          
representative of ,type ,i41 very few   
lines are represented2 but the lines are
 very thick4  ,next we have the solar   
spectrum1 which is a representative of  
,type ,,ii41 one in which more lines are
 represented4 ,in ,type ,,iii4 fluted   
spaces begin to appear1 and in ,type    
,,iv41 which is that of the red stars1  
nothing but fluted spaces is visible2   
and this spectrum shows that something  

is at work in the atmosphere of those   
red stars different from what there is  
in the simpler atmosphere of ,type ,i4  
,lockyer holds that these differences of
 spectra are due simply to differences  
of temperature4 ,according to him1 the  
red stars1 which give the fluted        
spectra1 are of the lowest temperature2 
and the temperature of the stars of the 
different types gradually rises till we 
reach the first type1 in which the      
temperature is so high that the         
dissociation 7#afa7 of the elements is  
nearly if not quite complete4           
,,iii4 ,,nebul4                         
,classification of ,nebul4   #cgj4      
,planetary ,nebul4--,many nebul 7#cbh7  
present a well-defined circular disk1   
like that of a planet1 and are therefore
 called planetary nebul4 ,specimens of  
planetary nebul are shown in ,fig4 #dbi4
  ,illustration3 ,fig4 #dbi4  #cga4     
,circular and ,elliptical               
,nebul4--,while many nebul are circular 
in form1 others are elliptical4 ,the    

former are called circular nebul1 and   
the latter elliptical nebul4 ,elliptical
 nebul have been discovered of every    
degree of eccentricity4 ,examples of    
various circular and elliptical nebul   
are given in ,fig4 #dcj4  ,illustration3
 ,fig4 #dcj4  #cgb4 ,annular            
,nebul4--,occasionally ring-shaped nebul
 have been observed1 sometimes with1 and
 sometimes without1 nebulous matter     
within the ring4 ,they are called       
annular nebul4 ,they are both circular  
and elliptical in form4 ,several        
specimens of this class of nebul are    
given in ,fig4 #dca4  ,illustration3    
,fig4 #dca4  #cgc4 ,nebulous            
,stars4--,sometimes one or more minute  
stars are enveloped in a nebulous haze1 
and are hence called nebulous stars4    
,several of these nebul are shown in    
,fig4 #dcb4  ,illustration3 ,fig4 #dcb4 
#cgd4 ,spiral ,nebul4--,very many nebul 
disclose a more or less spiral          
structure1 and are known as spiral      
nebul4 ,they are illustrated in ,fig4   

#dcc4 ,there are1 however1 a great      
variety of spiral forms4 ,we shall have 
occasion to speak of these nebul again  
7#cha-chc74  ,illustration3 ,fig4 #dcc4 
#cge4 ,double and ,multiple             
,nebul4--,many double and multiple nebul
 have been observed1 some of which are  
represented in ,fig4 #dcd4              
,illustration3 ,fig4 #dcd4  ,fig4 #dce  
shows what appears to be a double       
annular nebula4 ,fig4 #dcf gives two    
views of a double nebula4 ,the change of
 position in the components of this     
double nebula indicates a motion of     
revolution similar to that of the       
components of double stars4             
,illustration3 ,fig4 #dce4              
,illustration3 ,fig4 #dcf4              
,irregular ,nebul4   #cgf4 ,irregular   
,forms4--,besides the more or less      
regular forms of nebul which have been  
classified as indicated above1 there are
 many of very irregular shapes1 and some
 of these are the most remarkable nebul 
in the heavens4 ,fig4 #dcg shows a      

curiously shaped nebula1 seen by ,sir   
,john ,herschel in the southern heavens2
 and ,fig4 #dch1 one in ,taurus1 known  
as the ,crab nebula4  ,illustration3    
,fig4 #dcg4  ,illustration3 ,fig4 #dch4 
#cgg4 ,the ,great ,nebula of            
,andromeda4--,this is one of the few    
nebul that are visible to the naked eye4
 ,we see at a glance that it is not a   
star1 but a mass of diffused light4     
,indeed1 it has sometimes been very     
naturally mistaken for a comet4 ,it was 
first described by ,marius in #afad1 who
 compared its light to that of a candle 
shining through horn4 ,this gives a very
 good idea of the impression it         
produces1 which is that of a translucent
 object illuminated by a brilliant light
 behind it4 ,with a small telescope it  
is easy to imagine it to be a solid like
 horn2 but with a large one the effect  
is more like fog or mist with a bright  
body in its midst4 ,unlike most of the  
nebul1 its spectrum is a continuous one1
 similar to that from a heated solid1   

indicating that the light emanates1 not 
from a glowing gas1 but from matter in  
the solid or liquid state4 ,this would  
suggest that it is really an immense    
star-cluster1 so distant that the       
highest telescopic power cannot resolve 
it2 yet in the largest telescopes it    
looks less resolvable1 and more like a  
gas1 than in those of moderate size4 ,if
 it is really a gas1 and if the spectrum
 is continuous throughout the whole     
extent of the nebula1 either it must    
shine by reflected light1 or the gas    
must be subjected to a great pressure   
almost to its outer limit1 which is     
hardly possible4 ,if the light is       
reflected1 we cannot determine whether  
it comes from a single bright star1 or a
 number of small ones scattered through 
the nebula4  ,with a small telescope    
this nebula appears elliptical1 as in   
,fig4 #dci4 ,fig4 #ddj shows it as it   
appeared to ,bond1 in the ,cambridge    
refractor4  ,illustration3 ,fig4 #dci4  
,illustration3 ,fig4 #ddj4  #cgh4 ,the  

,great ,nebula of ,orion4--,the nebula  
which1 above all others1 has occupied   
the attention of astronomers1 and       
excited the wonder of observers1 is the 
great nebula of ,orion1 which surrounds 
the middle star of the three which form 
the sword of ,orion4 ,a good eye will   
perceive that this star1 instead of     
looking like a bright point1 has a hazy 
appearance1 due to the surrounding      
nebula4 ,this object was first described
 by ,huyghens in #afei1 as follows3--   
,there is one phenomenon among the fixed
 stars worthy of mention1 which1 so far 
as ,i know1 has hitherto been noticed by
 no one1 and indeed cannot be well      
observed except with large telescopes4  
,in the sword of ,orion are three stars 
quite close together4 ,in #afef1 as ,i  
chanced to be viewing the middle one of 
these with the telescope1 instead of a  
single star1 twelve showed themselves 7a
 not uncommon circumstance74 ,three of  
these almost touched each other1 and    
with four others shone through a nebula1

 so that the space around them seemed   
far brighter than the rest of the       
heavens1 which was entirely clear1 and  
appeared quite black2 the effect being  
that of an opening in the sky1 through  
which a brighter region was visible4    
,illustration3 ,fig4 #dda4  ,the        
representation of this nebula in ,fig4  
#dda is from a drawing made by ,bond4   
,in brilliancy and variety of detail it 
exceeds any other nebula visible in the 
northern hemisphere4 ,in its centre are 
four stars1 easily distinguished by a   
small telescope with a magnifying power 
of forty or fifty1 together with two    
smaller ones1 requiring a nine-inch     
telescope to be well seen4 ,besides     
these1 the whole nebula is dotted with  
stars4  ,in the winter of #ahfd-fe the  
spectrum of this nebula was examined    
independently by ,secchi and ,huggins1  
who found that it consisted of three    
bright lines1 and hence concluded that  
the nebula was composed1 not of stars1  
but of glowing gas4 ,the position of one

 of the lines was near that of a line of
 nitrogen1 while another seemed to      
coincide with a hydrogen line4 ,this    
would suggest that the nebula is a      
mixture of hydrogen and nitrogen gas2   
but of this we cannot be certain4       
,illustration3 ,fig4 #ddb4  #cgi4 ,the  
,nebula in ,argus4--,there is a nebula  
7,fig4 #ddb7 surrounding the variable   
star ,eta ,argus 7#cee71 which is       
remarkable as exhibiting variations of  
brightness and of outline4  ,in many    
other nebul1 changes have been          
suspected2 but the indistinctness of    
outline which characterizes most of     
these objects1 and the very different   
aspect they present in telescopes of    
different powers1 render it difficult to
 prove a change beyond a doubt4  #chj4  
,the ,dumb-,bell ,nebula4--,this nebula 
was named from its peculiar shape4 ,it  
is a good illustration of the change in 
the appearance of a nebula when viewed  
with different magnifying powers4 ,fig4 
#ddc shows it as it appeared in         

,herschel's telescope1 and ,fig4 #ddd as
 it appears in the great ,parsonstown   
reflector 7#bj74  ,illustration3 ,fig4  
#ddc4  ,illustration3 ,fig4 #ddd4       
,spiral ,nebul4   #cha4 ,the ,spiral    
,nebula in ,canes ,venatici4--,the great
 spiral nebula in the constellation     
,canes ,venatici1 or the ,hunting-,dogs1
 is one of the most remarkable of its   
class4 ,fig4 #dde shows this nebula as  
it appeared in ,herschel's telescope1   
and ,fig4 #ddf shows it as it appears in
 the ,parsonstown reflector4            
,illustration3 ,fig4 #dde4              
,illustration3 ,fig4 #ddf4  #chb4       
,condensation of ,nebul4--,the          
appearance of the nebula just mentioned 
suggests a body rotating on its axis1   
and undergoing condensation at the same 
time4  ,it is now a generally received  
theory that nebul are the material out  
of which stars are formed4 ,according to
 this theory1 the stars originally      
existed as nebul1 and all nebul will    
ultimately become condensed into stars4 

,illustration3 ,fig4 #ddg4              
,illustration3 ,fig4 #ddh4              
,illustration3 ,fig4 #ddi4  #chc4 ,other
 ,spiral ,nebul4--,fig4 #ddg represents 
a spiral nebula of the ,great ,bear4    
,this nebula seems to have several      
centres of condensation4 ,fig4 #ddh is a
 view of a spiral nebula in ,cepheus1   
and ,fig4 #ddi of a singular spiral     
nebula in the ,triangle4 ,this also     
appears to have several points of       
condensation4 ,figs4 #dej and #dea      
represent oval and elliptical nebul     
having a spiral structure4              
,illustration3 ,fig4 #dej4              
,illustration3 ,fig4 #dea4  ,,the       
,,magellanic ,,clouds4  ,illustration3  
,fig4 #deb4  #chd4 ,situation and       
,general ,appearance of the ,magellanic 
,clouds4--,the ,magellanic clouds are   
two nebulous-looking bodies near the    
southern pole of the heavens1 as shown  
in the right-hand portion of ,fig4 #deb4
 ,in the appearance and brightness of   
their light they resemble portions of   

the ,milky-,way4  ,illustration3 ,fig4  
#dec4  ,the larger of these clouds is   
called the ,nubecula ,major4 ,it is     
visible to the naked eye in strong      
moonlight1 and covers a space about two 
hundred times the surface of the moon4  
,it is shown in ,fig4 #dec4 ,the smaller
 cloud is called the ,nubecula ,minor4  
,it has only about a fourth the extent  
of the larger cloud1 and is considerably
 less brilliant4 ,it is visible to the  
naked eye1 but it disappears in full    
moonlight4 ,this cloud is shown in ,fig4
 #ded4 ,the region around this cloud is 
singularly bare of stars2 but the       
magnificent cluster of ,toucan1 already 
described 7#cdf71 is near1 and is shown 
a little to the right of the cloud in   
the figure4  ,illustration3 ,fig4 #ded4 
,illustration3 ,fig4 #dee4  #che4       
,structure of the ,nubecul4--,fig4 #dee 
shows the structure of these clouds as  
revealed by a powerful telescope4 ,the  
general ground of both consists of large
 tracts and patches of nebulosity in    

every stage of resolution1--from that   
which is irresolvable with eighteen     
inches of reflecting aperture1 up to    
perfectly separated stars1 like the     
,milky-,way and clustering groups4      
,there are also nebul in abundance1 both
 regular and irregular1 globular        
clusters in every state of condensation1
 and objects of a nebulous character    
quite peculiar1 and unlike any thing in 
other regions of the heavens4 ,in the   
area occupied by the nubecula major two 
hundred and seventy-eight nebul and     
clusters have been enumerated1 besides  
fifty or sixty outliers1 which ought    
certainly to be reckoned as its         
appendages1 being about six and a half  
per square degree2 which very far       
exceeds the average of any other part of
 the nebulous heavens4 ,in the nubecula 
minor the concentration of such objects 
is less1 though still very striking4    
,the nubecul1 then1 combine1 each within
 its own area1 characters which in the  
rest of the heavens are no less         

strikingly separated2 namely1 those of  
the galactic and the nebular system4    
,globular clusters 7except in one region
 of small extent7 and nebul of regular  
elliptic forms are comparatively rare in
 the ,milky-,way1 and are found         
congregated in the greatest abundance in
 a part of the heavens the most remote  
possible from that circle2 whereas in   
the nubecul they are indiscriminately   
mixed with the general starry ground1   
and with irregular though small nebul4  
,,the ,,nebular ,,hypothesis4           
#chf4 ,the ,basis of the ,nebular       
,hypothesis4--,we have seen that the    
planets all revolve around the sun from 
west to east in nearly the     same     
plane1 and that the sun rotates on his  
axis from west to east4     ,the        
planets1 so far as known1 rotate on     
their axes from west to     east2 and   
all the moons1 except those of ,uranus  
and ,neptune1 revolve     around their  
planets from west to east4 ,these common
 features in the     motion of the sun1 

moons1 and planets1 point to the        
conclusion that     they are of a common
 origin4      #chg4 ,kant's             
,hypothesis4--,kant1 the celebrated     
,german philosopher1     seems to have  
the best right to be regarded as the    
founder of the     modern nebular       
hypothesis4 ,his reasoning has been     
concisely stated     thus3 ,examining   
the solar system1 we find two remarkable
 features     presented to our          
consideration4 ,one is1 that six planets
 and nine     satellites the entire     
number then known move around the sun in
     circles1 not only in the same      
direction in which the sun himself      
revolves on his axis1 but very nearly in
 the same plane4 ,this common           
feature of the motion of so many bodies 
could not by any reasonable             
possibility have been a result of       
chance3 we are therefore forced to      
believe that it must be the result of   
some common cause originally     acting 
on all the planets4      ,on the other  

hand1 when we consider the spaces in    
which the planets     move1 we find them
 entirely void1 or as good as void2 for1
 if there     is any matter in them1 it 
is so rare as to be without effect on   
the     planetary motions4 ,there is1   
therefore1 no material connection now   
existing between the planets through    
which they might have been     forced to
 take up a common direction of motion4  
,how1 then1 are we to     reconcile this
 common motion with the absence of all  
material     connection8 ,the most      
natural way is to suppose that there was
 once     some such connection1 which   
brought about the uniformity of motion  
which we observe2 that the materials of 
which the planets are formed     once   
filled the whole space between them4    
,there was no formation in     this     
chaos1 the formation of separate bodies 
by the mutual     gravitation of parts  
of the mass being a later occurrence4   
,but1     naturally1 some parts of the  
mass would be more dense than others1   

and would thus gather around them the   
rare matter which filled the            
intervening spaces4 ,the larger         
collections thus formed would draw      
the smaller ones into them1 and this    
process would continue until a     few  
round bodies had taken the place of the 
original chaotic mass4      ,kant1      
however1 failed to account              
satisfactorily for the motion of     the
 sun and planets4 ,according to his     
system1 all the bodies formed     out of
 the original nebulous mass should have 
been drawn to a common     centre so as 
to form one sun1 instead of a system of 
revolving     bodies like the solar     
system4      #chh4 ,herschel's          
,hypothesis4--,the idea of the gradual  
transmutation     of nebul into stars   
seems to have been suggested to         
,herschel1 not     by the study of the  
solar system1 but by that of the nebul  
themselves4 ,many of these bodies he    
believed to be immense masses of        
phosphorescent vapor2 and he conceived  

that these must be gradually            
condensing1 each around its own centre1 
or around the parts where it     is most
 dense1 until it should become a star1  
or a cluster of stars4     ,on          
classifying the nebul1 it seemed to him 
that he could see this     process going
 on before his eyes4 ,there were the    
large1 faint1     diffused nebul1 in    
which the condensation had hardly begun2
 the     smaller but brighter ones1     
which had become so far condensed that  
the central parts would soon begin to   
form into stars2 yet others1     in     
which stars had actually begun to form2 
and1 finally1     star-clusters in which
 the condensation was complete4 ,the    
spectroscopic revelations of the gaseous
 nature of the true nebul     tend to   
confirm the theory of ,herschel1 that   
these masses will all1     at some time1
 condense into stars4      #chi4        
,laplace's ,hypothesis4--,laplace was   
led to the nebular     hypothesis by    
considering the remarkable uniformity in

 the direction     of the rotation of   
the planets4 ,believing that this could 
not have     been the result of chance1 
he sought to investigate its cause4     
,this1     he thought1 could be nothing 
else than the atmosphere of the sun1    
which once extended so far out as to    
fill all the space now occupied     by  
the planets4 ,he begins with the sun1   
surrounded by this immense     fiery    
atmosphere4 ,since the sum total of     
rotary motion now seen in     the       
planetary system must have been there   
from the beginning1 he     conceives the
 immense vaporous mass forming the sun  
and his     atmosphere to have had a    
slow rotation on its axis4 ,as the      
intensely     hot mass gradually cooled1
 it would contract towards the centre4  
,as     it contracted1 its velocity of  
rotation would1 by the laws of          
mechanics1 constantly increase2 so that 
a time would arrive1 when1     at the   
outer boundary of the mass1 the         
centrifugal force due to the            

rotation would counterbalance the       
attractive force of the central         
mass4 ,then those outer portions would  
be left behind as a revolving     ring1 
while the next inner portions would     
continue to contract until     the      
centrifugal and attractive forces were  
again balanced1 when a     second ring  
would be left behind2 and so on4 ,thus1 
instead of a     continuous atmosphere1 
the sun would be surrounded by a series 
of     concentric revolving rings of    
vapor4 ,as these rings cooled1 their    
denser materials would condense first2  
and thus the ring would be     composed 
of a mixed mass1 partly solid and partly
 vaporous1 the     quantity of solid    
matter constantly increasing1 and that  
of vapor     diminishing4 ,if the ring  
were perfectly uniform1 this            
condensation     would take place       
equally all around it1 and the ring     
would thus be     broken up into a group
 of small planets1 like the asteroids4  
,but if1     as would more likely be the

 case1 some portions of the ring were   
much denser than others1 the denser     
portions would gradually attract     the
 rarer portions1 until1 instead of a    
ring1 there would be a     single mass  
composed of a nearly solid centre1      
surrounded by an     immense atmosphere 
of fiery vapor4 ,this condensation of   
the ring of     vapor around a single   
point would not change the amount of    
rotary     motion that had existed in   
the ring4 ,the planet with its          
atmosphere     would therefore be in    
rotation2 and would be1 on a smaller    
scale1     like the original solar mass 
surrounded by its atmosphere4 ,in the   
same way that the latter formed itself  
first into rings1 which     afterwards  
condensed into planets1 so the planetary
 atmospheres1 if     sufficiently       
extensive1 would form themselves into   
rings1 which     would condense into    
satellites4 ,in the case of ,saturn1    
however1 one     of the rings was so    
uniform throughout1 that there was no   

denser     portion to attract the rest  
around it2 and thus the ring of ,saturn 
retained its annular form4              
,illustration3 ,fig4 #def4      ,such is
 the celebrated nebular hypothesis of   
,laplace4 ,it starts1 not     with a    
purely nebulous mass1 but with the sun1 
surrounded by an     immense atmosphere1
 out of which the planets were formed by
 gradual     condensation4 ,fig4 #def   
represents the condensing mass according
 to     this theory4      #cij4 ,the    
,modern ,nebular                        
,hypothesis4--,according to the nebular 
hypothesis as held at the present time1 
the sun1 planets1 and     meteoroids    
originated from a purely nebulous mass4 
,this nebula first     condensed into a 
nebulous star1 the star being the sun1  
and its     surrounding nebulosity being
 the fiery atmosphere of ,laplace4 ,the 
original nebula must have been put into 
rotation at the beginning4     ,as it   
contracted and became condensed through 
the loss of heat by     radiation into  

space1 and under the combined attraction
 of gravity1     cohesion1 and affinity1
 its speed of rotation increased2 and   
the     nebulous envelop became1 by the 
centrifugal force1 flattened into a     
thin disk1 which finally broke up into  
rings1 out of which were     formed the 
planets and their moons4 ,according to  
,laplace1 the rings     which were      
condensed into the planets were thrown  
off in succession     from the          
equatorial region of the condensing     
nebula2 and so the     outer planets    
would be the older4 ,according to the   
more modern idea1     the nebulous mass 
was first flattened into a disk1 and    
subsequently     broken up into rings1  
in such a way that there would be no    
marked     difference in the ages of the
 planets4 ,the sun represents the       
central portion of the original nebula1 
and the comets and     meteoroids its   
outlying portion4 ,at the sun the       
condensation is     still going on1 and 
the meteoroids appear to be still       

gradually     drawn in to the sun and   
planets4      ,the whole store of energy
 with which the original solar nebula   
was     endowed existed in it in the    
potential form4 ,by the condensation and
     contraction this energy was        
gradually transformed into the kinetic  
energy of molar motion and of heat2 and 
the heat became gradually     dissipated
 by radiation into space4 ,this         
transformation of potential     energy  
into heat is still going on at the sun1 
the centre of the     condensing mass1  
by the condensation of the sun itself1  
and by the     impact of meteors as they
 fall into it4      ,it has been        
calculated1 that1 by the shrinking of   
the sun to the     density of the earth1
 the transformation of potential energy 
into     heat would generate enough heat
 to maintain the sun's supply1 at the   
present rate of dissipation1 for        
seventeen million years4 ,a             
shrinkage of the sun which would        
generate all the heat he has poured     

into space since the invention of the   
telescope could not be     detected by  
the most powerful instruments yet       
constructed4      ,the least velocity   
with which a meteoroid could strike the 
sun would     be two hundred and eighty 
miles a second2 and it is easy to       
calculate how much heat would be        
generated by the collision4 ,it has     
been shown1 that1 were enough meteoroids
 to fall into the sun to     develop its
 heat1 they would not increase his mass 
appreciably     during a period of two  
thousand years4      ,the sun's heat is 
undoubtedly developed by contraction and
 the fall     of meteoroids2 that is to 
say1 by the transformation of the       
potential energy of the original nebula 
into heat4      ,it must be borne in    
mind that the nebular hypothesis is     
simply a     supposition as to the way  
in which the present solar system may   
have     been developed from a nebula   
endowed with a motion of rotation and   
with certain tendencies to condensation4

 ,of course nothing could     have been 
developed out of the nebula1 the germs  
of which had not     been originally    
implanted in it by the ,creator4        
,,iv4 ,,the ,,structure ,,of ,,the      
,,stellar ,,universe4   #cia4 ,sir      
,william ,herschel's ,view4--,sir       
,william ,herschel assumed that the     
stars are distributed with tolerable    
uniformity throughout the space occupied
 by our stellar system4 ,he accounted   
for the increase in the number of stars 
in the field of view as he approached   
the plane of the ,milky-,way1 not by the
 supposition that the stars are really  
closer together in and about this plane1
 but by the supposition that our stellar
 system is in the form of a flat disk   
cloven at one side1 and with our sun    
near its centre4 ,a section of this disk
 is shown in ,fig4 #deg4  ,illustration3
 ,fig4 #deg4  ,an observer near ,s1 with
 his telescope pointed in the direction 
of ,s b1 would see comparatively few    
stars within the field of view1 because 

looking through a comparatively thin    
stratum of stars4 ,with his telescope   
pointed in the direction ,s a1 he would 
see many more stars within his field of 
view1 even though the stars were really 
no nearer together1 because he would be 
looking through a thicker stratum of    
stars4 ,as he directed his telescope    
more and more nearly in the direction ,s
 f1 he would be looking through a       
thicker and thicker stratum of stars1   
and hence he would see a greater and    
greater number of them in the field of  
view1 though they were everywhere in the
 disk distributed at uniform distances4 
,he assumed1 also1 that the stars are   
all tolerably uniform in size1 and that 
certain stars appear smaller than       
others1 only because they are farther   
off4 ,he supposed the faint stars of the
 ,milky-,way to be merely the most      
distant stars of the stellar disk2 that 
they are really as large as the other   
stars1 but appear small owing to their  
great distance4 ,the disk was assumed to

 be cloven on one side1 to account for  
the division of the ,milky-,way through 
nearly half of its course4 ,this theory 
of the structure of the stellar universe
 is often referred to as the cloven disk
 theory4  ,illustration3 ,fig4 #deh4    
#cib4 ,the ,cloven ,ring                
,theory4--,according to ,mdler1 the     
stars of the ,milky-,way are entirely   
separated from the other stars of our   
system1 belonging to an outlying ring1  
or system of rings4 ,to account for the 
division of the ,milky-,way1 the ring is
 supposed to be cloven on one side3     
hence this theory is often referred to  
as the cloven ring theory4 ,according to
 this hypothesis1 the stellar system    
viewed from without would present an    
appearance somewhat like that in ,fig4  
#deh4 ,the outlying ring cloven on one  
side would represent the stars of the   
,milky-,way2 and the luminous mass at   
the centre1 the remaining stars of the  
system4  #cic4 ,proctor's               
,view4--,according to ,proctor1 the     

,milky-,way is composed of an irregular 
spiral stream of minute stars lying in  
and among the larger stars of our       
system1 as represented in ,fig4 #dei4   
,the spiral stream is shown in the inner
 circle as it really exists among the   
stars1 and in the outer circle as it is 
seen projected upon the sky4 ,according 
to this view1 the stars of the          
,milky-,way appear faint1 not because   
they are distant1 but because they are  
really small4  ,illustration3 ,fig4     
#dei4  #cid4 ,newcomb's                 
,view4--,according to ,newcomb1 the     
stars of our system are all situated in 
a comparatively thin zone lying in the  
plane of the ,milky-,way1 while there is
 a zone of nebul lying on each side of  
the stellar zone4 ,he believes that so  
much is certain with reference to the   
structure of our stellar universe2 but  
he considers that we are as yet         
comparatively ignorant of the internal  
structure of either the stellar or the  
nebular zones4 ,the structure of the    

stellar universe1 according to this     
view1 is shown in ,fig4 #dfj4           
,illustration3 ,fig4 #dfj4              
,,index    ,a4   ,aberration of light1  
#ch4   ,aerolites1 #cjd4   ,aldebaran1  
star in ,taurus1 #cdj1 #cdb4   ,algol1 a
 variable star1 #cdc1 #ceh4   ,almanac1 
perpetual1 #hb4   ,alps1 lunar          
mountains1 #abf4   ,altair1 star in     
,aquila1 #ccf4   ,alt-azimuth           
instrument1 #ac4   ,altitude1 #ab4      
,andromeda 7constellation71 #cdc1 #cdf4 
nebula in1 #cgf4   ,angstrm's map of    
spectrum1 #afd4   ,antares1 star in     
,scorpio1 #cdg4   ,apennines1 lunar     
mountains1 #abb1 #abd4   ,aphelion1 #dg4
   ,apogee1 #dd4   ,aquarius1 or the    
,water-,bearer1 #cej4    cluster in1    
#ced4   ,aquila1 or the ,eagle1 #ccf4   
,arcturus1 star in ,botes1 #cce1 #cfe1  
#cgj4   ,argo1 or the ,ship1 #cfj4      
nebula in1 #chc4    variable star in1   
#cfj4   ,aries1 or the ,ram1 #cej4      
,asteroids1 #bbc1 #bda4   ,astra1 an    
asteroid1 #bda4   ,auriga1 or the       

,wagoner1 #cdb4   ,azimuth1 #ac4    ,b4 
,betelgeuse1 star in ,orion1 #cdj1 #cgj4
   ,berenice's ,hair 7constellation71   
#ccd4   ,bode's law1 #bda4    disproved1
 #bgc4   ,botes 7constellation71 #ccd1  
#cce4    ,c4   ,calendar1 the1 #hj4     
,callisto1 moon of ,jupiter1 #bej4      
,cancer1 or the ,crab1 #cej4    tropic  
of1 #fa4   ,canes ,venatici1 or the     
,hunting-,dogs1 #ccd4   ,canes          
,venatici1 nebula in1 #chd4   ,canis    
,major1 or the ,great ,dog1 #cdb4       
,canis ,minor1 or the ,little ,dog1     
#cdj4   ,capella1 star in ,auriga1 #cdj1
 #cdc4   ,capricorn1 tropic of1 #fa4    
,capricornus1 or the ,goat1 #cej4       
,cassiopeia 7constellation71 #ccb4      
new star in1 #cfb4   ,castor1 star in   
,gemini1 #cdj1 #cgj4   ,caucasus1 a     
lunar range1 #abd4   ,centaurus1        
star-cluster in1 #cee4   ,cepheus       
7constellation71 #ccd4    nebula in1    
#chg4   ,ceres1 the planet1 #bda4       
,cetus1 or the ,whale1 #cdf4    variable
 star in1 #cei4   ,charles's ,wain1     

#ccj4   ,circles1 great1 #d4    diurnal1
 #h4    hour1 #af4    small1 #d4        
vertical1 #ab4   ,clock1 astronomical1  
#ah4    time1 #gh4   ,coma ,berenices1  
or ,berenice's ,hair1 #ccd4   ,comet1   
,biela's1 #bic4    and earth1 collision 
of1 #caf4    ,coggia's1 #big4           
,donati's1 #bif4    ,encke's1 #bic4     
,halley's1 #bia4    of #afhj1 #bij4     
of #ahaa1 #bij4    of #ahdc1 #bie4    of
 #ahfa1 #big4    of ,june1 #ahha1 #cjj4 
,comets1 appearance of1 #bgd4    and    
meteors1 #cac4    bright1 #bgd4         
chemical constitution of1 #cah4         
development of1 #bgg4    number of1     
#bhh4    orbits of1 #bhb4    origin of1 
#bhg4    periodic1 #bhf4    physical    
constitution of1 #cad4    tails of1     
#bgi4    telescopic1 #bge1 #bha4        
visibility of1 #bha4   ,conic sections1 
#dh4   ,conjunction1 #ia4    inferior1  
#acj4    superior1 #acj1 #acf4          
,constellations1 #cbe4    zodiacal1 #cb4
   ,copernican system1 the1 #dd1 #ec4   
,copernicus1 a lunar crater1 #abj1 #abi4

   ,corona ,borealis1 or the ,northern  
,crown1 #ccf4   ,corona ,borealis1 new  
star in1 #cfc4   ,corvus1 or the ,crow1 
#cci4   ,crystalline spheres1 #da4      
,cycles and epicycles1 #db4   ,cygnus1  
or the ,swan1 #cch4    ,d4   ,day and   
night1 #eg4    civil1 #gg4    lunar1    
#ajh4    sidereal1 #gd4    solar1 #gd4  
,declination1 #af4   ,deimos1 satellite 
of ,mars1 #bci4   ,delphinus1 or the    
,dolphin1 #cch4   ,deneb1 star in       
,cygnus1 #cch1 #cgj4   ,dione1 satellite
 of ,saturn1 #bei4   ,dipper1 the       
,great1 #ccj1 #cff1 #cfi1 #cgj4    the  
,little1 #cca4    the ,milk1 #cdg4      
,dissociation1 #afc4   ,dominical       
,letter1 the1 #ha4   ,draco1 or the     
,dragon1 #cca4    ,e4   ,earth1 density 
of1 #he4    flattened at poles1 #ee4    
form of1 #ec4    in space1 #ef4    seen 
from moon1 #aji4    size of1 #ee4       
weight of1 #hc4   ,eccentric1 the #dc4  
,eccentricity1 #df4   ,eclipses1 #baj4  
annular1 #bai4    lunar1 #baj1 #bad4    
solar1 #baf4   ,ecliptic1 the1 #bg4     

obliquity of1 #bh4   ,ellipse1 the1 #de1
 #di4   ,elongation1 of planet1 #acj4   
,enceladus1 moon of ,saturn1 #bei4      
,epicycles1 #db4   ,epicycloid1 #ajg4   
,epsilon ,lyr1 a double star1 #cef4     
,equator1 the celestial1 #g4            
,equinoctial1 the1 #g4    colure1 #af4  
elevation of1 #i4   ,equinox1 autumnal1 
#bi4    vernal1 #af1 #bi4   ,equinoxes1 
precession of1 #ca1 #he4   ,eta ,argus1 
a variable star1 #cfj1 #chc4   ,europa1 
moon of ,jupiter1 #bej4   ,f4   ,facul1 
solar1 #agg4   ,fomalhaut1 star in      
,southern ,fish1 #cej4   ,fraunhofer's  
lines1 #afd1 #cga4    ,g4   ,galaxy1    
the1 #cbf4   ,ganymede1 moon of         
,jupiter1 #bej4   ,gemini1 or the       
,twins1 #cdj4   ,georgium ,sidus1 #bga4 
,h4   ,hercules 7constellation71 #ccf4  
cluster in1 #cec4    orbit of double    
star in1 #ceg4    solar system moving   
towards1 #cfg4   ,herschel1 the planet  
7see ,uranus74   ,herschel's hypothesis1
 #cib1 #cif4   ,horizon1 the1 #e4       
,hyades1 the1 #cdb1 #cej4   ,hydra1 or  

the ,water-,snake1 #cdj4   ,hyperbola1  
the1 #di4   ,hyperion1 moon of ,saturn1 
#bei4    ,i4   ,io1 moon of ,jupiter1   
#bej4   ,irradiation1 #ij1 #aac4    ,j4 
,japetus1 moon of ,saturn1 #bei4        
,job's ,coffin 7asterism71 #cch4        
,juno1 the planet1 #bda4   ,jupiter1    
apparent size of1 #bde4    distance of1 
#bde4    great red spot of1 #bdi4       
orbit of1 #bdd4    periods of1 #bdf4    
physical constitution of1 #bdf4         
rotation of1 #bdh4    satellites of1    
#bej4      eclipses of1 #beb4           
transits of1 #bed4    volume of1 #bde4  
without satellites1 #bee4    ,k4        
,kant's hypothesis1 #cia4   ,kepler1 a  
lunar crater1 #abi4   ,kepler's system1 
#dd4    laws1 #df4    star1 #cfb4       
,kirchhoff's map of spectrum1 #afd4     
,l4   ,laplace's hypothesis1 #cib4      
,latitude1 celestial1 #cj4   ,leap year1
 #ha4   ,leo1 or the ,lion1 #ccd4       
,leonids 7meteors71 #cab4   ,libra1 or  
the ,balances1 #cdg4   ,libration1 #ajb4
   ,longitude1 celestial1 #cj4   ,lyra1 

or the ,lyre1 #cch4    double star in1  
#cef4    ,m4   ,magellanic clouds1 the1 
#chi4   ,magnetic storms1 #aij4         
,magnetism and sun-spots1 #aij4   ,mars1
 apparent size of1 #bcf4    brilliancy  
of1 #bcg4    distance of1 #bce4    orbit
 of1 #bce4    periods of1 #bcg4         
rotation of1 #bci4    satellites of1    
#bci4    volume of1 #bcf4   ,mercury1   
apparent size of1 #bbf4    atmosphere   
of1 #bbh4    distance of1 #bbe4         
elongation of1 #bbg4    orbit of1 #bbe4 
periods of1 #bbg4    volume of1 #bbf4   
,meridian1 the1 #ab4   ,meridian circle1
 #ag4   ,meridians1 celestial1 #ca4     
,meteoric iron1 #cje1 #cjg4    showers1 
#caj4    stones1 #cje4   ,meteors1 #cjj4
    ,august1 #caa4    light of1 #cji4   
,november1 #cab4    sporadic1 #caj4     
,meteoroids1 #cjh4   ,micrometers1 #bj1 
#aec4   ,milky-,way1 the1 #cbf4         
,mimas1 moon of ,saturn1 #bei4   ,mira1 
a variable star1 #cei4   ,moon1 apparent
 size of1 #hg1 #hi4    aspects of1 #ia4 
atmosphere of1 #aji4    chasms in1 #abc4

    craters in1 #aai4    day of1 #ajh4  
distance of1 #hf4    eclipses of1 #baj4 
form of orbit1 #ig4    harvest1 #aja4   
hunter's1 #ajb4    inclination of orbit1
 #ig4    kept in her path by gravity1   
#ea4    librations of1 #ajb4    mass of1
 #ij4    meridian altitude of1 #ih4     
mountains of1 #aaf4    orbital motion   
of1 #ia4    phases of1 #ic4    real size
 of1 #hh4    rising of1 #ii4    rotation
 of1 #ajb4    sidereal period of1 #ib4  
surface of1 #aae4    synodical period   
of1 #ib4    terminator of1 #aae4    wet 
and dry1 #ih4    ,n4   ,nadir1 the1 #f4 
,neap-tides1 #gb4   ,nebula1 in         
,andromeda1 #cgf4    crab1 #cgf4        
dumb-bell1 #chc4    in ,argus1 #chc4    
in ,canes ,venatici1 #chd4    in        
,cepheus1 #chg4    in ,orion1 #cgh4     
in the ,triangle1 #chg4    in ,ursa     
,major1 #chf4   ,nebul1 #bha1 #ccj1     
#cgc4    annular1 #cgc4    circular1    
#cgc4    condensation of1 #che4         
double1 #cge4    elliptical1 #cgc4      
irregular1 #cgf4    multiple1 #cge4     

spiral1 #cgc1 #chd4   ,nebular          
hypothesis1 the1 #cia4   ,neptune1      
discovery of1 #bga4    orbit of1 #bga4  
satellite of1 #bgd4   ,new style1 #hj4  
,newcomb's theory of the stellar        
universe1 #cih4   ,newton's system1 #dh4
   ,nodes1 #ig4   ,nubecula1 ,major1    
#chi4    ,minor1 #chi4   ,nutation1 #cd4
    ,o4   ,olbers's hypothesis1 #bda4   
,old style1 #hj4   ,ophiuchus           
7constellation71 #cdg4    new star in1  
#cfb4   ,opposition1 #ia1 #acf4         
,orion1 #cda4    nebula in1 #cgh4    the
 trapezium of1 #cef4    ,p4   ,pallas1  
the planet1 #bda4   ,parabola1 the1 #di4
   ,parallax1 #cg4   ,pegasus           
7constellation71 #cdc1 #cdf4    triple  
star in1 #cef4   ,perigee1 #dd4         
,perihelion1 #dg4   ,perseids 7meteors71
 #caa4   ,perseus 7constellation71 #cdf4
    cluster in1 #cec4   ,phobos1        
satellite of ,mars1 #bci4   ,pico1 a    
lunar mountain1 #abg4   ,pisces1 or the 
,fishes1 #cej4   ,piscis ,australis1 or 
the ,southern ,fish1 #cej4   ,planets1  

#ci4    inferior1 #acj4      periods of1
 #acb4      phases of1 #acb4    inner   
group of1 #bba4    intra-,mercurial1    
#bcj4    minor1 #bbc4    outer group of1
 #bbb1 #bdd4    superior1 #acd4         
motion of1 #acd4      periods of1 #acg4 
phases of1 #acg4    three groups of1    
#bba4   ,pleiades1 the1 #cbh1 #cdb1     
#cea4   ,pointers1 the1 #ccj4   ,polar  
distance1 #af4   ,pole ,star1 the1 #g1  
#ccj1 #cfe4   ,poles1 celestial1 #g1 #i4
   ,pollux1 star in ,gemini1 #cdj1 #cgj4
   ,prsepe1 or the ,beehive1 #cej4      
,precession of equinoxes1 #ca1 #he4     
,prime vertical1 the1 #ab4   ,proctor's 
theory of the stellar universe1 #cih4   
,procyon1 star in ,canis ,minor1 #cdj4  
,ptolemaic system1 the1 #da4    ,q4     
,quadrature1 #ia1 #acg4    ,r4          
,radiant point 7meteors71 #caj4         
,radius vector1 #dg4   ,refraction1 #ce4
   ,regulus1 star in ,leo1 #ccd1 #cgj4  
,rhea1 moon of ,saturn1 #bei4   ,rigel1 
star in ,orion1 #cdj1 #cgj4   ,right    
ascension1 #af4    ,s4   ,sagittarius1  

or the ,archer1 #cdg4   ,saturn1        
apparent size of1 #bef4    distance of1 
#bef4    orbit of1 #bee4    periods of1 
#bef4    physical constitution of1 #beg4
    ring of1 #bfa4      changes in1     
#bfh4      constitution of1 #bfi4       
phases of1 #bfc4    rotation of1 #beh4  
satellites of1 #bei4    volume of1 #bef4
   ,scorpio1 or the ,scorpion1 #cdg4    
cluster in1 #cee4   ,seasons1 the1 #fd4 
,sirius1 the ,dog-,star1 #cdj1 #cdb1    
#cfe1 #cgj1 #cga4   ,solar system1 the1 
#da4   ,solstices1 #bi1 #ei1 #fj4       
,sound1 effect of motion on1 #afh4      
,spectra1 bright-lined1 #aeh4           
comparison of1 #aed4    continuous1     
#aeh4    displacement of lines in1 #aga4
    of comets1 #cah4    reversed1 #afa4 
sun-spot1 #aic4    types of stellar1    
#cga4   ,spectroscope1 the1 #aeb4       
diffraction1 #aeg4    direct-vision1    
#aee4    dispersion1 #aeb4   ,spectrum  
analysis1 #aei4    solar1 #afd4         
,sphere1 defined1 #c4    the celestial1 
#e4      rotation of1 #g4               

,spring-tides1 #gb4   ,stars1           
circumpolar1 #g4    clusters of1 #cbh1  
#cej4    color of1 #ceg4                
constellations of1 #cbe4    constitution
 of1 #cga4    distance of1 #cfd4        
double1 #cee4    drift of1 #cfh4    four
 sets of1 #aj4    magnitude of1 #cbb4   
motion of1 in line of sight1 #cfi4      
multiple1 #cef4    names of1 #cbe4      
nebulous1 #cgc4    new1 #cfa4    number 
of1 #cbc4    parallax of1 #cfd4         
proper motion of1 #cfe4    secular      
displacement of1 #cff4    temporary1    
#cfa4    variable1 #ceh4   ,sun1        
atmosphere of1 #adi4    brightness of1  
#aea4    chemical constitution of1 #afd4
    chromosphere of1 #adi1 #aif4        
corona of1 #adi1 #aif1 #bjd4    distance
 of1 #adb4    facul of1 #agg4    heat   
radiated by1 #aej4    inclination of    
axis of1 #ahg4    mass of1 #adj4        
motion of1 among the stars1 #bf4      at
 surface of1 #afh4      in atmosphere   
of1 #agb4      secular1 #cff4           
photosphere of1 #adi1 #age4             

prominences of1 #adi1 #aig4    rotation 
of1 #ahf4    spectrum of1 #afd1 #aga4   
temperature of1 #adi4    volume of1     
#adj4    winds on1 #agd4   ,sun-spots1  
#agi4    and magnetism1 #aij4    birth  
and decay of1 #ahe4    cause of1 #aid4  
cyclonic motion in1 #ahb4               
distribution of1 #ahh4    duration of1  
#aha4    groups of1 #aha4    periodicity
 of1 #ahi4    proper motion of1 #ahg4   
size of1 #aha4    spectrum of1 #aic4    
,t4   ,taurus1 or the ,bull1 #cdb4      
quadruple star in1 #cef4   ,telescope1  
,cassegrainian1 #bc4    equatorial1 #ai4
    front-view1 #bb4    ,gregorian1 #bc4
    ,herschelian1 #bb4    ,lord         
,rosse's1 #be4    ,melbourne1 #be4      
,newall1 #bj4    ,newtonian1 #bb4       
,paris1 #bf4    reflecting1 #ba4        
,washington1 #bj4    ,vienna1 #bj4      
,telespectroscope1 the1 #aee4           
,telluric lines of spectrum1 #afe4      
,tethys1 moon of ,saturn1 #bei4         
,tides1 #fg4   ,time1 clock1 #gh4       
sun1 #gh4   ,titan1 moon of ,saturn1    

#bei1 #bfa4   ,toucan1 star cluster in1 
#ced1 #chi4   ,transit instrument1 #ag4 
,transits of ,venus1 #ade4              
,triesneker1 lunar formation1 #abc4     
,tropics1 #fa4   ,twilight1 #fb4        
,tycho ,brahe's star1 #cfa4    system1  
#dd4   ,tycho1 a lunar crater1 #abi4    
,u4   ,universe1 structure of the       
stellar1 #cif4    ,uranus1 discovery of1
 #bga4    name of1 #bgj4    orbit of1   
#bfi4    satellites of1 #bga4   ,ursa   
,major1 or the ,great ,bear1 #ccj4      
nebula in1 #chf4   ,ursa ,minor1 or the 
,little ,bear1 #ccj4    ,v4   ,vega1    
star in ,lyra1 #ccf1 #cfe1 #cgj4        
,venus1 apparent size of1 #bca4         
atmosphere of1 #bcd4    brilliancy of1  
#bcb4    distance of1 #bca4             
elongation of1 #bca4    orbit of1 #bcj4 
periods of1 #bcb4    volume of1 #bca4   
transits of1 #ade1 #bcd4   ,vernier1    
the1 #ae4   ,virgo1 or the ,virgin1     
#cch4   ,vesta1 the planet1 #bda4       
,vulcan1 the planet1 #bcj4    ,y4       
,year1 the1 #gh4    anomalistic1 #gi4   

,julian1 #hj4    sidereal1 #gi4         
tropical1 #gi4    ,z4   ,zenith1 the1   
#f4    distance1 #ab4   ,zodiac1 the1   
#cb4   ,zodiacal constellations1 #cb4   
light1 #cah4   ,zones1 #fa4           99
      99      99      99      99      99
     ,transcriber's note3  ,missing or  
obscured punctuation was corrected4     
,typographical errors were silently     
corrected4