,illu/ration3 ,! ,lun> 8,crat]0 ,cop]nicus ,photograph$ f ,nasmy? & ,c>p5t]'s pla/]-(-p>is model ( ! moon4 ,9 ?is model ! topography ( ! moon is fai?fully repres5t$ z se5 ) p[]ful telescopes4 ---------------------------------------- -------------------------------- ,a/ronomy 9 a ,nut%ell ,! ,*ief ,facts & ,pr9ciples ,expla9$ 9 ,popul> ,language = ! ,g5]al ,read] & = ,s*ools ,by ,g>rett ,p4 ,s]viss ,) #dg ,illu/rations ,g4 ,p4 ,putnam's ,sons ,new ,york & ,london ,! ,knick]bock] ,press #aiab ---------------------------------------- -------------------------------- ,,copyright1 #aiab ,,by ,,g>rett ,p4 ,,s]viss ,! ,knick]bock] ,press1 ,new ,york ---------------------------------------- -------------------------------- ,,preface ,h[ _m ?\s&s ( educat$ p1 tra9$ 9 ! be/ s*ools1 or ev5 graduates ( ! great univ]sities1 h made ! confession3 8,i nev] got a grip on a/ronomy 9 my /ud5t days4 ,!y didn't make x ei!r pla9 or int]e/+ to me2 & n[ ,i am sorry = x40 ,! purpose ( ! writ] ( ?is book is to supply ! ne$ ( su* p]sons1 ei!r 9 s*ool1 or at home1 aft] s*ool-days are end$4 ,he does n address himself to special /ud5ts ( ! subject--al?\gh !y1 too1 may f9d ! book useful at ! beg9n+--b to t va/1 intellig5t public = :om a/ronomy is1 m or less1 a 8my/ical midl&10 f :i*1 occasionally1 fasc9at+ news comes to _! e>s4 ,! ord9>y text-book is too ov]lad5 ) te*nical details1 & too summ>y 9 its treatm5t ( ! g5]al subject1 to cat* & hold ! att5tion ( ^? :o h no special prelim9>y int]e/ 9 a/ronomy4 ,! aim "h is to tell all t really ne$s to be told1 & no m1 & to put x z p]spicu\sly1 compactly1 & int]e/+ly1 z possible4 ,= t reason ! book is call$ a 8nut%ell40 99 99 99 99 99 ,! au?or has be5 sp>+ 9 ! use ( diagrams1 because he believes t1 9 _m cases1 !y h be5 ov]-press$4 ,!re is a t5d5cy to try to repres5t ev]y?+ to ! eye4 ,?is is well to a c]ta9 ext5t1 b !re is dang] t by pursu+ ?is me?od too f> ! p[] ( m5tal compreh5sion w be weak5$4 ,aft] all1 x is only by an intellig5t use ( ! imag9ation t progress c be made 9 su* a sci5ce z a/ronomy4 ,! read] is urg$ to make a s]i\s ef=t to und]/& :at is said 9 ! text1 & to picture x 9 8 m9d's eye1 be=e ref]r+ to ! diagrams4 ,aft] he has ?us pres5t$ ! subject to 8 imag9ation1 he may ref] to ! illu/rations1 & correct ) _! aid any misappreh5sion4 ,= ?is reason ! cuts1 ) _! descriptions1 h be5 made indep5d5t ( ! regul> text1 al?\gh !y are plac$ 9 _! prop] connections ?r\gh\t ! book4 ,g4 ,p4 ,s4 ,april1 #aiab4 ---------------------------------------- -------------------------------- ,,cont5ts ,,"p ,i--,,! ,,cele/ial ,,sph]e4 ,,pages ,def9ition ( ,a/ronomy--,fundam5tal ,law ( ! #c-fd ,/>s--,relations ( ! ,e>? to ! ,univ]se--,ord9>y ,appe>ance ( ! ,sky--,! ,horizon1 ! ,z5i?1 & ! ,m]idian--,locat+ ! ,/>s--,altitude & ,azimu?--,circul> or ,angul> ,measure--,altitude ,circles & ,v]tical ,circles--,! ,app>5t ,motion ( ! ,heav5s--,! ,nor? ,/> & ,ph5om5a ,connect$ ) x--,revolution ( ! ,/>s r\nd ! ,pole--,locat+ ! ,/>s on ! ,cele/ial ,sph]e--,a/ronomical ,equival5ts ( ,latitude & ,longitude--,p>allels ( ,decl9ation & ,h\r ,circles--8,! ,gre5wi* ( ! ,sky0--,effects ,produc$ by ,*ang+ ! ,obs]v]'s ,place on ! ,e>?--,! ,p>allel ,sph]e1 ! ,"r ,sph]e1 & ! ,oblique ,sph]e--,! ,a/ronomical ,clock--,! ,ecliptic--,app>5t ,annual ,revolution ( ! ,sun r\nd ! ,e>?--,incl9ation ( ! ,ecliptic to ! ,equator3 its ,cause & its ,effects--,! ,equ9oxes--,importance ( ! ,v]nal ,equ9ox--,! ,equ9octial ,colure--,! ,sol/ices--,poles ( ! ,ecliptic--,cele/ial ,latitude & ,longitude--,! ,zodiac--,! ,precession ( ! ,equ9oxes3 its ,cause & ,effects--,revolution ( ! ,cele/ial ,poles--,pa/ & ,future ,pole ,/>s ,,"p ,,ii--,,! ,,e>?4 ,,pages ,nature1 ,%ape1 & ,size ( ! ,e>?--,! ,pol> #fg-abc ,compression & ,equatorial ,protub]ance1 & _! ,cause--,! ,attraction ( ,gravitation--,! ,mass ( ! ,e>?3 h[ ,f\nd--,h[ ! ,e>? ,holds ! ,moon1 & ! ,sun ! ,planets--,! ,tides--,h[ ! ,moon & ,sun ,produce ,tides--,spr+ ,tides & ,neap ,tides--,! ,atmosph]e--,! ,law & ,effects ( ,refraction--,dip ( ! ,horizon--,! ,ab]ration ( ,light--,"t3 h[ ,measur$--,sid]eal1 ,app>5t ,sol>1 & ,mean ,sol> ,"t--,! ,clock & ! ,sun--,day & ,night--,:]e ! ,days ,beg9--,! ,seasons--,effects ( ! ,v>y+ ,decl9ation ( ! ,sun--,pol> & ,equatorial ,day & ,night--,! ,tropics & ! ,pol> ,circles--,inequality ( ,l5g? ( ! ,seasons--,:5 ! ,seasons 9 ! ,two ,hemisph]es w be ,rev]s$--,! ,cal5d>1 ! ,ye>1 & ! ,mon?--,re=mations ( ! ,cal5d>--,diff]5t ,measures ( ! ,mon? ,,"p ,,iii--,,! ,,sol> ,,sy/em4 ,,pages ,! ,sun--,di/ance1 ,size1 & ,condition ( ! #abg-bae ,sun--,temp]ature ( ! ,sun--,sol> ,heat on ! ,e>?1 & its ,me*anical ,equival5t--,peculi> ,rotation ( ! ,sun--,sun-spots1 _! ,appe>ance & ,probable ,cause--,facul--,! ,photosph]e--,sol> ,prom95ces--,explosive ,prom95ces--,! ,sol> ,corona--,p>allax1 & ! ,measure ( ,di/ances--,spectroscopic ,analysis--,h[ ! ,elem5ts 9 ! ,sun ,reveal _! ,pres5ce--,li/ ( ! ,pr9cipal ,sol> ,elem5ts--,! ,moon--,orig9 ( ! ,moon--,appe>ance ( its ,surface--,gravity on ! ,moon--,! ,phases ( ! ,moon--,causes ( ! ,abs5ce ( a ,lun> ,atmosph]e--,eclipses--,h[ ! ,moon ,causes ,eclipses ( ! ,sun--,! ,laws ,gov]n+ ,eclipses--,! ,%ad[ dur+ a ,sol> ,eclipse--,eclipses ( ! ,moon--,numb] ( ,eclipses 9 a ,ye>--,! ,s>os--,! ,planets--,kepl]'s ,laws ( ,planet>y ,motion--,m]cury--,v5us--,m>s1 & its ,s-call$ ,canals--,!ories ab\t ,m>s--,jupit]1 its ,belts & its ,satellites--,! ,great ,r$ ,spot0--,saturn1 its ,r+s & its ,satellites--,composition ( ! ,r+s--,uranus & ,neptune--,comets1 & ! ,laws ( _! ,motion--,composition ( ,comets--,! ,pressure ( ,light & its ,connection ) ,comets' ,tails--,break+ up ( ,comets--,meteors & _! ,relations to ,comets--,! ,novemb] ,meteors & ,o!r ,celebrat$ ,%[]s--,meteorites or ,bolides :i* ,fall ^u ! ,e>?--,! ,"q ( _! ,orig9 ,,"p ,,iv--,,! ,,fix$ ,,/>s4 ,,pages ,division ( ! ,/>s into ,magnitudes--,division ( #bai-beg ! ,/>s accord+ to _! ,spectra--,/>s ,l>g] & ,small] ?an ! ,sun--,! ,di/ances ( ! ,/>s--,v>iable ,/>s--,d\ble & ,b9>y ,/>s--,spectroscopic ,b9>ies & h[ !y are ,discov]$--,prop] ,motions ( ! ,/>s--,numb] ( ! ,/>s--,new1 or ,tempor>y ,/>s--,! ,milky ,way--,! ,nebul--,! ,two ,k9ds ( ,nebul--,spiral ,nebul--,! ,nebul> ,hypo!sis--,applications ( ,photography to ,/>s & ,nebul--,! ,con/ellations--,h[ to ,le>n ! ,con/ellations--,_! ,antiquity--,description ( ! ,pr9cipal ,con/ellations ,visible f ! ,nor!rn ,hemisph]e at ,v>i\s ,times ( ! ,ye> ,,index #bei ---------------------------------------- -------------------------------- ,,illu/rations ,,page ,,! ,,lun> 8,,crat]0 ,,cop]nicus ,frontispiece ,,photograph ,,( ,,s\? ,,pol> #h ,,region ,,( ,,! ,,moon ,,! ,,moon ,,ne> ,,! 8,,crat]0 #bj ,,ty*o ,,draw+ ,,( ,,jupit] #bh ,,draw+ ,,( ,,jupit] #bh ,,jupit] #ch ,,saturn #df ,,saturn #df ,,! ,,milky ,,way ,,ab\t ,,*i ,,cygni #eh ,,! ,,great ,,s\!rn #fd ,,/>-,,clu/] ,,9 ,,c5tauri ,,photograph ,,( ,a ,,gr\p ,,( #gf ,,sun-,,spots ,,pol> ,,/ream]s ,,( ,,! ,,sun1 #hi ,,eclipse ,,( #ahhi ,,sol> ,,corona ,,at ,,! ,,eclipse ,,( #hi #ahga ,,moreh\se',s ,,comet1 ,,octob] #ae1 #if #aijh ,,moreh\se',s ,,comet1 ,,novemb] #if #ae1 #aijh ,,head ,,( ,,! ,,great ,,comet ,,( #aje #ahfa ,,halley',s ,,comet1 ,,may #e1 #aiaj #aje ,,! ,,six-,,tail$ ,,comet ,,( #agdd #aab ,,spiral ,,nebula ,,9 ,,ursa ,,major 7,m #abd #aja7 ,,! ,,:irlpool ,,nebula ,,9 ,,canes #abd ,,v5atici ,,tress ,,nebula 7,n4 ,g4 ,c4 #fiib7 #acb ,,9 ,,cygnus ,,! ,,great ,,&rom$a ,,nebula #adj ,,spiral ,,nebula ,,9 ,,cepheus 7,h4 #aed ,,iv4 #gf7 ,,nebul\s ,,gr\ndwork ,,9 ,,taurus #aed ,,nebula ,,9 ,,sagitt>ius 7,m4 #h7 #afb ,,! ,,great ,,nebula ,,9 ,,orion #ahj ,,photographs ,,( ,,m>s #bjj ,,s*iap>relli',s ,,*>t ,,( #bbj ,,m>tian 8,,canals0 ,,illu/rations ,,9 ,,! ,,text ,,! ,,rational ,,& ,,! ,,s5sible #ab ,,horizon ,,altitude ,,& ,,azimu? #ad ,,"r ,,asc5sion ,,& #ce ,,decl9ation ,,! ,,ecliptic ,,& ,,cele/ial #ea ,,latitude ,,& ,,longitude ,,h[ ,,! ,,e>? ,,controls ,,! #ge ,,moon ,,! ,,tidal ,,=ce ,,( ,,! ,,moon #gi ,,refraction #he ,,dip ,,( ,,! ,,horizon #hg ,,sid]eal ,,& ,,sol> ,,"t #ic ,,! ,,*ange ,,( ,,day #aja ,,! ,,seasons #ajg ,,p>allax ,,( ,,! ,,moon #aci ,,p>allax ,,( ,,! ,,sun ,,f #ada ,,transit ,,( ,,v5us ,,spectrum ,,analysis #adg ,,! ,,phases ,,( ,,! ,,moon #afj ,,orbits ,,( ,,m>s ,,& ,,! ,,e>? #ahc ,,ellipse1 ,,p>abola1 ,,& #bjc ,,hyp]bola ,,! ,,nor? ,,circumpol> ,,/>s #bdd ,,key ,,to ,,nor? ,,circumpol> ,,/>s #bde ---------------------------------------- -------------------------------- ,,"p ,i4 ,,! ,,cele/ial ,,sph]e4 ---------------------------------------- -------------------------------- ,,"p ,i4 ,,! ,,cele/ial ,,sph]e4 #a4 ,def9ition ( ,a/ronomy4 ,a/ronomy has to d ) ! e>?1 sun1 moon1 planets1 comets1 meteors1 />s1 & nebul2 9 o!r words1 ) ! univ]se1 or 8! aggregate ( exi/+ ?+s40 ,x is ! mo/ anci5t ( all sci5ces4 ,! d]ivation ( ! "n f two ,greek words1 a/]1 8/>10 & nomos1 8law10 indicates its nature4 ,x deals ) ! law ( ! />s--! ^w 8/>0 be+ und]/ood1 9 its wide/ signification1 z includ+ e heav5ly body ( :atev] k9d4 ,! e>? itself is su* a body4 ,s9ce we happ5 to live on ! e>?1 x becomes our /&po9t 9 space1 f :i* we look out at ! o!rs4 ,b1 if we liv$ on "s o!r planet1 we w\ld see ! e>? z a di/ant body 9 ! sky1 j z we n[ see ,jupit] or ,m>s4 ,a/ronomy tea*es u t ev]y?+ 9 ! univ]se1 f ! sun & ! moon to ! mo/ remote /> or ! mo/ extraord9>y nebula1 is relat$ to ! e>?4 ,all are made ( simil> elem5t>y sub/ances & all obey simil> physical laws4 ,! same sub/ance :i* is a solid ^u ! e>? may be a gas or a vap\r 9 ! sun1 b t does n alt] its ess5tial nature4 ,iron appe>s 9 ! sun 9 ! =m ( a hot vap\r1 b fundam5tally x is ! same sub/ance :i* exi/s on ! e>? z a h>d1 t\gh1 & heavy metal4 ,its diff]5t /ates dep5d ^u ! temp]ature to :i* x is subject$4 ,! e>? is a cool body1 :ile ! sun is an int5sely hot "o2 consequ5tly iron is solid on ! e>? & vapor\s 9 ! sun1 j z 9 w9t] wat] is solid ice on ! surface ( a pond & /eamy vap\r ov] ! boil] 9 ! kit*54 ,ev5 on ! e>? we c make iron liquid 9 a bla/ furnace1 & ) ! /ill great] temp]atures obta9able 9 a laboratory we c turn x into vap\r1 ?us r$uc+ x to some?+ l ! /ate 9 :i* x regul>ly exi/s 9 ! sun4 ,?is fact1 t ! entire univ]se is made up ( simil> sub/ances1 diff]+ only 9 /ate accord+ to ! local circum/ances affect+ !m1 is ! greate/ ?+ t a/ronomy has to tell u4 ,x may be reg>d$ z ! fundam5tal law ( ! />s4 #b4 ,! ,situation ( ! ,e>? 9 ! ,heav5s4 ,"o ( ! greate/ triumphs ( human intellig5ce is ! discov]y ( ! real place :i* ! e>? occupies 9 ! univ]se4 ,?is discov]y has be5 made 9 spite ( ! mo/ deceptive appe>ances4 ,if we accept$ ! sole evid5ce ( our eyes1 z m5 once did1 we c\ld only conclude t ! e>? 0 ! c5tre ( ! univ]se4 ,9 ! daytime we see ! sun app>5tly mov+ ?r\gh ! sky f ea/ to we/1 z if x 7 travell+ 9 a circle r\nd ! e>?1 ov]head by day & und]foot at night4 ,9 ! night-"t1 we see ! />s app>5tly travell+ r\nd ! e>? 9 ! same way z ! sun4 ,! fact is1 t all ( !m are virtually motionless ) reg>d to ! e>?1 & _! app>5t movem5ts ?r\gh ! sky are produc$ by ! e>?'s rotation on its axis4 ,! e>? turns r\nd on itself once e tw5ty-f\r h\rs1 l a sp9n+ ball4 ,imag9e a fly on a rotat+ s*ool globe2 ! :ole room w\ld appe> to ! fly to be revolv+ r\nd x z ! heav5s appe> to revolve r\nd ! e>?4 ,x w\ld h to be a v intellig5t insect to correct ! deceptive evid5ce ( its eyes4 ,! actual facts1 reveal$ by _m c5turies ( obs]vation & reason+1 are t ! e>? is a rotat+ globe1 turn+ once on its axis e tw5ty-f\r h\rs & revolv+ once r\nd ! sun e ?ree hundr$ & sixty-five days4 ,! sun is also a globe1 #a1cjj1jjj times l>g] ?an ! e>?1 b s hot t x gl[s ) int5se brilliance1 :ile ! sub/ances ( :i* x consi/s are kept 9 a gase\s or vapor\s /ate4 ,besides ! e>? !re are sev5 o!r pr9cipal globes1 or planets1 :i* revolve r\nd ! sun1 at v>i\s di/ances & 9 v>i\s p]iods1 &1 9 addition to ^!1 !re are hundr$s ( small] bodies1 call$ a/]oids or small planets4 ,!re are also _m s+ul> bodies call$ comets1 & sw>ms ( /ill small] ones call$ meteors1 :i* likewise revolve r\nd ! sun4 ,! e>? & ! o!r bodies ( :i* we h j spok5 are n only cool] ?an ! sun1 b mo/ ( !m are 9 a solid /ate & d n %9e ) light ( _! own4 ,! sun furni%es bo? heat & light to ! small] & cool] bodies revolv+ r\nd x4 ,9 fact1 ! sun is simply a />1 resembl+ ! ?\s&s ( o!r />s :i* surr\nd u 9 ! sky1 & its app>5t sup]iority to !m is due only to ! fact t x is relatively ne>-by :ile !y are f> away4 ,x is probable t all1 or mo/1 ( ! />s also h planets1 comets1 & meteors revolv+ r\nd !m1 b invisible ow+ to _! imm5se di/ance4 ,! 8pa?s0 9 :i* ! e>? & ! o!r planets & bodies travel 9 _! revolutions r\nd ! sun are call$ _! orbits4 ,^! orbits are all elliptical 9 %ape1 b ^? ( ! e>? & ! o!r l>ge planets are n v diff]5t f circles4 ,"s ( ! a/]oids1 & all ( ! comets1 h[ev]1 travel 9 elliptical orbits ( consid]able ecc5tricity1 i4 e41 :i* diff] m>k$ly f circles4 ,! orbit ( ! e>? diff]s s slightly f a circle t ! ecc5tricity am\nts to only ab\t "o-sixtie?4 ,! di/ance ( ! e>? f ! sun be+1 on ! av]age1 #ic1jjj1jjj miles1 ! ecc5tricity ( its orbit causes x to approa* to )9 ab\t #ia1ejj1jjj miles 9 w9t] 7( ! nor!rn hemisph]e7 & to rec$e to ab\t #id1ejj1jjj miles 9 summ]4 ,! po9t 9 its orbit :]e ! e>? is ne>e/ ! sun is call$ p]ihelion1 & ! po9t :]e x is f>!/ f ! sun1 aphelion4 ,! e>? is at p]ihelion ab\t ,jan4 #a1 & at aphelion ab\t ,july #d4 ,n[1 9 ord] to make a g5]al picture 9 ! m9d ( ! e>?'s situation1 let ! read] suppose himself to be plac$ out 9 space z f> f ! sun z f ! o!r />s4 ,!n1 if he c\ld see x1 he w\ld obs]ve ! e>? z a little speck1 %9+ l a mote 9 ! sunlight1 & circl+ 9 its orbit close ar\nd ! sun4 ,! univ]se w\ld appe> to him to be some:at l an imm5se sph]ical room fill$ ) scatt]$ electric-light bulbs1 susp5d$ above1 bel[1 & all ar\nd him1 ea* ( ^! bulbs repres5t+ a sun1 & if !re 7 m9ute insects fly+ ar\nd ea* light1 ^! insects w\ld repres5t ! planets belong+ to ! v>i\s suns4 ,"o ( ! gl[+ bulbs among ! multitude w\ld /& = our sun1 & "o ( ! insects circl+ r\nd x w\ld be ! e>?4 ,illu/ration3 ,photograph ( ! ,s\? ,pol> ,region ( ! ,moon ,made by ,g4 ,w4 ,rit*ey ) ! =ty-in* refractor ( ! ,y]kes ,obs]vatory4 ,we h already rem>k$ t ! rotation ( ! e>? on its axis causes all ! o!r heav5ly bodies to appe> to revolve r\nd x once e tw5ty-f\r h\rs1 & we mu/ n[ add t ! e>?'s revolution r\nd ! sun causes ! same bodies to appe> to make ano!r1 sl[] revolution r\nd x once e ye>4 ,?is introduces a complication ( app>5t motions :i* x is ! bus9ess ( a/ronomy to deal )1 & :i* we %all endeav\r to expla94 #c4 ,! ,horizon1 ! ,z5i?1 & ! ,m]idian4 ,fir/1 let u consid] :at is ! ord9>y appe>ance ( ! sky4 ,:5 we g out ( doors on a cle> night we see ! heav5s 9 ! %ape ( a great dome ar*$ above u & fill$ ) />s4 ,:at we ?us see is "o half ( ! sph]ical %ell ( ! heav5s :i* surr\nds u on all sides1 ! e>? be+ app>5tly plac$ at its c5tre4 ,! o!r half is conceal$ f our sight beh9d1 or bel[1 ! e>?4 ,?is sph]ical %ell1 ( :i* only "o half is visible to u at a "t1 is call$ ! cele/ial sph]e4 ,n[1 ! surface ( ! e>? seems to u 7= ?is is ano!r ( ! deceptive appe>ances :i* a/ronomy has to correct7 to be a va/ flat expanse1 ^: level is brok5 by hills & m\nta9s1 & ! visible half ( ! cele/ial sph]e seems to b5d d[n on all sides & to re/ ^u ! e>? 9 a circle :i* ext5ds all ar\nd u4 ,?is circle1 :]e ! heav5s & ! e>? appe> to meet1 is call$ ! horizon4 ,z we ord9>ily see x1 ! horizon appe>s irregul> & brok5 on acc\nt ( ! unev5ness ( ! e>?'s surface1 b if we are at sea1 or 9 ! mid/ ( a great level prairie1 ! horizon appe>s z a smoo? circle1 ev]y:]e equally di/ant f ! eye4 ,?is circle is call$ ! s5sible horizon4 ,b !re is ano!r1 ideal1 horizon1 us$ 9 a/ronomy1 :i* is call$ ! rational horizon4 ,x is ( ! utmo/ importance t we %\ld cle>ly und]/& :at is meant by ! rational horizon1 & = ?is purpose we mu/ consid] ano!r fact conc]n+ ! dome ( ! sky4 ,we n[ turn our att5tion to ! c5tre ( t dome1 :i*1 ( c\rse1 is ! po9t directly ov]head4 ,?is po9t1 :i* is ( prim>y importance1 is call$ ! z5i?4 ,! position ( ! z5i? is indicat$ by ! direction ( a plumb-l9e4 ,if we imag9e a plumb-l9e to be susp5d$ f ! c5tre ( ! sky ov]head1 & to pass into ! e>? at our feet1 x w\ld run ?r\gh ! c5tre ( ! e>?1 &1 if x 7 cont9u$ onw>d 9 ! same direction1 x w\ld1 aft] em]g+ f ! o!r side ( ! e>?1 rea* ! c5tre ( ! invisible half ( ! sky-dome at a po9t diametrically opposite to ! z5i?4 ,?is c5tral po9t ( ! invisible half ( ! cele/ial sph]e1 ly+ "u our feet1 is call$ ! nadir4 ,keep+ 9 m9d ! def9itions ( z5i? & nadir t h j be5 giv51 we are 9 a position to und]/& :at ! rational horizon is4 ,x is a great circle ^: plane cuts ?r\gh ! c5tre ( ! e>?1 & :i* is situat$ exactly half-way betwe5 ! z5i? & ! nadir4 ,?is plane is necess>ily p]p5dicul>1 or at "r angles1 to ! plumb-l9e jo9+ ! z5i? & ! nadir4 ,9 o!r words1 ! rational horizon divides ! cele/ial sph]e into two precisely equal halves1 an upp] & a l[] half4 ,9 a hilly or m\nta9\s c\ntry ! s5sible or visible horizon diff]s widely f ! rational1 or true horizon1 b at sea ! two are ne>ly id5tical4 ,?is arises f ! fact1 t ! e>? is s excessively small 9 comp>ison ) ! di/ances ( mo/ ( ! heav5ly bodies t x may be reg>d$ z a m]e po9t 9 ! mid/ ( ! cele/ial sph]e4 ,illu/ration3 ,fig4 #a4 ,! ,rational & ! ,s5sible ,horizon4 ,let ,c be ! e>?'s c5tre1 ,o ! place ( ! obs]v]1 & ,h ,d ! rational horizon pass+ ?r\gh ! c5tre ( ! e>?4 ,= an object situat$ ne> ! e>?1 z at ,a1 ! s5sible horizon makes a l>ge angle ) ! rational horizon4 ,if ! object is f>!r away1 z at ,b1 ! angle becomes less2 & /ill less1 aga91 if ! object is at ,d4 ,x is evid5t t if ! object be imm5sely di/ant1 l a />1 ! s5sible horizon ,o ,s w be practically p>allel ) ! rational horizon1 & w bl5d ) x1 because ! radius1 or semi-diamet]1 ( ! e>?1 ,o ,c1 is virtually no?+ 9 comp>ison ) ! di/ance ( ! />4 ,besides ! horizon & ! z5i? !re is "o o!r ?+ ( fundam5tal importance :i* we mu/ le>n ab\t be=e proce$+ fur!r1--! m]idian4 ,! m]idian is an imag9>y l9e1 or semicircle1 beg9n+ at ! nor? po9t on ! horizon1 runn+ up ?r\gh ! z5i?1 & !n curv+ d[n to ! s\? po9t4 ,x ?us divides ! visible sky into two exactly equal halves1 an ea/]n & a we/]n half4 ,9 ! ord9>y affairs ( life we usually ?9k only ( t "p ( ! m]idian :i* ext5ds f ! z5i? to ! s\? po9t on ! horizon 7:i* is sometimes call$ ! noon-l9e0 because ! sun crosses x at noon71 b 9 a/ronomy ! nor!rn half ( ! m]idian is z important z ! s\!rn4 #d4 ,altitude & ,azimu?4 ,n[1 suppose t we wi% to indicate ! location ( a />1 or o!r object1 9 ! sky4 ,to d s1 we mu/ h "s fix$ basis ( ref]5ce1 & su* a basis is furni%$ by ! horizon & ! z5i?4 ,if we tri$ to describe ! position ( a />1 ! mo/ natural ?+ w\ld be1 fir/1 to e/imate1 or measure1 its height above ! horizon1 &1 second1 to indicate ! direction 9 :i* x 0 situat$ ) reg>d to ! po9ts ( ! compass4 ,^! two measures1 if !y 7 accurately made1 w\ld enable ano!r p]son to f9d ! /> 9 ! sky4 ,& ?is is precisely :at is done 9 a/ronomy4 ,! height above ! horizon is call$ altitude1 & ! be>+ ) ref]5ce to ! po9ts ( ! compass is call$ azimu?4 ,toge!r ^! are kn[n z co-ord9ates4 ,9 ord] to sy/ematise ?is me?od ( measur+ ! location ( a />1 ! a/ronom] uses imag9>y circles drawn on ! cele/ial sph]e4 ,! horizon & ! m]idian are two ( ^! circles4 ,9 addition to ^!1 o!r imag9>y circles are drawn p>allel to ! horizon & becom+ small] & small] until ! upp]mo/ "o may run close r\nd ! z5i?1 :i* is ! common c5tre ( ! entire set4 ,^! are call$ altitude circles1 because ea* "o ?r\gh\t its :ole ext5t is at an unv>y+ height1 or altitude1 above ! horizon4 ,su* circles may be drawn any:]e we please1 s z to pass ?r\gh any *os5 /> or />s4 ,if two />s 9 diff]5t qu>t]s ( ! sky are f\nd to lie on ! same circle1 !n we "k t bo? h ! same altitude4 ,illu/ration3 ,fig4 #b4 ,altitude & ,azimu?4 ,c is ! place ( ! obs]v]4 ,n ,c ,s1 a nor?-&-s\? l9e drawn 9 ! plane ( ! horizon4 ,e ,c ,w1 an ea/-&-we/ l9e 9 ! plane ( ! horizon4 ,n ,e ,s ,w1 ! circle ( ! horizon4 ,z1 ! obs]v]'s z5i?4 ,n ,z ,s1 v]tically above ,n ,c ,s1 ! m]idian4 ,e ,z ,w1 ! prime v]tical4 ,z s s1 "p ( a v]tical circle drawn ?r\gh ! /> s4 ,! circle ?r\gh s p>allel to ! horizon is an altitude circle4 ,! angle s ,c s1 or ! arc s s1 repres5ts ! />'s altitude4 ,! angle s ,c ,z1 or ! arc ,z s1 is ! />'s z5i? di/ance4 ,to f9d ! azimu?1 ! angul> di/ance r\nd ! horizon f ,s 7#j71 ?r\gh ,w1 ,n1 ,e1 to ! po9t :]e ! />'s v]tical circle meets ! horizon1 is measur$4 ,9 ?is case x is #cae4 ,b if we measur$ x ea/w>d f ! s\? po9t x w\ld be--#de4 ,!n ano!r set ( circles is drawn p]p5dicul> to ! horizon1 & all int]sect+ at ! z5i? & ! nadir4 ,^! are call$ v]tical circles1 f ! fact t !y are upright to ! horizon4 ,t "o ( ! v]tical circles :i* cuts ! horizon at ! nor?-&-s\? po9ts co9cides ) ! m]idian1 :i* we h already describ$4 ,! v]tical circle at "r angles to ! m]idian is call$ ! prime v]tical4 ,x cuts ! horizon at ! ea/ & we/ po9ts1 divid+ ! visible sky into a nor!rn & a s\!rn half4 ,l ! altitude circles1 v]tical circles may be drawn any:]e we please s z to pass ?r\gh a /> 9 any qu>t] ( ! sky--b ! m]idian & ! prime v]tical are fix$4 ,) ! two sets ( circles t h j be5 describ$1 x is possible to indicate accurately ! location ( any heav5ly body1 at any p>ticul> mom5t4 ,its altitude is asc]ta9$ by measur+1 along ! v]tical circle pass+ ?r\gh x1 its di/ance f ! horizon4 7,sometimes x is conv5i5t to measure1 in/ead ( ! altitude ( a />1 its z5i? di/ance1 :i* is also reckon$ on ! v]tical circle47 ,to asc]ta9 ! azimu?1 we mu/ fir/ *oose a po9t ( beg9n+ on ! horizon4 ,any ( ! c>d9al po9ts1 i4e41 ea/1 we/1 nor?1 or s\?1 may be employ$ = ?is purpose1 b 9 a/ronomy x is cu/om>y to use only ! s\? po9t1 & to c>ry ! measure we/w>d all r\nd ! circle ( ! horizon1 & s back to ! po9t ( beg9n+ 9 ! s\?4 ,?is involves circul>1 or degree1 measure1 to :i* a few words mu/ n[ be devot$4 ,e circle1 no matt] h[ l>ge or h[ small1 is divid$ into #cfj equal p>ts1 call$ degrees1 usually indicat$ by ! sign 772 ea* degree is subdivid$ into #fj equal p>ts call$ m9utes1 indicat$ by ! sign 772 & ea* m9ute is subdivid$ into #fj equal p>ts call$ seconds1 indicat$ by ! sign 774 ,?us !re are #cfj1 or #ba1fjj1 or #a1bif1jjj 9 e complete circle4 ,! actual l5g? ( a degree 9 in*es1 y>ds1 or miles1 dep5ds ^u ! size ( ! circle1 b no circle ev] has m ?an #cfj1 & a degree ( any p>ticul> circle is precisely equal to any o!r degree ( t same circle4 ,?us1 if a circle is #cfj miles 9 circumf]5ce1 e "o ( its degrees w be "o mile long4 ,9 ma!matics1 a degree usually means n a di/ance measur$ along ! circumf]5ce ( a circle1 b an angle =m$ at ! c5tre ( ! circle betwe5 two l9es call$ radii 7radius 9 ! s+ul>71 :i* l9es1 :]e !y int]sect ! circumf]5ce1 are sep>at$ by a di/ance equal to "o #cfj? ( ! entire circle4 ,b1 = ord9>y purposes1 x is simpl] to ?9k ( a degree z an arc equal 9 l5g? to "o #cfj? ( ! circle4 ,n[1 s9ce ! horizon1 & ! o!r imag9>y l9es drawn 9 ! sky1 are all circles1 x is evid5t t ! pr9ciple ( circul> measure may be appli$ to !m1 & inde$ mu/ be s appli$ 9 ord] t !y %all be ( use to u 9 indicat+ ! position ( a />4 ,to return1 !n1 to ! measurem5t ( ! azimu? ( a />4 ,s9ce ! s\? po9t is ! place ( beg9n+1 we m>k x #j1 & we divide ! circle ( ! horizon into #cfj1 c\nt+ r\nd we/w>d4 ,suppose we see a /> some:]e 9 ! s\?-we/]n qu>t] ( ! sky2 !n ! po9t :]e ! v]tical circle pass+ ?r\gh t /> int]sects ! horizon w indicate its azimu?4 ,suppose t ?is po9t is f\nd to be #be we/ ( s\?2 !n #be w be ! />'s azimu?4 ,suppose x is #ij2 !n ! azimu? is #ij1 & ! /> mu/ be on ! prime v]tical 9 ! we/1 because we/1 be+ "o qu>t] ( ! way r\nd ! horizon f s\?1 is #ij 9 angul> di/ance f ! s\? po9t4 ,suppose ! azimu? is #ahj2 !n ! /> mu/ be on ! m]idian nor? ( ! z5i?1 because nor? is exactly half-way1 or #ahj r\nd ! horizon f ! s\? po9t4 ,suppose ! azimu? is #bgj2 !n ! /> mu/ be on ! prime v]tical 9 ! ea/1 because ea/ is #bgj1 or ?ree qu>t]s ( ! way r\nd f ! s\? po9t4 ,if ! /> is on ! m]idian 9 ! s\? its azimu? may be call$ ei!r #j or #cfj1 because on any graduat$ circle ! m>k indicat+ #cfj co9cides 9 position ) #j1 t be+ at ! same "t ! po9t ( beg9n+ & ! po9t ( end+4 ,! same sy/em ( angul> measure is appli$ 9 asc]ta9+ a />'s altitude4 ,s9ce ! horizon is half-way betwe5 ! z5i? & ! nadir x mu/ be j #ij f ei!r4 ,if a /> is 9 ! z5i?1 !n its altitude is #ij1 & if x is bel[ ! z5i? its altitude lies some:]e betwe5 #j & #ij4 ,9 any case x _c be less ?an #j nor m ?an #ij4 ,hav+ measur$ ! altitude & ! azimu? we h ! two co-ord9ates :i* are ne$$ to indicate accurately ! place ( a /> 9 ! sky4 ,b1 z we %all see 9 a mom5t1 o!r co-ord9ates beside altitude & azimu? are ne$$ = a complete description ( ! places ( ! />s on ! cele/ial sph]e4 ,ow+ to ! app>5t revolution ( ! heav5s r\nd ! e>?1 ! altitudes & azimu?s ( ! cele/ial bodies are cont9ually *ang+4 ,we %all n[ /udy ! causes ( ^! *anges4 #e4 ,! ,app>5t ,motion ( ! ,heav5s4 ,we h lik5$ ! e>? to a rotat+ s*ool globe4 ,z su* a globe turns1 any p>ticul> spot on x is pres5t$ 9 succession t[>d ! v>i\s sides ( ! room4 ,9 precisely ! same way any spot on ! e>? is turn$ by its rotation successively t[>d v>i\s p>ts ( ! surr\nd+ sky4 ,to und]/& ! effect ( ?is1 a little pati5t wat*+ ( ! actual heav5s w be requir$1 b ?is has ! *>m ( all out-(-doors obs]vation ( nature1 & x w be f\nd ( fasc9at+ int]e/ z ! facts beg9 to unfold !mselves4 ,illu/ration3 ,! ,moon ,ne> ! 8,crat]0 ,ty*o ,photograph$ at ! ,lick ,obs]vatory "u ! direction ( ,e4 ,s4 ,hold54 ,ty*o is ! regul> oval depression a little bel[ ! c5tre ( ! view4 ,! va/ depression1 #adj miles across1 ) a r[ ( small] crat]s )91 bel[ ! c5tre ( ! view at ! top1 is ,clavius4 ,! photograph 0 made :5 sund[n 0 approa*+ on t "p ( ! moon4 ,obs]ve ! jagg$ l9e ( advanc+ night ly+ across ! rugg$ surface on ! we/]n 7left-h&7 side4 ,x is be/ to beg9 by f9d+ ! ,nor? ,/>1 or pole />4 ,if y are liv+ n f> f latitude #dj nor?1 :i* is ! m$ian latitude ( ! ,unit$ ,/ates1 y mu/1 aft] det]m9+ z closely z y c ! situation ( ! nor? po9t1 look upw>d along ! m]idian 9 ! nor? until y\r eyes are direct$ to a po9t ab\t #dj above ! horizon4 ,=ty degrees is some:at less ?an half-way f ! horizon to ! z5i?1 :i*1 z we h se51 are sep>at$ by an arc ( #ij4 ,at t po9t y w notice a lone /> ( :at a/ronom]s call ! second magnitude4 ,?is is ! celebrat$ ,nor? ,/>4 ,x is ! mo/ useful to man ( all ! />s1 except ! sun1 & x diff]s f all ! o!rs 9 a way pres5tly to be expla9$4 ,b fir/ x is ess5tial t y %\ld make no mi/ake 9 id5tify+ x4 ,!re are c]ta9 l&m>ks 9 ! sky :i* make su* id5tification c]ta94 ,9 ! fir/ place1 x is always s close to ! m]idian 9 ! nor?1 t by nak$-eye obs]vation y w\ld probably nev] suspect t x 0 n exactly on ! m]idian4 ,!n1 its altitude is always equal1 or v ne>ly equal1 to ! latitude ( ! place :]e y happ5 to be on ! e>?1 s t if y "k y\r latitude y "k h[ high to c>ry y\r eye above ! nor!rn horizon4 ,if y are 9 latitude #ej1 ! /> w be at #ej altitude1 & if y\r latitude is #cj1 ! altitude ( ! /> w be #cj4 ,next y w notice t ! ,nor? ,/> is situat$ at ! end ( ! h&le ( a k9d ( dipp]-%ap$ figure =m$ by />s1 ! h&le be+ b5t ! wrong way4 ,all ( ! />s =m+ ?is 8dipp]0 are fa9t1 except ! two :i* are f>!/ f ! ,nor? ,/>1 9 ! out] edge ( ! b[l1 "o ( :i* is ab\t z bright z ! ,nor? ,/> itself4 ,aga91 if y c>ry y\r eye along ! h&le to ! b[l1 & !n cont9ue onw>d ab\t z mu* f>!r1 y w be l$ to ano!r1 l>g]1 m conspicu\s1 & m p]fect1 dipp]-%ap$ figure1 :i* is 9 ! fam\s con/ellation ( ,ursa ,major1 or ! ,great ,be>4 ,?is /rik+ figure is call$ ! ,great ,dipp] 7kn[n 9 ,engl& z ,! ,wa974 ,x conta9s sev5 conspicu\s />s1 all ( :i*1 ) "o exception1 are equal 9 brightness to ! ,nor? ,/>4 ,n[1 look p>ticul>ly at ! two />s :i* indicate ! out] side ( ! b[l ( ?is dipp]1 & y w f9d t if y draw an imag9>y l9e ?r\gh !m t[>d ! m]idian 9 ! nor?1 x w lead y\r eye directly back to ! ,nor? ,/>4 ,^! two significant />s are (t5 call$ ,! ,po9t]s4 ,) _! aid y c make sure t y h really f\nd ! ,nor? ,/>4 ,hav+ f\nd x1 beg9 by not+ ! v>i\s gr\ps ( />s1 or con/ellations1 9 ! nor!rn "p ( ! sky1 &1 z ! night we>s on1 obs]ve :e!r any *ange takes place 9 _! position4 ,to make our description m def9ite1 we w suppose t ! obs]vations beg9 at n9e o'clock ,p4,m4 ab\t ! #a/ ( ,july4 ,at t h\r & date1 & f ! middle latitudes ( ! ,unit$ ,/ates1 ! ,great ,dipp] is se5 9 a s\?-we/]ly direction f ! ,nor? ,/>1 ) its h&le po9t+ ov]head4 ,at ! same "t1 on ! opposite side ( ! ,nor? ,/>1 & l[ 9 ! nor?-ea/1 appe>s ! rem>kable con/ellation ( ,cassiopeia1 easily recognisable by a zigzag figure1 r\ghly resembl+ ! lett] ,w1 =m$ by its five pr9cipal />s4 ,fix ! relative positions ( ^! con/ellations 9 ! memory1 & an h\r lat]1 at #aj ,p4,m41 look at !m aga94 ,y w f9d t !y h mov$1 ! ,great ,dipp] s9k+1 :ile ,cassiopeia rises4 ,make a ?ird obs]vation at #aa ,p4,m41 & y w p]ceive t ! motion has cont9u$1 ,! ,po9t]s hav+ desc5d$ 9 ! nor?-we/1 until !y are on a level ) ! ,nor? ,/>1 :ile ,cassiopeia has ris5 to ne>ly ! same level 9 ! nor?-ea/4 ,9 ! meantime1 ! ,nor? ,/> has rema9$ app>5tly motionless 9 its orig9al position4 ,if y repeat ! obs]vation at midnight1 y w f9d t ! ,great ,dipp] has desc5d$ s f> t its c5tre is on a level ) ! ,nor? ,/>1 & t ,cassiopeia has proportionally ris5 9 ! nor?-ea/4 ,x is j z if ! two con/ellations 7 atta*$ to ! ends ( a rod pivot$ at ! c5tre ^u ! ,nor? ,/> & twirl+ ab\t x4 ,9 ! meantime y w h notic$ t ! figure ( ! 8,little ,dipp]10 atta*$ to ! ,nor? ,/>1 :i* _h its b[l t[>d ! z5i? at #i ,p4,m41 has swung r\nd s t at midnight x is ext5d$ t[>d ! s\?-we/4 ,?us1 y w p]ceive t ! ,nor? ,/> is l a hub r\nd :i* ! heav5s appe> to turn1 c>ry+ ! o!r />s ) !m4 ,to conv9ce y\rself t ?is motion is common to ! />s 9 all p>ts ( ! sky1 y %\ld also wat* ! conduct ( ^? :i* pass ov]head1 & ^? :i* are 9 ! s\!rn qu>t] ( ! heav5s4 ,= in/ance1 at #i ,p4,m4 7same date71 y w see ne> ! z5i? a beautiful coronet :i* makes a /rik+ appe>ance al?\gh all b "o ( its />s are relatively fa9t4 ,?is is ! con/ellation ,corona ,borealis1 or ! ,nor!rn ,cr[n4 ,z ! h\rs pass y w see ! ,cr[n sw+ sl[ly we/w>d1 desc5d+ gradually t[>d ! horizon1 & if y p]sev]e 9 y\r obs]vations until ab\t #e ,a4,m41 y w see x set 9 ! nor?-we/4 ,! curve t x describes is conc5tric ) ^? foll[$ by ! ,great ,dipp] & ,cassiopeia1 b1 be+ f>!r f ! ,nor? ,/> ?an !y are1 & at a di/ance great] ?an ! altitude ( ! ,nor? ,/>1 x s9ks bel[ ! horizon be=e x c arrive at a po9t directly und]nea? t />4 ,!n take a /> f> 9 ! s\?4 ,at #i o'clock1 y w p]ceive ! bright r$di% /> ,ant>es1 9 ! con/ellation ,scorpio1 r l[ 9 ! s\? & consid]ably ea/ ( ! m]idian4 ,h\r aft] h\r x w move we/w>d1 9 a curve l>g] ?an t ( ! ,nor!rn ,cr[n1 b /ill conc5tric ) x4 ,a little be=e #aj ,p4,m41 x w cross ! m]idian1 & betwe5 #a & #b ,a4,m41 x w s9k b5ea? ! horizon at a po9t s\? ( we/4 ,s1 no matt] 9 :at "p ( ! heav5s y wat* ! />s1 y w see n only t !y move f ea/ to we/1 b t ?is motion is p]=m$ 9 curves conc5tric r\nd ! ,nor? ,/>1 :i* alone appe>s to ma9ta9 its place un*ang$4 ,along ! ea/]n horizon y w p]ceive />s cont9ually ris+2 9 ! middle ( ! sky y w see o!rs cont9ually cross+ ! m]idian--a maje/ic m>* ( con/ellations1--& along ! we/]n horizon y w f9d /ill o!rs cont9ually sett+4 ,if y c\ld wat* ! />s un9t]rupt$ly ?r\gh\t ! tw5ty-f\r h\rs 7if daylight did n hide !m f sight dur+ half t p]iod71 y w\ld p]ceive t !y g entirely r\nd ! cele/ial sph]e1 or r t x goes r\nd ) !m1 & t at ! end ( tw5ty-f\r h\rs !y return to _! orig9al positions4 ,b y c d ?is j z well by look+ at !m on two successive nights1 :5 y w f9d t at ! same h\r on ! second night !y are back aga91 practically 9 ! places :]e y saw !m on ! fir/ night4 ,( c\rse1 :at happ5s on a ,july night happ5s on any o!r night ( ! ye>4 ,we h tak5 a p>ticul> date m]ely 9 ord] to make ! description cle>]4 ,x is only necess>y to f9d ! ,nor? ,/>1 ! ,great ,dipp]1 & ,cassiopeia1 & y c obs]ve ! app>5t revolution ( ! heav5s at any "t ( ! ye>4 ,^! con/ellations1 be+ s ne> ! ,nor? ,/> t !y nev] g entirely bel[ ! horizon 9 middle nor!rn latitudes1 are always visible on "o side or ano!r ( ! ,nor? ,/>4 ,n[1 call ^u y\r imag9ation to deal ) :at y h be5 obs]v+1 & y w h no difficulty 9 expla9+ :at all ?is app>5t motion ( ! />s means4 ,y already "k t ! heav5s =m a sph]e surr\nd+ ! e>?4 ,y h simply to suppose ! ,nor? ,/> to be situat$ at1 or close to1 ! nor? end1 or nor? pole1 ( an imag9>y axle1 or axis1 r\nd :i* ! cele/ial sph]e seems to turn1 & in/antly ! :ole s]ies ( ph5om5a w fall into ord]1 & ! explanation w />e y 9 ! face4 ,t explanation is t ! motion ( all ! />s 9 conc5tric circles r\nd ! ,nor? ,/> is due to an app>5t revolution ( ! :ole cele/ial sph]e1 l a huge holl[ ball1 ab\t an axis1 ! position ( "o ( ^: poles is graphically indicat$ 9 ! sky by ! ,nor? ,/>4 ,! circles 9 :i* ! />s seem to move are p]p5dicul> to ?is axis1 & incl9$ to ! horizon at an angle dep5d+ ^u ! altitude ( ! ,nor? ,/> at ! place on ! e>? :]e ! obs]vations are made4 ,ano!r important fact dem&s our att5tion1 al?\gh ! ?\ghtful read] w already h guess$ x--! nor? pole ( ! cele/ial sph]e1 ^: position 9 ! sky is closely indicat$ by ! ,nor? ,/>1 is situat$ directly ov] ! nor? pole ( ! e>?4 ,?is foll[s f ! fact t ! app>5t revolution ( ! cele/ial sph]e is due to ! real rotation ( ! e>?4 ,y c see t ! two poles1 t ( ! e>? & t ( ! heav5s1 mu/ necess>ily co9cide1 by tak+ a s*ool globe & imag9+ t y are an intellig5t little be+ dwell+ on its surface4 ,z ! globe turn$ on its axis y w\ld see ! walls ( ! room revolv+ r\nd y1 & ! poles ( ! app>5t axis r\nd :i* ! room turn$1 w\ld1 evid5tly1 be directly ov] ! correspond+ poles ( ! globe itself4 ,ano!r ?+ :i* y c\ld make cle> by ?is exp]im5t is t1 z ! poles ( ! cele/ial sph]e are ov] ! e>?'s poles1 s ! cele/ial equator1 or equator ( ! heav5s1 mu/ be directly ov] ! equator ( ! e>?4 ,we c det]m9e ! location ( ! poles ( ! heav5s by wat*+ ! revolution ( ! />s ar\nd !m1 & we c fix ! position ( ! circle ( ! equator ( ! heav5s1 by draw+ an imag9>y l9e r\nd ! cele/ial sph]e1 half-way betwe5 ! two poles4 ,we h spok5 specifically only ( ! nor? pole1 b1 ( c\rse1 !re is a correspond+ s\? pole situat$ ov] ! s\? pole ( ! e>?1 b ^: position is invisible f ! nor!rn hemisph]e4 ,x happ5s t ! place ( ! s\? cele/ial pole is n indicat$ to ! eye1 l t ( ! nor!rn1 by a conspicu\s />4 ,illu/ration3 ,draw+ ( ,jupit] ,illu/ration3 ,draw+ ( ,jupit] ,note ! *ange ( details betwe5 ! two draw+s1 made at diff]5t times4 ,simil> *anges are cont9ually occurr+4 #f4 ,locat+ ! ,/>s on ! ,cele/ial ,sph]e4 ,hav+ f\nd ! poles & ! equator ( ! cele/ial sph]e1 we beg9 to see h[ x is possible to make a map1 or globe1 ( ! heav5s j z we d ( ! e>?1 on :i* ! objects t !y conta9 may be repres5t$ 9 _! prop] positions4 ,:5 we wi% to describe ! location ( an object on ! e>?1 a city = in/ance1 we h to ref] to a sy/em ( imag9>y circles1 drawn r\nd ! e>? & bas$ ^u ! equator & ! poles4 ,^! circles enable u to fix ! place ( any po9t on ! e>? ) accuracy4 ,"o set ( circles call$ p>allels ( latitude are drawn ea/ & we/ r\nd ! globe p>allel to ! equator1 & becom+ small] & small] until ! smalle/ runs close r\nd _! common c5tral po9t1 :i* is "o ( ! poles4 ,ea* pole ( ! e>? is ! c5tre ( su* a set ( circles all p>allel to ! equator4 ,s9ce ea* circle is unv>y+ 9 its di/ance f ! equator1 all places :i* are situat$ any:]e on t circle h ! same latitude1 or di/ance f ! equator1 ei!r nor? or s\?4 ,b to "k ! latitude ( any place on ! e>? is n suffici5t2 we mu/ also "k :at is call$ its longitude1 or its angul> di/ance ea/ or we/ ( "s *os5 po9t on ! equator4 ,?is k is obta9$ ) ! aid ( ano!r set ( circles drawn nor?-&-s\? r\nd ! e>?1 & all meet+ & cross+ at ! poles4 ,^! are call$ m]idians ( longitude4 ,9 ord] to make use ( !m we mu/1 z already intimat$1 select "s p>ticul> m]idian ^: cross+ po9t on ! equator w s]ve z a place ( beg9n+4 ,by common cons5t ( ! civilis$ _w1 ! m]idian :i* passes ?r\gh ! obs]vatory at ,gre5wi*1 ne> ,london1 has be5 *os5 = ?is purpose4 ,x is1 l all ! m]idians1 p]p5dicul> to ! equator1 & x is call$ ! prime m]idian ( ! e>?4 ,9 locat+ any place on ! e>?1 !n1 we asc]ta9 by means ( ! p>allel ( latitude pass+ ?r\gh x h[ f> 9 degrees1 x is nor? or s\? ( ! equator1 & by means ( its m]idian ( longitude h[ f> x is ea/ or we/ ( ! prime m]idian1 or m]idian ( ,gre5wi*4 ,^! two ?+s be+ kn[n1 we h ! exact location ( ! place on ! e>?4 ,let u n[ see h[ a simil> sy/em is appli$ to asc]ta9 ! location ( a heav5ly body on ! cele/ial sph]e4 ,we h obs]v$ t ! poles ( ! heav5s correspond 9 position1 or direction1 ) ^? ( ! e>?1 & t ! equator ( ! heav5s runs r\nd ! sky directly ov] ! e>?'s equator4 ,x foll[s t we c divide ! cele/ial sph]e j z we d ! surface ( ! e>? by means ( p>allels & m]idians1 correspond+ to ! simil> circles ( ! e>?4 ,on ! e>?1 di/ance f ! equator is call$ latitude1 & di/ance f ! prime m]idian1 longitude4 ,9 ! heav5s1 di/ance f ! equator is call$ decl9ation1 & di/ance f ! prime m]idian1 "r asc5sion2 b !y are ess5tially ! same ?+s z latitude & longitude1 & are measur$ virtually 9 ! same way4 ,9 place ( p>allels ( latitude1 we h on ! cele/ial sph]e circles drawn p>allel to ! equator & c5tr+ ab\t ! cele/ial poles1 :i* are call$ p>allels ( decl9ation1 & 9 place ( m]idians ( longitude1 we h circles p]p5dicul> to ! equator1 & drawn ?r\gh ! cele/ial poles1 :i* are call$ h\r circles4 ,! orig9 ( ?is "n w be expla9$ 9 a mom5t4 ,= ! pres5t x is only necess>y to fix firmly 9 ! m9d ! fact t ^! two sy/ems ( circles1 "o on ! e>? & ! o!r 9 ! heav5s1 are fundam5tally id5tical4 ,j z on ! e>? geograph]s h *os5 a p>ticul> place1 viz4 ,gre5wi*1 to fix ! location ( ! t]re/rial prime m]idian1 s a/ronom]s h agre$ ^u a p>ticul> po9t 9 ! heav5s :i* s]ves to det]m9e ! location ( ! cele/ial prime m]idian4 ,?is po9t1 :i* lies on ! cele/ial equator1 is kn[n z ! v]nal equ9ox4 ,we %all expla9 its orig9 aft] hav+ indicat$ its use4 ,! h\r circle :i* passes ?r\gh ! v]nal equ9ox is ! prime m]idian ( ! heav5s1 & ! v]nal equ9ox itself is sometimes call$ ! 8,gre5wi* ( ! ,sky40 ,if1 n[1 we wi% to asc]ta9 ! exact location ( a /> on ! cele/ial sph]e1 z we w\ld t ( ,new ,york1 ,london1 or ,p>is1 on ! e>?1 we measure along ! h\r circle runn+ ?r\gh x1 its decl9ation1 or di/ance f ! cele/ial equator1 & !n1 along its p>allel ( decl9ation1 we measure its "r asc5sion1 or di/ance f ! v]nal equ9ox4 ,hav+ ^! two co-ord9ates1 we possess all t is necess>y to enable u to describe ! position ( ! />1 s t someone else look+ = x1 may f9d x 9 ! sky1 z a navigator f9ds "s lone isl& 9 ! sea by kn[+ its latitude & longitude4 ,decl9ation1 z we h se51 is simply ano!r "n = latitude1 b "r asc5sion1 :i* corresponds to longitude1 ne$s a little additional explanation4 ,x diff]s f longitude1 fir/1 9 t1 in/ead ( be+ reckon$ bo? ea/ & we/ f ! prime m]idian1 x is reckon$ only t[>d ! ea/1 ! reckon+ be+ cont9u$ un9t]rupt$ly entirely r\nd ! circle ( ! equator2 &1 second1 9 t x is usually c\nt$ n 9 degrees1 m9utes1 & seconds ( arc1 b 9 h\rs1 m9utes1 & seconds ( "t4 ,! reason = ?is is t1 s9ce ! cele/ial sph]e makes "o complete revolution 9 tw5ty-f\r h\rs1 x is conv5i5t to divide ! circuit into tw5ty-f\r equal p>ts1 ea* correspond+ to ! di/ance ?r\gh :i* ! heav5s appe> to turn 9 "o h\r4 ,?is expla9s ! orig9 ( ! t]m h\r circles appli$ to ! cele/ial m]idians1 :i*1 by int]sect+ ! equator1 divide x into tw5ty-f\r equal p>ts1 ea* "p correspond+ to an h\r ( "t4 ,9 express+ "r asc5sion 9 "t1 ! ,roman num]als--,i1 ,,ii1 ,,iii1 ,,iv1 ,v1 ,,vi1 ,,vii1 ,,viii1 ,,ix1 ,x1 ,,xi1 ,,xii1 ,,xiii1 ,,xiv1 ,,xv1 ,,xvi1 ,,xvii1 ,,xviii1 ,,xix1 ,,xx1 ,,xxi1 ,,xxii1 ,,xxiii1 ,,xxiv--are employ$ = ! h\rs1 & ! lett]s m & s respectively = ! m9utes & seconds4 ,s9ce !re are #cfj 9 e circle1 x is pla9 t "o h\r ( "r asc5sion corresponds to #ae4 ,s1 too1 "o m9ute ( "r asc5sion corresponds to #ae1 & "o second to #ae4 ,x w be f\nd useful to memorise ^! relations4 ,illu/ration3 ,fig4 #c4 ,"r ,asc5sion & ,decl9ation4 ,! plane ( ! horizon1 ) ! nor?1 s\?1 ea/1 & we/ po9ts1 & ! z5i?1 are repres5t$ z 9 ,fig4 #b4 ,p & ,p are ! poles ( ! cele/ial sph]e1 ! dott$ l9e connect+ !m repres5t+ ! direction ( ! axis1 bo? ( cele/ial sph]e & ! e>?4 ,! circle ,eq ,eq is ! equator4 ,v is ! v]nal equ9ox1 or ! po9t on ! equator :5ce "r asc5sion is reckon$ r\nd t[>d ! ea/4 ,! circle pass+ ?r\gh s1 p>allel to ! equator1 is a decl9ation circle4 ,! circle ,p s ,p is ! h\r circle ( ! /> s4 ,! arc ( ?is h\r circle conta9$ betwe5 s & ! po9t :]e x meets ! equator is ! />'s decl9ation4 ,its "r asc5sion is measur$ by ! arc ( ! equator conta9$ betwe5 ,v & ! po9t :]e its h\r circle meets ! equator1 or by ! angle ,v ,p s4 ,! h\r circle ,p ,v ,p1 pass+ ?r\gh ! v]nal equ9ox1 is ! equ9octial colure4 ,:5 ?is has mov$ up to co9cid5ce ) ! m]idian1 ,n ,z ,s1 x w be a/ronomical noon4 #g4 ,effects ,produc$ by ,*ang+ ! ,obs]v]'s ,place on ! ,e>?4 ,! read] w recall t 9 ,sect4 #d we describ$ ano!r sy/em ( circles = det]m9+ ! places ( />s1 a sy/em bas$ on ! horizon & ! z5i?4 ,?is horizon-z5i? sy/em takes no acc\nt ( ! *anges produc$ by ! app>5t motion ( ! heav5s1 & consequ5tly x is n applicable to det]m9+ ! absolute positions ( ! />s on ! cele/ial sph]e4 ,x simply %[s _! positions 9 ! visible half ( ! sky1 z se5 at "s p>ticul> "t f "s def9ite po9t on ! e>?4 ,9 ord] to %[ ! *ang+ relations ( ?is sy/em to t :i* we h j be5 describ+1 let u consid] ! effects produc$ by %ift+ our place ( obs]vation on ! e>?4 ,s9ce ! z5i? is ! po9t ov]head & ! nadir ! po9t und]foot1 & ! horizon is a great circle drawn exactly half-way betwe5 ! z5i? & ! nadir1 x is evid5t1 ^u a mom5t's consid]ation1 t e place on ! e>? has its own z5i? & its own horizon4 ,x is also cle> t e place mu/ h its own m]idian1 because ! m]idian is a nor?-&-s\? l9e runn+ directly ov] ! obs]v]'s head4 ,y c see h[ ?is is1 if y reflect t = an obs]v] situat$ on ! o!r side ( ! e>? :at is ov]head = y w be und]foot = him1 & vice v]sa4 ,?us ! direction ( our z5i? is ! direction ( ! nadir = our antipodes1 & ! direction ( _! z5i? is ! direction ( our nadir4 ,!y see ! half ( ! sky :i* is invisible to u1 & we ! half :i* is invisible to !m4 ,n[1 suppose t we %\ld g to ! nor? pole4 ,! cele/ial nor? pole w\ld !n be 9 our z5i?1 & ! equator w\ld correspond ) ! horizon4 ,?us1 = an obs]v] at ! nor? pole ! two sy/ems ( circles t we h describ$ w\ld fall into co9cid5ce4 ,! z5i? w\ld correspond ) ! pole ( ! heav5s2 ! horizon w\ld correspond ) ! cele/ial equator2 ! v]tical circles w\ld correspond ) ! h\r circles2 & ! altitude circles w\ld correspond ) ! circles ( decl9ation4 ,! ,nor? ,/>1 be+ close to ! pole ( ! heav5s1 w\ld appe> directly ov]head4 ,be+ at ! z5i?1 its altitude w\ld be #ij 7see ,sect4 #d74 ,pe>y1 if he _h visit$ ! pole dur+ ! pol> night1 w\ld h se5 ! ,nor? ,/> ov]head1 & x w\ld h enabl$ him ) relatively little tr\ble to det]m9e 8 exact place on ! e>?1 or1 9 o!r words1 ! exact location ( ! nor? t]re/rial pole4 ,) ! pole /> 9 ! z5i?1 x is evid5t t ! o!r />s w\ld be se5 revolv+ r\nd x 9 circles p>allel to ! horizon4 ,all ! />s situat$ nor? ( ! cele/ial equator w\ld be simultane\sly & cont9u\sly visible4 ,none ( !m w\ld ei!r rise or set1 b all1 9 ! c\rse ( tw5ty-f\r h\rs1 w\ld appe> to make a complete circuit horizontally r\nd ! sky4 ,?is pol> pres5tation ( ! cele/ial sph]e is call$ ! p>allel sph]e1 because ! />s appe> to move p>allel to ! horizon4 ,no man has yet beheld ! nocturnal ph5om5a ( ! p>allel sph]e1 b if 9 ! future "s explor] %\ld visit "o ( ! e>?'s poles dur+ ! pol> night1 he w\ld behold ! spectacle 9 all its /range spl5d\r4 ,illu/ration3 ,jupit] ,photograph$ at ! ,lick ,obs]vatory4 ,obs]ve on ! left ! ,great ,r$ ,spot1 :i* fir/ appe>$ 9 #ahgh4 ,next1 suppose t y are some:]e on ! e>?'s equator4 ,s9ce ! equator is ev]y:]e #ij1 or "o qu>t] ( a circle1 f ea* pole1 x is evid5t t look+ at ! sky f ! equator y w\ld see ! two poles 7if !re 0 any?+ to m>k _! places7 ly+ on ! horizon "o exactly 9 ! nor? & ! o!r exactly 9 ! s\?4 ,! cele/ial equator w\ld correspond ) ! prime v]tical1 pass+ ea/ & we/ directly ov] y\r head1 & all ! />s w\ld rise & set p]p5dicul>ly to ! horizon1 ea* describ+ a semicircle 9 ! sky 9 ! c\rse ( twelve h\rs4 ,dur+ ! o!r twelve h\rs1 ! same />s w\ld be bel[ ! horizon4 ,/>s situat$ ne> ei!r ( ! poles w\ld describe little semi-circles ne> ! nor? or ! s\? po9t2 ^? f>!r away w\ld describe l>g] semi-circles2 ^? close to ! cele/ial equator w\ld describe semi-circles pass+ ov]head4 ,b all1 no matt] :]e situat$1 w\ld describe _! visible c\rses 9 ! same p]iod ( "t4 ,?is equatorial pres5tation ( ! cele/ial sph]e is call$ ! "r sph]e1 because ! />s rise & set at a "r angle to ! plane ( ! horizon4 ,comp>+ x ) ! sy/em ( circles on :i* "r asc5sion & decl9ation are bas$1 we see t1 z ! prime v]tical corresponds ) ! cele/ial equator1 s ! horizon mu/ repres5t an h\r circle4 ,! m]idian also repres5ts an h\r circle4 ,x may require a little ?\ght to make ?is cle>1 b x w be a good ex]cise4 ,f9ally1 if y are some:]e betwe5 ! equator & "o ( ! poles1 :i* is ! actual situation ( ! va/ majority ( mank9d1 y see ei!r ! nor? or ! s\? pole ( ! heav5s elevat$ to an altitude correspond+ ) y\r latitude1 & ! />s app>5tly revolv+ r\nd x 9 circles incl9$ to ! horizon at an angle dep5d+ ^u ! latitude4 ,! ne>] y are to ! equator1 ! /eep] ?is angle w be4 ,?is ord9>y pres5tation ( ! cele/ial sph]e is call$ ! oblique sph]e4 ,its horizon does n correspond ) ei!r ! equator or ! prime v]tical1 & its z5i? & nadir lie at po9ts situat$ betwe5 ! cele/ial poles & ! cele/ial equator4 #h4 ,! ,a/ronomical ,clock & ! ,ecliptic4 ,x w be rememb]$ t ! m]idian ( any place on ! e>? is a /raight nor? & s\? l9e runn+ ?r\gh ! z5i? & p]p5dicul> to ! horizon4 ,m /rictly speak+1 ! m]idian is a circle pass+ f nor? to s\? directly ov]head & correspond+ exactly ) ! m]idian ( longitude ( ! place ( obs]vation4 ,n[1 let u consid] ! h\r circles on ! cele/ial sph]e4 ,!y are drawn 9 ! same way z ! m]idians on ! e>?4 ,b ! cele/ial sph]e appe>s to revolve r\nd ! e>?1 & z x does s x mu/ c>ry ! h\r circles ) x1 s9ce !y are fix$ 9 position ^u its surface4 ,fix y\r att5tion ^u ! fir/ ( ^! h\r circles1 i4 e41 ! "o :i* runs ?r\gh ! v]nal equ9ox4 ,its "r asc5sion is call$ #j h\rs1 because x is ! />t+ po9t4 ,suppose t at "s "t we f9d ! v]nal equ9ox exactly 9 ! s\?2 !n ! #j h\r circle1 or ! prime m]idian ( ! heav5s1 w1 at t in/ant1 co9cide ) ! m]idian ( ! place ( obs]vation4 ,b "o h\r lat]1 9 consequ5ce ( ! motion ( ! heav5s1 ! v]nal equ9ox1 toge!r ) ! circle ( #j h\rs1 w be #ae1 or "o h\r ( "r asc5sion1 we/ ( ! m]idian1 & ! h\r circle m>k$ ,i w h come up to1 & = an in/ant w be bl5d$ )1 ! m]idian4 ,an h\r lat] /ill1 ! circle ( ,,ii h\rs "r asc5sion w h tak5 its place on ! m]idian1 :ile ! v]nal equ9ox & ! circle ( #j h\rs w be ,,ii h\rs1 or #cj1 we/ ( ! m]idian4 ,& s on1 ?r\gh\t ! entire circuit ( ! sky4 ,:at has j be5 said makes x evid5t t ! app>5t motion ( ! heav5s resembles ! movem5t ( a clock1 ! v]nal equ9ox1 or ! circle ( #j h\rs1 s]v+ z a h&1 or po9t]1 on ! dial4 ,a/ronom]s use x 9 exactly t way1 = a/ronomical clocks are made ) dials divid$ into tw5ty-f\r h\r spaces1 & ! "t reckon+ runs cont9u\sly f #j h\rs to ,,xxiv h\rs4 ,! 8a/ronomical day0 beg9s :5 ! v]nal equ9ox is on ! m]idian4 ,at t in/ant ! h&s ( ! a/ronomical clock m>k #j h\rs1 #j m9utes1 #j seconds4 ,?us ! clock foll[s ! motion ( ! heav5s1 & ! a/ronom] c tell by simply glanc+ at ! dial1 & )\t look+ at ! sky1 :]e ! v]nal equ9ox is1 & :at is ! "r asc5sion ( any body :i* may at t mom5t be on ! m]idian4 ,we mu/ n[ expla9 a little m fully :at ! v]nal equ9ox is1 & :y x has be5 *os5 z ! 8,gre5wi* ( ! ,sky40 ,its position is n m>k$ by any />1 b is det]m9$ by means ( ! int]sect+ circles t we h describ$4 ,!re is "o o!r su* circle1 t we h n yet m5tion$1 :i* be>s a peculi> relation to ! v]nal equ9ox4 ,?is is ! ecliptic4 ,j z ! daily1 or diurnal1 rotation ( ! e>? on its axis causes ! :ole cele/ial sph]e to appe> to make "o revolution e day1 s ! ye>ly or annual revolution ( ! e>? 9 its orbit ab\t ! sun causes ! sun to appe> to make "o revolution ?r\gh ! sky e ye>4 ,z ! e>? moves onw>d 9 its orbit1 ! sun seems to move 9 ! opposite direction4 ,inasmu* z !re are #cfj 9 a complete circle & #cfe days 9 a ye>1 ! app>5t motion ( ! sun am\nts to ne>ly #a p] day1 or #cj p] mon?4 ,9 twelve mon?s1 !n1 ! sun comes back aga9 to ! place 9 ! sky :i* x occupi$ at ! beg9n+ ( ! ye>4 ,s9ce ! motion ( ! e>? 9 its orbit is f we/ to ea/ 7! same z t ( its rotation on its axis71 x foll[s t ! direction ( ! sun's app>5t annual motion 9 ! sky is f ea/ to we/ 7l its daily motion74 ,?us1 :ile 9 fact ! e>? pursues a pa?1 or orbit1 r\nd ! sun1 ! sun seems to pursue a pa? r\nd ! e>?4 ,?is app>5t pa? ( ! sun1 project$ aga9/ ! backgr\nd ( ! sky1 is call$ ! ecliptic4 ,! "n arises f ! fact t eclipses only occur :5 ! moon is 9 or ne> ! plane ( ! sun's app>5t pa?4 ,z ! app>5t motion ( ! sun r\nd ! ecliptic is caus$ by ! real motion ( ! e>? r\nd ! sun1 we may reg>d ! ecliptic z a circle m>k+ ! int]section ( ! plane ( ! e>?'s orbit ) ! cele/ial sph]e4 ,9 o!r words1 if we 7 situat$ on ! sun in/ead ( on ! e>?1 we w\ld see ! e>? travell+ r\nd ! sky 9 ! circle ( ! ecliptic4 ,we mu/ keep ?is fact1 t ! ecliptic indicates ! plane ( ! e>?'s orbit1 firmly 9 m9d1 9 ord] to und]/& :at foll[s4 ,! ecliptic is n co9cid5t ) ! cele/ial equator1 = ! foll[+ reason3 ,! axis ( ! e>?'s daily rotation is n p>allel to1 or does n po9t 9 ! same direction z1 ! axis ( its ye>ly revolution r\nd ! sun4 ,z ! axis ( rotation is p]p5dicul> to ! equator1 s ! axis ( ! ye>ly revolution is p]p5dicul> to ! ecliptic1 & s9ce ^! two axes are incl9$ to "o ano!r1 x results t ! equator & ! ecliptic mu/ lie 9 diff]5t planes4 ,! incl9ation ( ! plane ( ! ecliptic to t ( ! equator am\nts to ab\t #bc4 ,z x is v important to h a cle> conception ( ?is subject1 we may illu/rate x 9 ?is way3 ,take a ball to repres5t ! e>?1 & ar\nd x draw a circle to repres5t ! equator4 ,!n1 ?r\gh ! c5tre ( ! ball1 & at "r angles to its equator1 put a long p9 to repres5t ! axis4 ,set x afloat 9 a tub ( wat]1 weight+ x s t x w be half subm]g$1 & plac+ x 9 su* a position t ! p9 w be n upright b incl9$ at a consid]able angle f ! v]tical4 ,n[1 imag9e t ! sun is situat$ 9 ! c5tre ( ! tub1 & cause ! ball to circle sl[ly r\nd x1 :ile ma9ta9+ ! p9 always 9 ! same position4 ,!n ! surface ( ! wat] w repres5t ! plane ( ! ecliptic1 or plane ( ! e>?'s orbit1 & y w see t1 9 consequ5ce ( ! incl9ation ( ! p91 ! plane ( ! equator does n co9cide ) t ( ! ecliptic 7or ! surface ( ! wat]71 b is tipp$ ) reg>d to x 9 su* a mann] t "o half ( ! equator is bel[ & ! o!r half above x4 ,in/ead ( actually try+ ?is exp]im5t1 x w be a useful ex]cise ( ! imag9ation to repres5t x to ! m9d's eye j z if x 7 tri$4 ,we h said t ! incl9ation ( ! equator to ! ecliptic am\nts to #bc1 & ?is angle %\ld be memoris$4 ,n[1 s9ce bo? ! ecliptic & ! equator are great circles ( ! cele/ial sph]e1 i4 e41 circles ^: planes cut ?r\gh ! c5tre ( ! sph]e1 !y mu/ int]sect "o ano!r at two opposite po9ts4 ,9 ! exp]im5t j describ$1 ^! two po9ts lie on opposite sides ( ! ball1 :]e ! equator cuts ! level ( ! wat]4 ,^! po9ts ( int]section ( ! equator & ! ecliptic on ! cele/ial sph]e are call$ ! equ9oxes1 or equ9octial po9ts1 because :5 ! sun appe>s at ei!r ( ^? po9ts x is p]p5dicul> ov] ! equator1 & :5 x is 9 t position day & night are ( equal l5g? all ov] ! e>?4 7,equ9ox is f two ,lat9 words mean+ 8equal night407 ,illu/ration3 ,saturn ,f a draw+ by ,tr\velot4 ,illu/ration3 ,saturn ,photograph$ at ! ,lick ,obs]vatory4 ,we %all h m to say ab\t ! equ9oxes lat]1 b = ! pres5t x is suffici5t to rem>k t "o ( ^! po9ts--t "o :]e ! sun is ab\t ! #ba/ ( ,m>*1 :i* is ! beg9n+ ( a/ronomical spr+--is ! 8,gre5wi* ( ! ,sky10 or ! v]nal equ9ox4 ,! o!r1 opposite1 po9t is call$ ! autumnal equ9ox1 because ! sun arrives !re ab\t ! #bc;d ( ,septemb]1 ! beg9n+ ( a/ronomical autumn4 ,! v]nal equ9ox1 z we h already se51 s]ves z a po9t] on ! dial ( ! sky4 ,:5 x crosses ! m]idian ( any place x is a/ronomical noon at t place4 ,its position 9 ! sky is n m>k$ by any p>ticul> />1 b x is situat$ 9 ! con/ellation ,pisces1 & lies exactly at ! cross+ po9t ( ! cele/ial equator & ! ecliptic4 ,! h\r circle1 runn+ ?r\gh ?is po9t1 & ?r\gh its opposite1 ! autumnal equ9ox1 is ! prime m]idian ( ! heav5s1 call$ ! equ9octial colure4 ,! h\r circle at "r angles to ! equ9octial colure1 i4 e41 be>+ to x ! same relation t ! prime v]tical does to ! m]idian 7see ,sect4 #d71 is call$ ! sol/itial colure4 ,?is latt] circle cuts ! ecliptic at two opposite po9ts1 call$ ! sol/ices1 :i* lie half-way betwe5 ! equ9oxes4 ,s9ce ! ecliptic is incl9$ #bc to ! plane ( ! equator1 & s9ce ! sol/ices lie half-way betwe5 ! two cross+ po9ts ( ! ecliptic & ! equator1 x is evid5t t ! sol/ices mu/ be situat$ #bc f ! equator1 "o above & ! o!r bel[1 or "o nor? & ! o!r s\?4 ,! nor!rn "o is call$ ! summ] sol/ice1 because ! sun arrives !re at ! beg9n+ ( a/ronomical summ]1 ab\t ! #bb;d ( ,june1 & ! s\!rn "o is call$ ! w9t] sol/ice1 because ! sun arrives !re at ! beg9n+ ( ! a/ronomical w9t]1 ab\t ! #bb;d ( ,decemb]4 ,! "n sol/ice comes f two ,lat9 words mean+ 8! /&+ /ill ( ! sun10 because :5 x is at ! sol/itial po9ts its app>5t c\rse ?r\gh ! sky is = sev]al days ne>ly horizontal & its decl9ation *anges v sl[ly4 ,n[1 j z !re are two opposite po9ts 9 ! sky at equal di/ances f ! equator1 :i* m>k ! poles ( ! imag9>y axis ab\t :i* ! cele/ial sph]e makes its diurnal revolution1 s !re are two opposite po9ts at equal di/ances f ! ecliptic :i* m>k ! poles ( ! imag9>y axis ab\t :i* ! ye>ly revolution ( ! sun takes place4 ,^! are call$ ! poles ( ! ecliptic1 & !y are situat$ #bc f ! cele/ial poles--a di/ance necess>ily correspond+ ) ! incl9ation ( ! ecliptic to ! equator4 ,! nor!rn pole ( ! ecliptic is 9 ! con/ellation ,draco1 :i* y may see any night circl+ r\nd ! ,nor? ,/>1 toge!r ) ! ,great ,dipp] & ,cassiopeia4 #i4 ,cele/ial ,latitude & ,longitude4 ,we h se5 t ! cele/ial sph]e is m>k$ ) imag9>y circles resembl+ ! circles ( latitude & longitude on ! e>?1 & t 9 bo? cases ! circles are us$ = a simil> purpose1 viz41 to det]m9e ! location ( objects1 9 "o case on ! globe ( ! e>? & 9 ! o!r on ! sph]e ( ! heav5s4 ,x has also be5 expla9$ t :at corresponds to latitude on ! cele/ial sph]e is call$ decl9ation1 & :at corresponds to longitude is call$ "r asc5sion4 ,x happ5s1 h[ev]1 t ^! same t]ms1 latitude & longitude1 are also employ$ 9 a/ronomy4 ,b1 un=tunately1 !y are bas$ ^u a diff]5t set ( circles f t :i* has be5 describ$1 & !y d n correspond 9 ! way t "r asc5sion & decl9ation d to t]re/rial longitude & latitude4 ,a few words mu/ !re=e be devot$ to cele/ial latitude & longitude1 z di/+ui%$ f decl9ation & "r asc5sion4 ,cele/ial latitude & longitude !n1 in/ead ( be+ bas$ ^u ! equator & ! poles1 are bas$ ^u ! ecliptic & ! poles ( ! ecliptic4 ,cele/ial latitude means di/ance nor? or s\? ( ! ecliptic 7n ( ! equator71 & cele/ial longitude means di/ance f ! v]nal equ9ox reckon$ along ! ecliptic 7n along ! equator74 ,cele/ial longitude runs1 ! same z "r asc5sion1 f we/ t[>d ea/1 b x is reckon$ 9 degrees in/ead ( h\rs4 ,cele/ial latitude is measur$ ! same z decl9ation1 b along circles runn+ ?r\gh ! poles ( ! ecliptic in/ead ( ! cele/ial poles1 & drawn p]p5dicul> to ! ecliptic in/ead ( to ! equator4 ,circles ( cele/ial latitude are drawn p>allel to ! ecliptic & c5tr+ r\nd ! poles ( ! ecliptic1 & m]idians ( cele/ial longitude are drawn ?r\gh ! poles ( ! ecliptic & p]p5dicul> to ! ecliptic itself4 ,! m]idian ( cele/ial longitude t passes ?r\gh ! two equ9oxes is ! ecliptic prime m]idian4 ,?is int]sects ! equ9octial colure at ! equ9octial po9ts1 mak+ ) x an angle ( #bc4 ,! sol/itial colure1 :i* x w be rememb]$ runs r\nd ! cele/ial sph]e half-way betwe5 ! equ9oxes1 is p]p5dicul> to ! ecliptic z well z to ! equator1 & s is common to ! two sy/ems ( circles4 ,x passes alike ?r\gh ! cele/ial poles & ! poles ( ! ecliptic4 ,x w also be obs]v$ t ! v]nal equ9ox is common to ! two sy/ems ( co-ord9ates1 because x lies at "o ( ! int]sections ( ! ecliptic & ! equator4 ,9 pass+ f "o sy/em to ! o!r1 ! a/ronom] employs ! me?ods ( sph]ical trigonometry4 ,illu/ration3 ,fig4 #d4 ,! ,ecliptic & ,cele/ial ,latitude & ,longitude4 ,c1 z 9 ! o!r figures1 is ! place ( ! obs]v] & ,z is ! z5i?1 b to avoid complication ( details ! circle ( ! horizon is n drawn1 only ! nor?-&-s\? l9e1 ,n ,c ,s1 be+ %[n4 ,eq ,eq is ! equator4 ,ec ,ec is ! ecliptic4 ,p & ,p are ! cele/ial poles4 p & p are ! poles ( ! ecliptic4 ,na is ! nadir4 ,v is ! v]nal equ9ox1 & ,a ! autumnal equ9ox4 ,! circle ?r\gh s1 p>allel to ! ecliptic1 is a latitude circle4 ,! circle p s p' is ! ecliptic m]idian ( ! /> s4 ,! circle ,p ,v ,p ,a is ! equ9octial colure4 ,! circle p ,v p ,a is ! prime ecliptic m]idian4 ,! arc ( ! ecliptic m]idian conta9$ betwe5 ! ecliptic & s measures ! />'s latitude4 ,! arc ( ! ecliptic conta9$ betwe5 ,v & ! po9t :]e ! ecliptic m]idian p s p meets ! ecliptic 7or ! angle ,v p s7 measures ! />'s longitude ea/ f ,v1 ! v]nal equ9ox4 #aj4 ,! ,zodiac & ! ,precession ( ! ,equ9oxes4 ,! next ?+ ) :i* we mu/ make acqua9tance is ! zodiac4 ,we h le>n$ t ! ecliptic is a great circle ( ! cele/ial sph]e incl9$ at an angle ( #bc to ! equator1 & cross+ ! latt] at two opposite po9ts call$ ! equ9oxes1 & t ! sun 9 its annual j\rney r\nd ! sky foll[s ! circle ( ! ecliptic4 ,consequ5tly1 ! place :i* ! sun occupies at any "t mu/ be some:]e on ! c\rse ( ! ecliptic4 ,! fact has be5 m5tion$ t z se5 f ! sun ! e>? w\ld appe> to travel r\nd ! ecliptic1 :5ce ! ecliptic may be reg>d$ z ! projection ( ! e>?'s orbit1 or pa?1 aga9/ ! backgr\nd ( ! heav5s4 ,b1 besides ! e>? !re are sev5 o!r l>ge planets1 ,m]cury1 ,v5us1 ,m>s1 ,jupit]1 ,saturn1 ,uranus1 & ,neptune1 :i*1 l x1 revolve r\nd ! sun1 "s ne>] & "s f>!r away4 ,n[1 ! orbits ( all ( ^! planets lie 9 planes ne>ly co9cid5t ) t ( ! e>?'s orbit4 ,none ( !m is incl9$ m ?an #g f ! ecliptic & mo/ ( !m are incl9$ only "o or two degrees4 ,consequ5tly1 z we wat* ^! planets mov+ sl[ly r\nd 9 _! orbits we f9d t !y are always q close to ! circle ( ! ecliptic4 ,?is fact %[s t ! sol> sy/em1 i4 e41 ! sun & its att5dant planets1 occupies a disk-%ap$ area 9 space1 ! outl9es ( :i* w\ld be l ^? ( a v ?9 r\nd *eese1 ) ! sun 9 ! c5tre4 ,! ecliptic indicates ! m$ian plane ( ?is imag9>y disk4 ,! moon1 too1 travels ne>ly 9 ?is common plane1 its orbit r\nd ! e>? be+ incl9$ only a little m ?an #e to ! ecliptic4 ,ev5 ! e>ly a/ronom]s notic$ ^! facts1 & 9 anci5t times !y gave to ! app>5t road r\nd ! sky 9 :i* ! sun & planets travel1 9 tracks relatively z close toge!r z ! p>allel m>ks ( :eels on a highway1 ! "n zodiac4 ,!y assign$ to x a c]ta9 arbitr>y wid?1 suffici5t to include ! orbits ( all ! planets kn[n to !m4 ,?is wid? is #h on ea* side ( ! circle ( ! ecliptic1 or #af 9 all4 ,!y also divid$ ! r+ ( ! zodiac into twelve equal p>ts1 correspond+ ) ! numb] ( mon?s 9 a ye>1 & ea* "p 0 call$ a sign ( ! zodiac4 ,s9ce !re are #cfj 9 a circle1 ea* sign ( ! zodiac has a l5g? ( j #cj4 ,to indicate ! c\rse ( ! zodiac to ! eye1 its inv5tors obs]v$ ! con/ellations ly+ along x1 assign+ "o con/ellation to ea* sign4 ,beg9n+ at ! v]nal equ9ox1 & runn+ r\nd ea/w>d1 !y gave to ^! zodiacal con/ellations1 z well z to ! correspond+ signs1 names drawn f fancy resemblances ( ! figures =m$ by ! />s to m51 animals1 or o!r objects4 ,! fir/ sign & con/ellation 7 call$ ,aries1 ! ,ram1 indicat$ by ! symbol 2 ! second1 ,taurus1 ! ,bull1 2 ! ?ird1 ,gem9i1 ! ,tw9s1 2 ! f\r?1 ,canc]1 ! ,crab1 2 ! fif?1 ,leo1 ! ,lion1 2 ! six?1 ,virgo1 ! ,virg91 2 ! sev5?1 ,libra1 ! ,balance1 2 ! eigh?1 ,scorpio1 ! ,scorpion1 2 ! n9?1 ,sagitt>ius1 ! ,ar*]1 2 ! t5?1 ,capricornus1 ! ,goat1 2 ! elev5?1 ,aqu>ius1 ! ,wat]-,be>]1 2 & ! twelf?1 ,pisces1 ! ,fi%es1 4 ,! "n zodiac comes f a ,greek ^w = animal1 s9ce mo/ ( ! imag9>y figures =m$ by ! />s ( ! zodiacal con/ellations are ^? ( animals4 ,! signs & _! correspond+ con/ellations be+ suppos$ fix$ 9 ! sky1 ! planets1 toge!r ) ! sun & ! moon1 7 obs]v$ to run ?r\gh !m 9 succession f we/ to ea/4 ,:5 ?is sy/em 0 inv5t$1 ! signs & _! con/ellations co9cid$ 9 position1 b 9 ! c\rse ( "t x 0 f\nd t !y 7 drift+ ap>t1 ! signs1 ^: />t+ po9t rema9$ ! v]nal equ9ox1 back+ we/w>d ?r\gh ! sky until !y became disjo9$ f _! prop] con/ellations4 ,at pres5t ! sign ,aries is f\nd 9 ! con/ellation next we/ ( its orig9al position1 viz41 ,pisces1 & s on r\nd ! entire circle4 ,?is motion1 z already intimat$1 c>ries ! equ9oxes along ) ! signs1 s t ! v]nal equ9ox1 :i* 0 once at ! beg9n+ ( ! con/ellation ,aries 7z x /ill is at ! beg9n+1 or 8fir/ po9t10 ( ! sign ,aries71 is n[ f\nd 9 ! con/ellation ,pisces4 ,to expla9 ! %ift+ ( ! signs ( ! zodiac on ! face ( ! sky we mu/ consid] ! ph5om5on kn[n z ! precession ( ! equ9oxes1 :i* is "o ( ! mo/ int]e/+ ?+s 9 a/ronomy4 ,let u ref] aga9 to ! fact t ! axis ( ! e>?'s daily rotation is incl9$ #bc f a p]p5dicul> to ! plane ( its ye>ly revolution r\nd ! sun1 f :i* x results t ! ecliptic is tipp$ at ! same angle to ! plane ( ! equator4 ,?us ! sun1 mov+ 9 ! ecliptic1 appe>s half ! ye> above 7or nor? (7 ! equator1 & half ! ye> bel[ 7or s\? (7 x1 ! cross+ po9ts be+ ! two equ9oxes4 ,n[1 ?is incl9ation ( ! e>?'s axis is ! key to ! explanation we are seek+4 ,! direction 9 :i* ! axis lies 9 space is a fix$ direction1 :i* c be *ang$ only by "s outside =ce int]f]+4 ,:at we mean by ?is w become cle>] if we ?9k ( ! e>?'s axis z resembl+ ! peg ( a top1 or ! axis ( a gyroscope4 ,:5 a top is sp9n+ smoo?ly1 ) its peg v]tical1 ! peg w rema9 v]tical z long z ! sp9 is n dim9i%$1 & no outside =ce int]f]es4 ,s1 too1 ! axis ( ! sp9n+-:eel ( a gyroscope keeps po9t+ 9 ! same direction s p]si/5tly t ! :eel is kept f fall+4 ,if x is s m\nt$ t x is free to move 9 any direction1 & if !n y take ! in/rum5t 9 y\r h& & turn r\nd ) x1 ! axis w adju/ itself 9 su* a mann] z to reta9 its orig9al direction 9 space4 ,?is t5d5cy ( a rotat+ body to keep its axis ( rotation fix$ applies equally to ! e>?1 ^: axis1 also1 ma9ta9s a con/ant direction 9 space1 except = a sl[ *ange produc$ by outside =ces1 :i* *ange con/itutes ! ph5om5on ( ! precession ( ! equ9oxes4 ,we _c too (t5 recall ! fact t ! axis ( ! e>? is co9cid5t 9 direction ) t ( ! cele/ial sph]e1 s t ! e>?'s poles are situat$ directly "u ! cele/ial poles4 ,b ! poles ( ! ecliptic are #bc aside f ! cele/ial poles4 ,if ! direction ( ! e>?'s axis & ) x t ( ! cele/ial sph]e1 did n *ange at all1 !n ! cele/ial poles & ! poles ( ! ecliptic w\ld always reta9 ! same relative positions 9 ! sky2 b1 9 fact1 an ext]ior =ce1 act+ ^u ! e>?1 causes a gradual *ange 9 ! direction ( its axis1 & 9 consequ5ce ( ?is *ange ! cele/ial poles1 ^: position dep5ds ^u t ( ! e>?'s poles1 h a sl[ motion ( revolution ab\t ! poles ( ! ecliptic1 9 a circle ( #bc radius4 ,! =ce :i* produces ?is effect is ! attraction ( ! sun & ! moon ^u ! protub]ant "p ( ! e>? r\nd its equator4 ,if ! e>? 7 a p]fect sph]e1 ?is =ce c\ld n act1 or w\ld n exi/1 b s9ce ! e>? is an oblate sph]oid1 slightly flatt5$ at ! poles1 & bulg$ r\nd ! equator1 ! attraction acts ^u ! equatorial protub]ance 9 su* a way z to /rive to pull ! e>?'s axis into an upright position ) respect to ! plane ( ! ecliptic4 ,b1 9 consequ5ce ( its sp9n+ motion1 ! e>? resi/s ?is pull1 & tries1 s to speak1 to keep ! incl9ation ( its axis un*ang$4 ,! result is t ! axis sw+s sl[ly r\nd :ile ma9ta9+ ne>ly ! same incl9ation to ! plane ( ! ecliptic4 ,"h1 aga91 we may employ ! illu/ration ( a top4 ,if ! peg ( ! top is tipp$ a little aside1 s t ! attraction ( gravitation w\ld cause ! top to fall flat on ! table if x 7 n sp9n+1 x w1 z long z x cont9ues to sp91 sw+ r\nd & r\nd 9 a circle in/ead ( fall+4 ,we _c ent] into a ma!matical explanation ( ?is ph5om5on "h1 b ! read] w f9d a cle> popul> acc\nt ( ! :ole matt] 9 ,pr(4 ,john ,p]ry's little book on ,sp9n+ ,tops4 ,x is suffici5t "h to say t ! attraction ( gravitation1 t5d+ to make ! top fall1 b really caus+ ! peg to turn r\nd & r\nd1 resembles1 9 its effect1 ! attraction ( ! sun & ! moon ^u ! equatorial protub]ance ( ! e>?1 :i* makes ! e>?'s axis turn r\nd 9 space4 ,illu/ration3 ,! ,milky ,way ab\t ,*i ,cygni ,photograph$ at ! ,lick ,obs]vatory by ,e4 ,e4 ,b>n>d1 ) ! six-in* ,will>d l5s4 ,obs]ve ! cl\d-l =ms4 ,n[1 z we h said1 ?is sl[ sw++ r\nd ( ! axis ( ! e>? produces ! s-call$ precession ( ! equ9oxes4 ,9 a p]iod ( ab\t #be1hjj ye>s1 ! axis makes "o complete sw+ r\nd1 s t 9 t space ( "t ! cele/ial poles describe a revolution ab\t ! poles ( ! ecliptic1 :i* rema9 fix$4 ,b s9ce ! equator is a circle situat$ half-way betwe5 ! poles1 x is evid5t t x mu/ turn also4 ,to illu/rate ?is1 take a r\nd flat disk ( t91 or pa/ebo>d1 to repres5t ! equator & its plane1 & p]p5dicul>ly ?r\gh its c5tre run a /raight rod to repres5t ! axis4 ,put "o end ( ! axis on ! table1 &1 hold+ x at a fix$ incl9ation1 turn ! upp] end r\nd 9 a circle4 ,y w see t z ! axis ?us revolves1 ! disk revolves ) x1 & if y imag9e a plane1 p>allel to ! surface ( ! table1 pass+ ?r\gh ! c5tre ( ! disk at ! po9t :]e ! rod pi]ces x1 y w p]ceive t ! two opposite po9ts1 :]e ! edge ( ! disk int]sects ?is imag9>y plane1 revolve ) ! disk4 ,9 "o position ( ! axis1 = in/ance1 ^! po9ts may lie 9 ! direction ( ! nor?-&-s\? sides ( ! room4 ,:5 y h revolv$ ! axis1 & ) x ! disk1 "o qu>t] way r\nd1 ! po9ts w lie t[>d ! ea/ & we/ sides ( ! room4 ,:5 y h produc$ a half revolution !y w once m lie t[>d ! nor?-&-s\?1 b n[ ! direction ( ! slope ( ! disk w be ! rev]se ( t :i* x _h at ! beg9n+4 ,f9ally1 :5 ! revolution is complet$1 ! two po9ts w aga9 lie nor?-&-s\? & ! slope ( ! disk w be 9 ! same direction z at ! />t4 ,9 ?is illu/ration ! disk /&s = ! plane ( ! cele/ial equator1 ! rod = ! axis ( ! cele/ial sph]e1 ! imag9>y plane p>allel to ! surface ( ! table = ! plane ( ! ecliptic1 & ! two opposite po9ts :]e ?is plane is int]sect$ by ! edge ( ! disk = ! equ9oxes4 ,! motion ( ^! po9ts z ! incl9$ disk revolves repres5ts ! precession ( ! equ9oxes4 ,?is t]m means t ! direction ( ! motion ( ! equ9oxes1 z !y %ift _! place on ! ecliptic1 is su* t !y seem to precess1 or move =w>d1 z if to meet ! sun 9 its annual j\rney r\nd ! ecliptic4 ,! direction is f ea/ to we/1 & ?us ! zodiacal signs are c>ri$ f>!r & f>!r we/w>d f ! con/ellations orig9ally associat$ ) !m2 = ^! signs1 z we h said1 are s arrang$ t !y beg9 at ! v]nal equ9ox1 & if ! equ9ox moves1 ! :ole sy/em ( signs mu/ move ) x4 ,! am\nt ( ! motion is ab\t #ej4b p] ye>1 & s9ce !re are #a1bif1jjj 9 a circle1 simple division %[s t ! "t requir$ = "o complete revolution ( ! equ9oxes mu/ be1 z already /at$ 9 ref]5ce to ! poles1 ab\t #be1hjj ye>s4 ,a little ov] #bjjj ye>s ago ! signs & ! con/ellations 7 9 accord2 x foll[s1 !n1 t ab\t #bc1hjj ye>s 9 ! future1 !y w be 9 accord aga94 ,9 ! mean:ile ! signs w h back$ entirely r\nd ! circle ( ! ecliptic4 ,! att5tive read] w p]ceive t ! precession ( ! equ9oxes1 ) its att5dant revolution ( ! cele/ial poles r\nd ! poles ( ! ecliptic1 mu/ affect ! position ( ! ,nor? ,/>4 ,we h already said t t /> only happ5s to occupy its pres5t comm&+ position 9 ! sky4 ,! /> itself is motionless1 or practically s1 ) reg>d to ! e>?1 & x is ! nor? pole t *anges its place4 ,at ! pres5t "t ! pole is ab\t #a #aj1 f ! ,nor? ,/>1 9 ! direction ( ! ,great ,dipp]1 & x is sl[ly draw+ ne>] s t 9 ab\t #bjj ye>s x w be less ?an half a degree f ! />4 ,aft] t ! precessional motion w c>ry ! pole 9 a circle dep>t+ f>!r & f>!r f ! />1 until ! latt] w h entirely lo/ its importance z a guide to ! position ( ! pole4 ,x happ5s1 h[ev]1 t sev]al o!r conspicu\s />s lie ne> ?is circle4 ,"o ( ^! is ,?uban1 or ,alpha ,draconis 7n n[ z bright z x once 071 & ?is /> at ! "t :5 x s]v$ z an indicator ( ! place ( ! pole1 "s #dfjj ye>s ago1 0 connect$ ) a v romantic *apt] 9 ! hi/ory ( a/ronomy4 ,9 ! great pyramid ( ,*eops 9 ,egypt1 !re is a long passage lead+ /raight t[>d ! nor? f a *amb] cut deep 9 ! rock "u ! c5tre ( ! pyramid1 & ! upw>d slope ( ?is passage is su* t x is believ$ to h be5 employ$ by ! ,egyptian a/ronom]-prie/s z a k9d ( telescope-tube = view+ ! !n pole />1 & obs]v+ ! times ( its passage ov] ! m]idian--= ev5 ! ,nor? ,/>1 s9ce x is n exactly at ! pole1 revolves e tw5ty-f\r h\rs 9 a t9y circle ab\t x1 & consequ5tly crosses ! m]idian twice a day1 once above & once b5ea? ! true pole4 ,ab\t #aa1ejj ye>s 9 ! future1 ! extremely brilliant /> ,vega1 or ,alpha ,lyr1 w s]ve z a pole />1 al?\gh x w n be z ne> ! pole z ! ,nor? ,/> n[ is4 ,at t "t ! ,nor? ,/> w be ne>ly #ej f ! pole4 ,9 ab\t #ba1jjj ye>s ! pole w h come r\nd aga9 to ! neighb\rhood ( ,alpha ,draconis1 ! /> ( ! pyramid1 & 9 ab\t #be1hjj ye>s ! ,nor? ,/> w h be5 re/or$ to its pres5t pre/ige z ! app>5t hub ( ! heav5s4 ,"o curi\s irregul>ity 9 ! motion ( ! e>?'s poles mu/ be m5tion$ 9 connection ) ! precession ( ! equ9oxes4 ,?is is a k9d ( 8nodd+10 kn[n z nutation4 ,x arises f v>iation 9 ! effect ( ! attraction ( ! sun & ! moon1 due to ! v>y+ directions 9 :i* ! attraction is ex]cis$4 ,z f> z ! sun is conc]n$1 ! precession is sl[] ne> ! "t ( ! equ9oxes ?an 9 o!r p>ts ( ! ye>2 9 o!r words1 x is mo/ rapid 9 mid-summ] & mid-w9t] :5 "o or ! o!r ( ! poles is turn$ sunw>d4 ,a simil>1 b mu* l>g]1 *ange takes place 9 ! effect ( ! moon's attraction ow+ to ! incl9ation ( h] orbit to ! ecliptic4 ,dur+ ab\t n9e & a half ye>s1 or half ! p]iod ( revolution ( h] nodes 7see ,"p ,,iii1 ,section #d71 ! moon t5ds to ha/5 ! precession1 & dur+ ! next n9e & a half ye>s to ret>d x4 ,! g5]al effect ( ! comb9ation ( ^! irregul>ities is to cause ! e>?'s poles to describe a slightly wav+ curve in/ead ( a smoo? circle r\nd ! poles ( ! ecliptic4 ,!re are ab\t #adjj ( ^! 8waves10 or 8nods10 9 ! motion ( ! poles 9 ! c\rse ( _! #bf1jjj-ye> circuit4 ,9 accurate obs]vation ! a/ronom] is compell$ to take acc\nt ( ! effects ( nutation ^u ! app>5t places ( ! />s4 ,a v rem>kable g5]al consequ5ce ( ! *ange 9 ! direction ( ! e>?'s axis w be m5tion$ :5 we come to deal ) ! seasons4 ,illu/ration3 ,! ,great ,s\!rn ,/>-,clu/] ,c5tauri ,photograph$ by ,s4 ,i4 ,bailey at ! ,s\? ,am]ican ,/ation ( ,h>v>d ,obs]vatory4 ,note ! /ream+ ( small />s ar\nd ! clu/]4 ,! clu/] itself is globul> & its />s are too num]\s to be c\nt$1 or ev5 to be sep>ately di/+ui%$ 9 ! c5tral "p4 ---------------------------------------- -------------------------------- ,,"p ,,ii4 ,,! ,,e>?4 ---------------------------------------- -------------------------------- ,,"p ,,ii4 ,,! ,,e>?4 #a4 ,nature1 ,%ape1 & ,size ( ! ,e>?4 ,! situation ( ! e>? 9 ! univ]se has be5 briefly describ$ 9 ,"p ,i2 x rema9s n[ to see :at ! e>? is 9 itself1 & :at are "s ( ! pr9cipal ph5om5a connect$ ) x z a cele/ial body inhabit$ by obs]vant & reason+ be+s4 ,we "k by ord9>y exp]i5ce t ! e>? is compos$ ( rock1 s&1 soil1 etc41 & g5]ally cov]$1 :]e !re is no runn+ or /&+ wat] 9 ! =m ( riv]s1 lakes1 or seas1 ) vegetation1 su* z trees & grass4 ,fur!r exp]i5ce tea*es u t ! e>? is v l>ge1 & t its surface is divid$ into wide areas ( l& & ( wat]4 ,! l>ge/ bodies ( wat]1 ! oceans1 tak5 all toge!r1 cov] ab\t #gb p] c5t41 or ne>ly ?ree-qu>t]s ( ! entire surface ( ! e>?4 ,inve/igations c>ri$ z f> d[n z we c g %[ t ! int]ior ( ! e>? consi/s ( v>i\s k9ds ( rock1 9 :i* are conta9$ _m diff]5t k9ds ( metals4 ,:ile !re is reason = ?9k+ t a high degree ( temp]ature prevails deep 9 ! e>?1 yet x appe>s evid5t1 = o!r reasons1 t1 tak5 z a :ole1 x is solid & v rigid ?r\gh\t4 ,by me?ods1 ! hi/ory & description ( :i* we h n "h suffici5t space to give1 x has be5 prov$ t ! e>? is1 9 =m1 a globe1 or m /rictly an ellipsoid1 slightly drawn 9 at ! poles & swoll5 r\nd ! equator4 ,! pol> diamet] is #ghii miles1 & ! equatorial diamet] #gibf miles1 ! diff]5ce am\nt+ to only #bg miles4 ,?us1 = ord9>y purposes1 we may reg>d ! e>? z be+ a true sph]e4 ,! level ( its surface1 h[ev]1 is v>i$ by hills & m\nta9s1 :i*1 ?\gh insignificant 9 comp>ison ) ! size ( ! :ole e>?1 are enorm\s :5 comp>$ ) ! /ructures ( human h&s4 ,! l(tie/ kn[n m\nta9 on ! e>?1 ,mt4 ,ev]e/ 9 ! ,himalayas1 has an elevation ( #bi1jjj feet above sea-level1 & ! deepe/ kn[n depression ( ! ocean bottom1 ne> ! isl& ( ,guam 9 ! ,pacific1 s9ks #ca1fad feet bel[ sea-level4 ,?us1 ! apex ( ! highe/ m\nta9 is ab\t elev5 & a half miles 9 v]tical elevation above ! bottom ( ! deepe/ pit ( ! sea--a di/ance v consid]ably less ?an half ! diff]5ce betwe5 ! equatorial & pol> diamet]s ( ! e>?4 ,x is believ$ t at ! beg9n+ ( its hi/ory ! e>? 0 a molt5 mass1 or p]haps a mass ( hot gases & vap\rs l ! sun1 & t x assum$ its pres5t %ape 9 ob$i5ce to me*anical laws1 z x cool$ (f4 ,! rotation caus$ x to swell r\nd ! equator & draw 9 at ! poles4 ,! out] "p ( ! e>? is call$ its cru/1 & geology %[s t ?is has be5 subject to viol5t *anges1 su* z upheavals & subsid5ces1 & t 9 _m places sea & l& h int]*ang$ places1 probably m ?an once4 ,geology also %[s t ! rocks ( ! e>?'s cru/ are fill$ ) ! rema9s1 or fossils1 ( plants & animals diff]+ f ^? n[ exi/+1 ?\gh relat$ to !m1 & t _m ( ^! mu/ h liv$ millions ( ye>s ago4 ,?us we see t ! e>? be>s m>ks ( an imm5se antiquity1 & t x 0 probably inhabit$ dur+ va/ ages be=e ! race ( man _h be5 develop$4 ,! orig9 ( life ^u ! e>? is unkn[n4 #b4 ,! ,attraction ( ,gravitation4 ,among ! ph5om5a ( life ^u ! e>?1 :i* are s famili> t only ?\ghtful p]sons see any?+ to wond] at 9 !m1 is :at we call ! 8weight0 ( bodies4 ,e p]son feels t he is held d[n to ! gr\nd by 8 weight1 & he kn[s t if he drops a heavy body x w fall /raight t[>d ! gr\nd4 ,b :at is ?is weight :i* causes ev]y?+ ei!r to re/ ^u ! e>? or to fall back to x if lift$ up & dropp$8 ,! answ] to ?is "q involves a pr9ciple1 or 8law10 :i* affects ! :ole univ]se1 & makes x :at we see x4 ,?is pr9ciple is "o ( ! great f\ndation /ones ( a/ronomy4 ,x is call$ ! law ( gravitation1 ! ^w gravitation be+ d]iv$ f ! ,lat9 gravis1 8heavy40 ,briefly /at$1 ! law is t e body1 or e p>ticle ( matt]1 attracts1 or /rives to draw to itself1 e o!r body1 or p>ticle ( matt]4 ,?is =ce is call$ ! attraction ( gravitation4 ,a l>ge body possesses m attractive =ce ?an a small "o1 9 proportion to ! mass1 or quantity ( matt]1 t x conta9s4 ,! e>?1 be+ extremely l>ge1 holds all bodies on its surface ) a =ce proportionate to its great mass4 ,?is expla9s :y we possess :at we call weight1 :i* is simply ! effect ( ! attraction ( ! e>? ^u our bodies4 ,a l>ge body is heavi]1 or drawn ) m =ce by ! e>?1 ?an a small "o 7compos$ ( ! same k9d ( matt]71 because x has a great] mass4 ,! body really attracts ! e>? z mu* z ! e>? attracts ! body1 b ! am\nt ( motion caus$ by ! attraction is proportional to ! respective masses ( ! attract+ bodies1 & s9ce ! mass ( ! e>? is almo/ inf9itely great 9 comp>ison ) t ( any body t we c h&le1 ! motion :i* ! latt] imp>ts to ! e>? is imp]ceptible1 & x is ! small body only t is se5 to move "u ! =ce ( ! attraction4 ,n[ we are go+ to see h[ va/ly important 9 its effects is ! fact t ! e>? is sph]ical 9 =m4 ,sir ,isaac ,newton1 :o fir/ work$ out ma!matically ! law ( gravitation1 prov$ t a sph]ical body attracts1 & is attract$1 z if its entire mass 7 conc5trat$ 9 a po9t at its c5tre4 ,f ?is x foll[s t ! attraction ( ! e>? is ex]cis$ j z if ! :ole attractive =ce emanat$ f a middle po9t1 &1 t be+ s1 ! effect ( ! attraction is to draw bodies f all sides t[>d ! c5tre ( ! e>?4 ,?is expla9s :y p on ! opposite side ( ! e>?1 or "u our feet1 z we say1 exp]i5ce ! same attractive =ce1 or h ! same weight1 t we d4 ,all r\nd ! e>?1 no matt] :]e !y may be situat$1 objects are drawn t[>d ! c5tre4 ,if at any po9t on ! e>? y susp5d a plumb-l9e1 & !n1 go+ "o qu>t] way r\nd1 susp5d ano!r plumb-l9e1 ea* ( ! l9es w be v]tical at ! place :]e x hangs1 & yet1 ! directions ( ! two l9es w be at "r angles to "o ano!r1 s9ce bo? po9t t[>d ! c5tre ( ! e>?4 ,kn[+ ! mann] 9 :i* ! e>? attracts1 we h ! means ( det]m9+ its entire mass1 or1 z x is sometimes call$1 ! weight ( ! e>?4 ,! pr9ciple on :i* ?is is done is easily und]/ood1 ,suppose1 = in/ance1 t a small ball ( lead1 ( kn[n weight1 is br\ght ne> a l>ge ball1 & delicately susp5d$ 9 su* a way t1 by microscopic obs]vation1 ! movem5t imp>t$ by ! attraction ( ! l>ge ball c be measur$4 ,! =ce requir$ to produce ?is movem5t c be comp>$ ) ! =ce ( ! e>?'s attraction :i* produces ! weight ( ! ball1 & ?us ! ratio ( ! mass ( ! e>? to t ( ! ball is det]m9$4 ,! total mass ( ! e>? has be5 f\nd to be equival5t to a 8weight0 ( ab\t #f1ejj1jjj1jjj1jjj1jjj1jjj1jjj tons4 ,! mean d5sity ( ! e>? comp>$ ) t ( wat] is f\nd to be ab\t #e1 t is to say1 ! e>? weighs #e times z mu* z a globe ( wat] ( equal size4 ,newton did n /op ) %[+ ! mann] ( ! e>?'s attraction ^u bodies on or ne> its surface2 he prov$ t ! e>? attract$ ! moon also1 & ?us reta9$ x 9 its orbit4 ,to und]/& ?is we mu/ notice ano!r fact conc]n+ ! mann] 9 :i* gravitation acts4 ,its =ce v>ies ) di/ance4 ,exp]im5t foll[$ by ma!matical demon/ration1 has prov$ t ! v>iation ( ! attraction is inv]sely proportional to ! squ>e ( ! di/ance4 ,?is simply means t if ! di/ance betwe5 ! two bodies conc]n$ is d\bl$1 ! =ce ( attraction w be dim9i%$ f\r times1 #d be+ ! squ>e ( #b2 & t if ! di/ance is halv$1 ! =ce w be increas$ f\rfold4 ,increase ! di/ance ?ree times1 & ! =ce dim9i%es n9e times2 dim9i% ! di/ance ?ree times1 & ! =ce increases n9e times1 because #i is ! squ>e ( #c1 &1 z we h said1 ! =ce v>ies inv]sely1 or contr>ily1 to ! *ange ( di/ance4 ,kn[+ ?is1 ,newton comput$ :at ! =ce ( ! e>?'s attraction mu/ be on ! moon1 & he f\nd t x 0 j suffici5t to keep ! latt] mov+ r\nd & r\nd ! e>?4 ,b :y does n ! moon fall directly to ! e>?8 ,because ! moon _h orig9ally ano!r motion across ! direction ( ! e>?'s attraction4 ,h[ x got ?is motion is a "q into :i* we _c "h ent]1 b1 if x 7 n attract$ by ! e>? 7or by ! sun71 ! moon w\ld travel 9 a /raight l9e ?r\gh space1 l a /one escap+ f a sl+4 ,! =ce ( ! attraction is j suffici5t to make ! moon move 9 an orbital pa? ab\t ! e>?4 ,illu/ration3 ,fig4 #e4 ,h[ ! ,e>? ,controls ! ,moon4 ,let ,c be ! c5tre ( ! e>? & ,m t ( ! moon4 ,suppose ! moon to be mov+ 9 a /raight l9e at su* a velocity t x w1 if n int]f]$ )1 g to ,a 9 "o day4 ,n[ suppose ! attraction ( ! e>? to act ^u x4 ,t attraction w draw x to ,m4 ,aga9 suppose t at ,m ! moon 7 sudd5ly releas$ f ! e>?'s attraction2 x w\ld !n %oot /raight ahead to ,b 9 ! c\rse ( ! next day4 ,b1 9 fact1 ! e>?'s attraction acts cont9ually1 & 9 ! second day ! moon is drawn to ,m4 ,9 o!r words ! moon is all ! "t fall+ away f ! /raight l9e t x w\ld pursue b = ! e>?'s attraction1 & yet x does n get ne>] ! e>? b simply travels 9 an endless curve r\nd x4 ,! same pr9ciple 0 ext5d$ by ,newton to expla9 ! motion ( ! e>? ar\nd ! sun4 ,! =ce ( ! sun's attraction1 calculat$ 9 ! same way1 c be %[n to be j suffici5t to reta9 ! e>? 9 its orbit & prev5t x f travell+ away into space4 ,& s ) all ! o!r planets :i* revolve r\nd ! sun4 ,& ?is applies ?r\gh\t ! univ]se4 ,!re are c]ta9 s-call$ d\ble1 or b9>y1 />s1 :i* are s close toge!r t _! attraction ^u "o ano!r causes !m to revolve 9 orbits ab\t _! common c5tre4 ,9 tru?1 all ! />s attract ! e>? & ! sun1 b ! =ce ( ?is attraction is s slight on acc\nt ( _! imm5se di/ance t we _c obs]ve its effects4 ,! read] :o wi%es to pursue ?is subject ( gravitational attraction %\ld consult m ext5sive works1 su* z ,pr(4 ,"y's ,g5]al ,a/ronomy1 or ,sir ,george ,airy's ,gravitation4 ,illu/ration3 ,photograph ( a ,gr\p ( ,sun-spots ,simil> gr\ps are frequ5tly se5 dur+ p]iods ( sun-spot maximum4 #c4 ,! ,tides4 ,! tides 9 ! ocean are a direct result ( ! attraction ( gravitation4 ,!y also involve 9 an int]e/+ way ! pr9ciple t a sph]ical body1 l ! e>?1 attracts & is attract$ z if its entire mass 7 conc5trat$ at its c5tre4 ,! cause ( ! tides is ! diff]5ce 9 ! attraction ( ! sun & moon ^u ! body ( ! e>? z a rigid sph]e1 & ^u ! wat] ( ! oceans1 z a fluid envelope ^: p>ticles1 :ile n free to escape f ! e>?1 are free to move1 or slide1 among "o ano!r 9 ob$i5ce to v>y+ =ces4 ,! diff]5ce ( ! =ce ( attraction arises f ! diff]5ce ( di/ance4 ,s9ce ! moon1 because ( h] relative ne>ness1 is ! *ief ag5t 9 produc+ tides we %all1 at fir/1 consid] h] tidal influ5ce alone4 ,! diamet] ( ! e>? is1 9 r\nd numb]s1 #hjjj miles2 !re=e1 its radius is #djjj miles4 ,f ?is x foll[s t ! c5tre ( ! e>? is #djjj miles f>!r f ! moon ?an t side ( ! e>? :i* is t[>d h] at any "t1 & #djjj miles ne>] ?an ! side :i* is away f h]4 ,consequ5tly1 h] attraction mu/ be /rong] ^u ! wat] ( ! ocean ly+ j "u h] ?an ^u ! c5tre ( ! e>?1 & x mu/ also be /rong] ^u ! c5tre ( ! e>? ?an ^u ! wat] ( ! ocean ly+ ^u ! side :i* is f>!/ f h]4 ,! result ( ^! diff]5ces 9 ! =ce ( ! moon's attraction is t ! wat] directly "u h] t5ds away f ! c5tre ( ! e>?1 :ile1 on ! o!r h&1 ! e>?1 consid]$ z a solid sph]e1 t5ds away f ! wat] on ! side opposite to t :]e ! moon is1 & ^! comb9$ t5d5cies cause ! wat] to rise1 ) reg>d to its g5]al level1 9 two protub]ances1 situat$ on opposite sides ( ! e>?4 ,^! we call tides4 ,illu/ration3 ,fig4 #f4 ,! ,tidal ,=ce ( ! ,moon4 ,! solid e>? is repres5t$ surr\nd$ by a %ell ( wat]4 ,! wat] on ! side t[>d ! moon is m attract$ ?an ! c5tre ( ! e>?1 ,c2 ! wat] on ! opposite side is less attract$4 ,! l9es ( =ce f ! moon to ! p>ts ( ! wat] ly+ t[>d ,a & ,b are incl9$ to ! direct l9e betwe5 ! c5tres ( ! e>? & ! moon1 & ! =ces act+ along ^! l9es t5d to draw ! wat] 9 ! directions %[n by ! arr[ po9ts4 ,^! are resultants ( ! horizontal & v]tical compon5ts ( ! moon's attraction at ! correspond+ po9ts on ! e>?1 & ! =ce act+ along !m t5ds to increase ! weight ( ! wat] :]ev] ! l9es are incl9$ m t[>d ! c5tre ( ! e>? ?an t[>d ! moon4 ,on ! side opposite ! moon ! same effects are produc$ 9 rev]se1 because on t side ! g5]al t5d5cy is to draw ! e>? away f ! wat]4 ,consequ5tly if ! e>? did n rotate1 & if x 7 surr\nd$ ) a complete %ell ( wat]1 ! latt] w\ld be drawn into an ellipsoidal %ape1 ) ! highe/ po9ts "u & opposite to ! moon1 & ! l[e/ at ! extremities ( ! diamet] ly+ at "r angles to ! direction ( ! moon4 ,"s p]sons1 :5 ?is /atem5t is made1 inquire3 8,:y1 !n1 does n ! moon take ! wat] entirely away f ! e>?80 ,! answ] is1 t ! effect ( ! tidal =ce is simply to dim9i% v slightly ! weight ( ! wat]1 or its t5d5cy t[>ds ! e>?'s c5tre1 b n to de/roy1 or ov]ma/]1 ! gravitational control ( ! e>?4 ,! wat] reta9s ne>ly all its weight1 = ! tidal =ce ( ! moon dim9i%es x less ?an "o "p 9 #h1jjj1jjj4 ,/ill1 ?is slight dim9ution is suffici5t to cause ! wat] to swell a little above its g5]al level1 at ! po9ts :]e x feels ! effect ( ! tidal =ce4 ,on ! o!r h&1 ar\nd t "p ( ! e>? :i* is situat$ half-way betwe5 ! two tides1 or along a diamet] at "r angles to ! direction ( ! moon1 ! latt]'s attraction increases ! weight ( ! wat]1 i4 e41 its t5d5cy t[>d ! e>?'s c5tre 7see ,fig4 #f74 ,p]haps ?is c bett] be und]/ood1 if we imag9e ! e>? to be entirely liquid4 ,9 t case ! diff]5ce 9 ! =ce ( ! moon's attraction ) diff]5ce ( di/ance w\ld be manife/$ 9 v>y+ degrees ?r\gh\t ! e>?'s :ole frame1 & ! result w\ld be to draw ! wat]y globe out into an ellipsoidal figure1 hav+ its greate/ diamet] 9 ! l9e ( ! moon's attraction1 & its smalle/ diamet] at "r angles to t l9e4 ,! proportions ( ! ellipsoid w\ld be su* t ! =ces w\ld be 9 equilibrium4 ,ow+ to a v>iety ( causes1 su* z ! rotation ( ! e>? on its axis1 :i* c>ries ! wat] rapidly r\nd ) x2 ! in]tia ( ! wat]1 prev5t+ x f in/antly respond+ to ! tidal =ce2 ! irregul> %ape ( ! oceans1 int]rupt$ on all sides by great areas ( l&2 _! v>y+ dep?1 produc+ diff]5ces ( friction1 & s on1 ! tidal waves d n appe> directly "u1 or directly opposite to1 ! moon1 & ! calculation ( ! c\rse & height ( ! actual tides1 at p>ticul> po9ts on ! e>?1 becomes "o ( ! mo/ difficult problems 9 a/ronomical physics4 ,we n[ turn to consid] ! effects ( ! sun's tidal =ce 9 connection ) t ( ! moon4 ,?is introduces fur!r complications4 ,! sol> tides are only ab\t two-fif?s z high z ! lun> tides1 b !y suffice to produce notable effects :5 !y are ei!r comb9$ )1 or act 9 opposition to1 ! o!rs4 ,!y are comb9$ twice a mon?--once :5 ! moon is betwe5 ! e>? & ! sun1 at ! "t ( new moon2 & aga9 :5 ! moon is 9 opposition to ! sun1 at ! "t ( full moon4 ,9 ^! two positions ! attractions ( ! sun & ! moon mu/1 s to speak1 act toge!r1 ) ! result t ! tides produc$ by !m bl5d into a s+le great] wave4 ,?is comb9ation produces :at are call$ spr+ tides1 ! highe/ ( ! mon?4 ,:51 on ! o!r h&1 ! moon is 9 a position at "r angles to ! direction ( ! sun1 :i* happ5s at ! lun> phases nam$ fir/ & la/ qu>t]s1 ! sol> & ! lun> tides h _! cre/s #ij ap>t1 &1 9 a s5se1 act aga9/ "o ano!r1 & !n we h ! neap tides1 :i* are ! l[e/ ( ! mon?4 ,)\t ent]+ into a demon/ration1 x may "h be /at$ z a fact to be memoris$1 t ! tidal =ce ex]t$ by any cele/ial body v>ies inv]sely z ! cube ( ! di/ance4 ,?is is ! reason :y ! sun1 al?\gh x exce$s ! moon 9 mass m ?an #be1jjj1jjj times1 & is situat$ only ab\t #djj times z f> away f ! e>?1 ex]cises comp>atively s slight a tidal =ce on ! wat] ( ! ocean4 ,if ! tidal =ce v>i$ z ! squ>e ( ! di/ance1 l ! ord9>y effects ( gravitation1 ! tides produc$ by ! sun w\ld be m ?an #aej times z high z ^? produc$ by ! moon1 & w\ld sweep ,new ,york1 ,london1 & all ! seaports ( ! _w to de/ruction4 ,9 t case x might be possible1 by delicate obs]vations1 to detect a tidal effect produc$ ^u ! oceans ( ! e>? by ! planet ,jupit]4 #d4 ,! ,atmosph]e4 ,! solid globe ( ! e>? is envelop$ 9 a mixture ( gases1 pr9cipally oxyg5 & nitrog51 :i* we call ! air1 or ! atmosph]e1 & ^u ^: pres5ce our life & mo/ o!r =ms ( life dep5d4 ,! atmosph]e is reta9$ by ! attraction ( ! e>?1 & x rotates toge!r ) ! e>?4 ,if ?is 7 n s--if ! atmosph]e /ood fa/ :ile ! e>? cont9u$ to sp9 )9 x--a t]rific w9d w\ld con/antly bl[ f ! ea/1 hav+ a velocity at ! equator ( m ?an a ?\s& miles an h\r4 ,exactly h[ high ! atmosph]e ext5ds we d n "k--x may n h any def9ite limits--b we d "k t its d5sity rapidly dim9i%es ) increase ( height above ! gr\nd1 s t above an elevation ( a few miles x becomes s r>e t x w\ld n support human life4 ,! ph5om5a ( meteors1 set afire by ! friction ( _! swift ru% ?r\gh ! upp] air1 prove1 h[ev]1 t !re is a p]ceptible atmosph]e at an elevation ( m ?an a hundr$ miles4 ,f an a/ronomical po9t ( view1 ! mo/ important effect ( ! pres5ce ( ! atmosph]e is its p[] ( refract+ light4 ,by refraction is meant ! prop]ty possess$ by e transp>5t m$ium ( b5d+1 "u p>ticul> circum/ances1 ! rays ( light :i* ent] x out ( _! orig9al c\rse4 ,! sci5ce ( physics tea*es u t if a ray ( light passes f any transp>5t m$ium into ano!r :i* is d5s]1 & if ! pa? ( ?is ray is n p]p5dicul> to ! surface ( ! second m$ium1 x w be turn$ f its orig9al c\rse 9 su* a way z to make x m ne>ly p]p5dicul>4 ,?us1 if a ray ( light passes f air into wat] at a c]ta9 slope to ! surface1 x w1 ^u ent]+ ! wat]1 be s *ang$ 9 direction t ! slope w become /eep]4 ,only if x falls p]p5dicul>ly ^u ! wat] w x cont9ue on )\t *ange ( direction4 ,conv]sely a ray pass+ f a d5s] into a r>] m$ium is b5t away f a p]p5dicul> to ! surface ( ! fir/ m$ium1 or its slope becomes less4 ,?is expla9s :y1 if we put a co9 9 a b[l1 ) ! eye 9 su* a position t x _c see ! co9 ov] ! edge1 & !n fill ! b[l ) wat]1 ! co9 seems to be lift$ up into sight4 ,moreov]1 if any transp>5t m$ium increases 9 d5sity ) dep?1 ! am\nt ( refraction w increase z ! ray goes deep]1 & ! direction ( ! ray w be *ang$ f a /raight l9e into a curve1 t5d+ to become m & m p]p5dicul>4 ,n[ all ?is applies to ! atmosph]e4 ,if a /> is se5 9 ! z5i?1 its light falls p]p5dicul>ly into ! atmosph]e & its c\rse is n deviat$1 or 9 o!r words !re is no refraction4 ,b if ! /> is some:]e betwe5 ! z5i? & ! horizon1 its light falls slop+ly into ! atmosph]e1 & is subject to refraction1 ! am\nt ( b5d+ increas+ ) approa* to ! horizon4 ,obs]vation %[s t ! refraction ( ! atmosph]e1 :i* is z]o at ! z5i?1 increases to ab\t half a degree 7& sometimes mu* m1 dep5d+ ^u ! /ate ( ! air71 ne> ! horizon4 ,x foll[s t a cele/ial object se5 ne> ! horizon w ord9>ily appe> ab\t half a degree above its true place4 ,s9ce ! app>5t diamet]s ( ! sun & ! moon are ab\t half a degree1 :5 !y are ris+ or sett+ !y c be se5 on ! horizon be=e !y h really ris5 above x1 or aft] !y h really sunk bel[ x4 ,tables ( refraction at v>i\s altitudes h be5 prep>$1 & !y h to be consult$ 9 all exact obs]vations ( ! cele/ial bodies4 ,illu/ration3 ,fig4 #g4 ,refraction4 ,suppose an obs]v] situat$ at ,o on ! e>?4 ,! sun1 at ,s1 has sunk bel[ ! level ( 8 horizon1 ,o ,h1 b s9ce ! sun s5ds out rays 9 all directions !re w be "s1 su* z ,s ,a ,b1 :i* w /rike ! atmosph]e at ,a1 & ! refraction1 t5d+ to make ! ray m ne>ly p]p5dicul> to ! surface ( ! atmosph]e1 w1 in/ead ( all[+ x to g on /raight ov] ! obs]v]'s head to ,b1 b5d x d[n along ! dott$ l9e ,a ,o1 & ! obs]v] w see ! sun z if x lay 9 ! direction ( ! dott$ l9e ,o ,a ,s1 :i* places ! sun app>5tly above ! horizon4 #e4 ,dip ( ! ,horizon4 ,ano!r correction :i* has to be appli$ 9 _m obs]vations dep5ds ^u ! sph]icity ( ! e>?4 ,we h describ$ ! rational horizon1 & po9t$ out h[ x diff]s f ! s5sible horizon4 ,we h also said t at sea ! s5sible horizon ne>ly accords ) ! rational horizon 7see ,"p ,i1 ,sect4 #c74 ,b ! accord is n complete1 ow+ to :at is call$ ! dip ( ! horizon4 ,9 fact1 ! sea horizon lies bel[ ! rational horizon by an am\nt v>y+ ) ! elevation ( ! eye above ! surface4 ,geometry enables u to det]m9e j :at ! dip ( ! horizon mu/ be = any giv5 elevation ( ! eye4 ,a r\gh & ready rule1 :i* may s]ve = _m purposes1 is t ! squ>e root ( ! elevation ( ! eye 9 feet equals ! dip ( ! horizon 9 m9utes ( arc1 or ( angul> measure4 ,! read] w readily see t ! dip ( ! horizon is a necess>y consequ5ce ( ! rotundity ( ! e>?4 ,x is because ( ?is t1 z a %ip rec$es at sea h] hull fir/ disappe>s bel[ ! horizon1 & !n h] l[] sails1 & f9ally h] top-sails4 ,! use ( a telescope does n help ! matt]1 because a telescope only sees /raight1 & _c b5d ! l9e ( sight ov] ! rim ( ! horizon4 ,atmosph]ic refraction1 h[ev]1 enables u to see an object :i* w\ld be hidd5 by ! horizon if !re 7 no air4 ,9 navigation1 :i*1 z a sci5ce1 is an outgr[? ( a/ronomy1 ^! ?+s h to be c>efully tak5 into acc\nt4 ,illu/ration3 ,fig4 #h4 ,dip ( ! ,horizon4 ,x is to be rememb]$ t x is ! s5sible horizon :i* dips1 & n ! rational horizon4 ,! s5sible horizon ( ! obs]v] at ! elevation ,a dips bel[ ! horizontal plane & he sees r\nd ! curv$ surface z f> z a2 9 o!r words 8 skyl9e is at a4 ,! obs]v] at ! elevation ,b has a s5sible horizon /ill m incl9$ & he sees z f> z b4 ,if ! obs]vation 7 made f an imm5se height ! obs]v] w\ld see practically half r\nd ! e>? j z we see half r\nd ! globe ( ! moon4 ,illu/ration3 ,pol> ,/ream]s ( ! ,sun1 ,eclipse ( #ahhi ,illu/ration3 ,! ,sol> ,corona at ! ,eclipse ( #ahga ,f draw+s4 #f4 ,ab]ration4 ,a few words mu/ be said ab\t ! ph5om5on kn[n z ab]ration ( light4 ,?is is an app>5t displacem5t ( a cele/ial object due to ! motion ( ! e>? 9 its orbit4 ,x is cu/om>y to illu/rate x by imag9+ oneself to be 9 a %[] ( ra91 ^: drops are fall+ v]tically4 ,9 su* a case1 if a p]son /&s fa/ ! ra9 w desc5d p]p5dicul>ly ^u 8 head1 b if he advances rapidly 9 any direction he w feel ! drops /rik+ him 9 ! face1 because 8 own =w>d motion is comp\nd$ ) ! d[nw>d motion ( ! ra9 s t ! latt] seems to be desc5d+ slant+ly t[>d him4 ,! same ?+ happ5s ) ! light fall+ f ! />s4 ,z ! e>? advances 9 its orbit x seems to meet ! light rays1 & !y appe> to come f a direction ahead ( ! fly+ e>?4 ,! result is t1 s9ce we see a /> 9 ! direction f :i* its light seems to come1 ! /> appe>s 9 advance ( its real position1 or ( ! position 9 :i* we w\ld see x if ! e>? /ood fa/4 ,! am\nt by :i* ! position ( a /> is %ift$ by ab]ration dep5ds ^u ! ratio ( ! e>?'s velocity to ! velocity ( light4 ,9 r\nd numb]s ?is ratio is z #a to #aj1jjj4 ,! motion ( ! e>? be+ 9 a slightly ecc5tric ellipse1 ! />s describe correspond+1 b v t9y1 ellipses once e ye> ^u ! backgr\nd ( ! sky4 ,b ! precise %ape ( ! ellipse dep5ds ^u ! position ( ! /> on ! cele/ial sph]e4 ,if x is ne> "o ( ! poles ( ! ecliptic1 x w describe an annual ellipse :i* w be almo/ a circle1 its great] diamet] be+ #da ( arc4 ,if x is ne> ! plane ( ! ecliptic1 x w describe a v ecc5tric ellipse1 b ! great] diamet] w always be #da1 al?\gh ! %ort] diamet] may be immeasurably small4 ,! effects ( ab]ration h to be all[$ = 9 all c>eful a/ronomical obs]vation ei!r ( ! sun or ! />s4 ,?is is done by r$uc+ ! app>5t place ( ! object to ! place x w\ld h if x 7 se5 at ! c5tre ( its annual ellipse4 #g4 ,"t4 ,)\t a/ronomical obs]vations we c\ld h no accurate k ( "t4 ,! basis ( ! measurem5t ( "t is furni%$ by ! rotation ( ! e>? on its axis4 ,we divide ! p]iod :i* ! e>? occupies 9 mak+ "o complete turn into tw5ty-f\r equal p>ts1 or h\rs4 ,! asc]ta9m5t ( ?is p]iod1 call$ a day1 dep5ds ^u obs]vations ( ! />s4 ,suppose we see a c]ta9 /> exactly on ! m]idian at "s mom5t2 j tw5ty-f\r h\rs lat] t /> w h gone entirely r\nd ! sky1 & w aga9 appe> on ! m]idian4 ,! revolv+ heav5s con/itute ! great clock ( clocks1 by ^: movem5ts all o!r clocks are regulat$4 ,we "k t x is n ! heav5s :i* revolve1 b ! e>? :i* rotates1 b = conv5i5ce we accept ! appe>ance z a sub/itute = ! fact4 ,! rotation ( ! e>? is s regul> t no measurable v>iation has be5 f\nd 9 two ?\s& ye>s4 ,we h reasons = ?9k+ t !re mu/ be a v sl[ & gradual ret>dation1 ow+ pr9cipally to ! brak+ action ( ! tides1 b x is s slight t we _c detect x ) any means at pres5t )9 our comm&4 ,9 ,"p ,i x 0 %[n h[ ! passage across ! m]idian ( ! po9t 9 ! sky call$ ! v]nal equ9ox s]ves to indicate ! beg9n+ ( ! a/ronomical 8day10 b ! position ( ! v]nal equ9ox itself has to be det]m9$ by obs]vations on ! />s4 ,by means ( a telescope1 s m\nt$ t x c only move up or d[n1 r\nd a horizontal axis1 & ) ! axis po9t+ exactly ea/ & we/ s t ! up & d[n movem5ts ( ! telescope tube foll[ ! l9e ( ! m]idian1 ! mom5t ( passage across ! m]idian ( a /> at any altitude c be obs]v$4 ,obs]vations ( ?is nature are cont9ually made at all great gov]nm5t obs]vatories1 su* z ! obs]vatory at ,wa%+ton or t at ,gre5wi*1 & at _m o!rs1 & by _! means clocks & *ronomet]s are correct$1 & a /&>d ( "t is furni%$ to ! :ole _w4 ,!re are1 h[ev]1 ?ree diff]5t ways ( reckon+ "t1 or1 z x is usually said1 ?ree k9ds ( "t4 ,"o is sid]eal "t1 :i* is indicat$ by ! passage ( />s across ! m]idian1 & :i* measures ! true p]iod ( ! e>?'s rotation2 ano!r is app>5t sol> "t1 :i* is indicat$ by ! passage ( ! sun across ! m]idian2 & a ?ird is mean sol> "t1 :i* is indicat$ by a c>efully regulat$ clock1 ^: errors are correct$ by /> obs]vations4 ,?is la/ k9d ( "t is t :i* is univ]sally us$ 9 ord9>y life 7! use ( sid]eal "t be+ conf9$ to a/ronomy71 s x is necess>y to expla9 :at x is & h[ x diff]s f app>5t sol> "t4 ,illu/ration3 ,fig4 #i4 ,sid]eal & ,sol> ,"t4 ,c is ! c5tre ( ! e>?1 & ,o ! place ( an obs]v] on ! e>?'s surface4 ,suppose ! sun at ,a to be 9 conjunction ) ! /> ,s4 ,!n1 at ! end ( tw5ty-f\r sid]eal h\rs1 :5 ! e>? has made "o turn on its axis & ! place ,o has aga9 come into conjunction ) ! />1 ! sun1 9 consequ5ce ( its ye>ly motion 9 ! ecliptic1 w h advanc$ to ,b1 & ! e>? w h to turn ?r\gh ! angle ,a ,c ,b be=e ,o w ov]take ! sun & complete a sol> day2 :]e=e ! sol> day is long] ?an ! sid]eal4 ,9 ! fir/ place1 ! reason :y sid]eal "t is n univ]sally & exclusively us$ is because1 al?\gh x measures ! true p]iod ( ! e>?'s rotation by ! app>5t motion ( ! />s1 x does n exactly accord ) ! app>5t motion ( ! sun2 &1 naturally1 ! sun1 s9ce x is ! s\rce ( light = ! e>?1 & ! cause ( ! diff]5ce betwe5 day & night1 is tak5 = all ord9>y purposes1 z ! /&>d indicator ( ! progress ( ! h\rs4 ,! fact t x is mid-day1 or noon1 at any place :5 ! sun crosses ! m]idian ( t place1 is a fact ( common k1 :i* _c be ignor$4 ,on ! o!r h&1 ! v]nal equ9ox1 :i* is ! 8noon m>k0 = sid]eal "t1 is indep5d5t ( ! alt]nation ( day & night1 & may be on ! m]idian z well at midnight z at mid-day4 ,be=e clocks & wat*es 7 p]fect$1 ! mom5t ( ! sun's passage ov] ! m]idian 0 det]m9$ by means ( a gnomon1 :i* %[s ! in/ant ( noon by ! l5g? ( a %ad[ ca/ by an upright rod4 ,s9ce ! app>5t c\rse ( ! sun ?r\gh ! sky is a curve1 ris+ f ! ea/]n horizon1 atta9+ its greate/ elevation :]e x meets ! m]idian1 & !nce decl9+ to ! we/]n horizon1 x is evid5t t ! l5g? ( ! %ad[ mu/ be lea/ :5 ! sun is on ! m]idian1 or at its maximum altitude4 ,! gnomon1 or ! sun-dial1 gives u app>5t sol> "t4 ,b ?is diff]s f sid]eal "t because1 z we saw 9 ,"p ,i1 ! sun1 9 consequ5ce ( ! e>?'s motion r\nd x1 moves ab\t "o degree ea/w>d e tw5ty-f\r h\rs1 &1 s9ce "o degree is equal to f\r m9utes ( "t1 ! sun rises ab\t f\r m9utes lat]1 ) ref]5ce to ! />s1 e morn+4 ,consequ5tly x comes f\r m9utes lat] to ! m]idian day aft] day4 ,or1 to put x 9 ano!r way1 suppose t ! sun & a c]ta9 /> are ^u ! m]idian at ! same in/ant4 ,! /> is fix$ 9 its place 9 ! sky1 b ! sun is n fix$2 on ! contr>y x moves ab\t "o degree ea/w>d 7! same direction z t ( ! e>?'s rotation7 9 tw5ty-f\r h\rs4 ,!n1 :5 ! rotation ( ! e>? has br\ght ! /> back to ! m]idian at ! end ( tw5ty-f\r sid]eal h\rs1 ! sun1 9 consequ5ce ( its motion1 w /ill be "o degree ea/ ( ! m]idian1 & ! e>? mu/ turn ?r\gh ! space ( ano!r degree1 :i* w take f\r m9utes1 be=e x c h ! sun aga9 ^u ! m]idian4 ,! true di/ance mov$ by ! sun 9 tw5ty-f\r h\rs is a little less ?an "o degree1 & ! exact "t requir$ = ! m]idian to ov]take x is #c m94 #ef4eee sec4 ,?us1 ! sid]eal day 7p]iod ( #bd h\rs7 is ne>ly f\r m9utes %ort] ?an ! sol> day4 ,x w\ld seem1 !n1 t by tak+ ! sun = a guide1 & divid+ ! p]iod betwe5 two ( its successive passages ov] ! m]idian into tw5ty-f\r h\rs1 we %\ld h a p]fect measure ( "t1 )\t reg>d to ! />s2 9 o!r words1 t app>5t sol> "t w\ld be entirely satisfactory = ord9>y use4 ,b1 un=tunately1 ! app>5t ea/w>d motion ( ! sun is n regul>4 ,x is sometimes great] ?an ! av]age & sometimes less4 ,?is v>iation is due almo/ entirely3 fir/1 to ! fact t its orbit n be+ a p]fect circle ! e>? moves fa/] :5 x is ne> p]ihelion1 & sl[] :5 x is ne> aphelion2 &1 second1 to ! effects ( ! incl9ation ( ! ecliptic to ! equator4 ,9 consequ5ce1 ano!r measure ( sol> "t is us$1 call$ mean sol> "t1 9 :i*1 by imag9+ a fictiti\s sun1 mov+ ) p]fect regul>ity ?r\gh ! ecliptic1 ! discrepancies are avoid$4 ,all ord9>y clocks are set to foll[ ?is fictiti\s1 or mean1 sun4 ,! result is t clock "t does n agree exactly ) sun-dial "t1 or1 :at is ! same ?+1 app>5t sol> "t4 ,! clock is ahead ( ! real sun at "s times ( ! ye>1 & beh9d x at o!r times4 ,?is diff]5ce is call$ ! equation ( "t4 ,f\r times 9 ! ye> ! equation is z]o1 i4e41 !re is no diff]5ce betwe5 ! clock & ! sun4 ,^! times are ,april #ae1 ,june #ad1 ,sept4 #a1 & ,dec4 #bd4 ,at f\r o!r times ( ! ye> ! diff]5ce is at a maximum1 viz4 ,feb4 #aa1 sun #ad m94 #bg sec4 beh9d clock2 ,may #ad1 sun #c m94 #di sec4 ahead ( clock2 ,july #bf1 sun #f m94 #af sec4 beh9d clock2 ,nov4 #b1 sun #af m94 #ah sec4 ahead ( clock4 ,^! dates & diff]5ces v>y v slightly f ye> to ye>4 ,b1 :atev] measures ( "t we may use1 x is obs]vation ( ! />s t furni%es ! means ( correct+ !m4 ,illu/ration3 ,moreh\se's ,comet1 ,octob] #ae1 #aijh ,photograph$ at ! ,y]kes ,obs]vatory by ,e4 ,e4 ,b>n>d ) ! t5-in* ,bruce telescope4 ,exposure "o h\r & a half4 ,note ! deta*$ portions :i* appe>$ to sep>ate f ! head & retreat up ! l9e ( ! tail at enorm\s velocity4 ,illu/ration3 ,moreh\se's ,comet1 ,novemb] #ae1 #aijh ,photograph$ at ! ,y]kes ,obs]vatory by ,e4 ,e4 ,b>n>d1 ) ! t5-in* ,bruce telescope4 ,exposure =ty m9utes4 #h4 ,day & ,night4 ,! p]iod ( tw5ty-f\r h\rs requir$ = "o turn ( ! e>? on its axis is call$ a day1 & 9 a/ronomical reckon+ x is treat$ z an undivid$ :ole1 ! h\rs be+ c\nt$ un9t]rupt$ly f #j to #bd2 b nature has divid$ ! p]iod into two v di/9ct portions1 "o *>act]is$ by ! pres5ce & ! o!r by ! abs5ce ( ! sun4 ,popul>ly we speak ( ! sunlight$ portion z day & ( ! o!r z night1 & !re are no two associat$ ph5om5a 9 nature m completely 9 contra/ "o to ! o!r4 ,! cause ( ! contra/ betwe5 day & night mu/ h be5 evid5t to ! e>lie/ human be+s :o 7 capable ( any ?\ght at all4 ,!y saw t day inevitably began :5ev] ! sun rose above ! horizon1 & z inevitably ceas$ :5ev] x sank b5ea? x4 ,9 all lit]atures1 imag9ative writ]s h pictur$ ! despair ( primeval man :5 he fir/ saw ! sun disappe> & night come on1 & 8 joy :5 he fir/ beheld ! sun rise1 br++ day back ) x4 ,ev5 8 un9/ruct$ m9d c\ld n h be5 9 d\bt ab\t ! causal connection ( ! sun ) daylight4 ,we n[ "k t ! cause ( ! alt]nate ris+ & sett+ ( ! sun1 & ( its app>5t motion ?r\gh ! sky1 is ! rotation ( ! e>?4 ,mak+ 9 our m9ds a picture ( ! e>? z a turn+ globe expos$ to ! sunbeams1 we are able to see t "o half ( x mu/ necess>ily be illum9at$1 :ile ! o!r half is 9 d>kness4 ,we also see t its rotation causes ^! two halves gradually to int]*ange places s t daylight progresses completely r\nd ! e>? once 9 ! c\rse ( tw5ty-f\r h\rs4 ,if ! e>? 7 n surr\nd$ by an atmosph]e1 exactly "o half ( x w\ld lie 9 ! sunlight & exactly "o half 9 d>kness1 b ! atmosph]e causes ! illum9at$ "p slightly to exce$ ! unillum9at$ "p4 ,! reason = ?is is tw(old3 fir/1 because ! atmosph]e1 be+ transp>5t & ext5d+ to a consid]able height above ! solid globe1 receives rays f ! sun aft] ! latt] has sunk bel[ ! horizon1 & ^! rays cause a fa9t illum9ation 9 ! sky aft] ! sun z view$ f ! surface ( ! gr\nd has disappe>$2 &1 second1 because ! air has ! prop]ty ( refract+ ! rays ( light1 9 consequ5ce ( :i* ! sun appe>s above ! horizon bo? a little "t be=e x has actually ris5 & a little "t aft] x has actually set4 ,! fa9t illum9ation at ! beg9n+ & ! end ( ! day is call$ twilight4 ,its cause is ! reflection ( light f ! air at a consid]able elevation above ! gr\nd4 ,obs]vation %[s t ev5+ twilight la/s until ! sun has sunk ab\t #ah bel[ ! we/]n horizon1 :ile morn+ twilight beg9s :5 ! sun is /ill #ah bel[ ! ne>e/ horizon4 ,! l5g? ( "t occupi$ by twilight1 or its duration1 dep5ds ^u ! obs]v]'s place on ! e>? & increases ) di/ance f ! equator4 ,! l5g? ( twilight at any p>ticul> place also v>ies ) ! seasons4 ,x w probably h occurr$ to ! read] t1 s9ce day & night are ceaselessly *as+ ea* o!r r\nd ! globe1 x mu/ be necess>y to *oose "s po9t ( beg9n+1 9 ord] to keep ! regul> succession ( ! days ( ! week4 ,! necessity = ?is is evid5t z soon z we reflect t :at is sunrise at "o place on ! e>?1 is sunset = a place situat$ half-way r\nd1 on ! o!r side4 ,to und]/& ?is x w be bett]1 p]haps1 to consid] ! ph5om5a ( noon at v>i\s places4 ,x is noon at any place :5 ! sun is on ! m]idian ( t place4 ,b we h se5 t e place has its own m]idian2 consequ5tly1 s9ce ! sun _c be on ! m]idian ( m ?an "o place at a "t1 ea* diff]5t place 7reckon+ ea/ & we/1 =1 ( c\rse1 all places ly+ exactly nor? or s\? ( "o ano!r h ! same m]idian71 mu/ h its own local noontime4 ,s9ce ! sun appe>s to move r\nd ! e>? f ea/ to we/1 x w arrive at ! m]idian ( a place ly+ ea/ ( u soon] ?an at our m]idian1 & x w arrive at our m]idian soon] ?an at t ( a place ly+ we/ ( u4 ,?us1 :5 x is noon at ,gre5wi*1 x is ab\t #g o'clock ,a4,m41 or five h\rs be=e noon1 at ,new ,york1 because ! angul> di/ance we/w>d r\nd ! e>?'s surface f ,gre5wi* to ,new ,york is1 9 r\nd numb]s1 #ge1 :i* corresponds ) five h\rs ( "t1 !re be+ #aej to e h\r4 ,at ! same mom5t x w be #e o'clock ,p4,m41 or five h\rs aft] noon1 at ,ca%m]e1 because ,ca%m]e lies #ge ea/ ( ,gre5wi*4 ,t is to say1 ! sun crosses ! m]idian ( ,ca%m]e five h\rs be=e x rea*es ! m]idian ( ,gre5wi*1 & x crosses ! m]idian ( ,gre5wi* five h\rs be=e x rea*es t ( ,new ,york4 ,at a place half-way r\nd ! circumf]5ce ( ! globe1 i4e4 #ahj ei!r ea/ or we/ ( ,gre5wi*1 x w be midnight at ! same in/ant :5 x w be mid-day1 or noon1 at ,gre5wi*4 ,n[ let u consid] ?is = a mom5t4 ,illu/ration3 ,! arr[s %[ ! direction 9 :i* ! e>? turns 7f we/ to ea/74 ,x is always noon at ! place :i* is directly "u ! sun4 ,call x ,sunday noon at ,gre5wi*1 at ! top ( ! circle2 !n x is #aj ,a4,m4 ,sunday at a po9t #cj we/ & #b ,p4,m4 ,sunday at a po9t #cj ea/1 & s on4 ,exactly opposite to ! noon po9t x is midnight4 ,by common cons5t we *ange ! "n ( ! day1 & ! date1 at midnight2 consequ5tly x is ,sunday midnight j ea/ ( ! v]tical l9e at ! bottom ( ! circle & ,monday morn+ j we/ ( x4 ,if we cross t l9e go+ we/w>d we %all pass directly f ,sunday to ,monday1 & if we cross x go+ ea/w>d we %all pass directly f ,monday to ,sunday4 ,s9ce1 by conv5tion1 ?is is a fix$ l9e on ! e>?'s surface1 ! same *ange w take place no matt] :at ! h\r ( ! day may be4 ,x is cu/om>y to *ange ! "n ( ! day at midnight4 ,?us at ! /roke ( midnight1 any:]e1 ,sunday gives place to ,monday4 ,suppose1 !n1 t ! day :5 we see ! sun on ! m]idian at ,gre5wi* happ5s to be ,sunday4 ,sunday w !n be1 s to speak1 twelve h\rs old at ,gre5wi*1 because x began !re at ! prec$+ midnight4 ,sunday w be only sev5 h\rs old at ,new ,york1 :]e x also began at ! prec$+ midnight4 ,9 ,cali=nia1 #de1 or ?ree h\rs1 /ill f>!r we/ ?an ,new ,york1 ,sunday w be only f\r h\rs old1 s9ce ! local "t !re is only f\r h\rs aft] midnight4 ,g on ov] ! ,pacific ,ocean1 until we arrive at a po9t #ahj1 or twelve h\rs1 we/ ( ,gre5wi*4 ,!re1 evid5tly1 ,sunday w j h be5 born1 ! prec$+ day1 ,saturday1 hav+ expir$ at ! /roke ( midnight4 ,n[ if we j /ep ov] t l9e ( #ahj 9 :at day %all we be8 ,x _c be ,sunday1 because ,sunday has j begun on ! l9e itself4 ,x _c be ,saturday1 because t w\ld be c\nt+ backw>d4 ,evid5tly x c be no o!r ?an ,monday4 ,let u exam9e ?is a little m closely4 ,x is ,sunday noon at ,gre5wi*4 ,we n[ g r\nd ! e>? ea/w>d in/ead ( we/w>d4 ,at #ij1 or six h\rs1 ea/ ( ,gre5wi*1 we f9d t x is #f ,p4,m4 ,sunday & at #ahj1 or twelve h\rs1 ea/ ( ,gre5wi* we f9d t x is ,sunday midnight1 or 9 o!r words ,monday morn+4 ,b ! l9e ( #ahj ea/ ( ,gre5wi* co9cides ) ! l9e ( #ahj we/ ( ,gre5wi*1 :i* we =m]ly approa*$ f ! opposite direction4 ,s we see t we 7 "r 9 conclud+ t 9 /epp+ ov] t l9e f ! ea/ to ! we/ side1 we 7 pass+ f ,sunday into ,monday4 ,x is on t l9e t ea* day vani%es & its successor takes its place4 ,x is ! 8date-l9e0 = ! :ole e>?1 *os5 by ! common cons5t ( e civilis$ nation1 j z we h se5 t ! m]idian ( ,gre5wi* is ! common ref]5ce l9e = reckon+ longitude4 ,x lies entirely 9 ! ,pacific ,ocean1 h>dly t\*+ any isl&1 & x 0 *os5 = ?is v reason1 because if x ran ov] inhabit$ l&s1 l ,europe or ,am]ica1 x w\ld cause endless confusion4 ,situat$ z x is1 x causes no tr\ble except to sea capta9s1 & v little to !m4 ,if a %ip crosses ! l9e go+ we/w>d ! capta9 jumps 8 log-book "o day =w>d4 ,if x is1 = in/ance1 ,w$nesday noon1 ea/ ( ! l9e he calls x ,?ursday noon1 z soon z he has pass$ ov]4 ,if he is go+ ea/w>d he drops back a day on cross+ ! l9e1 z f ,?ursday noon to ,w$nesday noon4 ,! date-l9e !oretically foll[s ! #ahj? m]idian1 b1 9 fact1 9 ord] to avoid c]ta9 gr\ps ( isl&s1 x b5ds ab\t a little1 :ile keep+ its g5]al direction f nor? to s\?4 #i4 ,! ,seasons4 ,we n[ recall aga9 :at 0 said 9 ,"p ,i1 ab\t ! incl9ation ( ! ecliptic1 or ! app>5t pa? ( ! sun 9 ! heav5s1 to ! equator4 ,because ( ?is incl9ation1 ! sun appe>s half ! ye> above ! equator & ! o!r half bel[ x4 ,:5 x is above ! equator = p liv+ 9 ! nor!rn hemisph]e1 x is bel[ ! equator = ^? liv+ 9 ! s\!rn hemisph]e1 & vice v]sa4 ,?is is because obs]v]s on opposite sides ( ! plane ( ! equator look at x f opposite po9ts ( view4 ,= ! nor!rn obs]v] ! cele/ial equator appe>s s\? ( ! z5i?2 = ! s\!rn obs]v] x appe>s nor? ( ! z5i?1 its di/ance f ! z5i?1 9 bo? cases1 increas+ ) ! obs]v]'s di/ance f ! equator ( ! e>?4 ,if he is on ! e>?'s equator1 ! cele/ial equator passes directly ?r\gh ! z5i?4 ,= conv5i5ce we %all suppose ! obs]v] to be some:]e 9 ! nor!rn hemisph]e4 ,illu/ration3 ,head ( ! ,great ,comet ( #ahfa ,f a draw+ by ,w>r5 ,de ,la ,rue4 ,illu/ration3 ,halley's ,comet1 ,may #e1 #aiaj ,photograph$ at ! ,y]kes ,obs]vatory by ,e ,e4 ,b>n>d1 ) ! t5-in* ,bruce telescope4 ,?is 0 %ortly be=e ! passage ( ! comet betwe5 ! e>? & ! sun1 :5 "s ?9k its tail 0 ?r[n ov] u4 ,let u beg9 ) t "t ( ! ye> :5 ! sun arrives at ! v]nal equ9ox4 ,?is occurs ab\t ! #ba/ ( ,m>*4 ,! sun is !n p]p5dicul> ov] ! equator1 daylight ext5ds1 un9t]rupt$1 f pole to pole1 & day & night 7neglect+ ! effects ( twilight & refraction7 are ( equal l5g? all ov] ! e>?4 ,ev]y:]e !re are ab\t twelve h\rs ( daylight & twelve h\rs ( d>kness4 ,?is is ! beg9n+ ( ! a/ronomical spr+4 ,z "t goes on1 ! motion ( ! sun 9 ! ecliptic c>ries x ea/w>d f ! v]nal equ9ox1 &1 at ! same "t1 ow+ to ! incl9ation ( ! ecliptic1 x rises gradually high] above ! equator1 increas+ its nor!rn decl9ation sl[ly1 day aft] day4 ,imm$iately ! equality ( day & night ceases1 & 9 ! nor!rn hemisph]e ! day becomes gradually long] 9 duration ?an ! night1 :ile 9 ! s\!rn hemisph]e x becomes %ort]4 ,moreov]1 because ! sun is n[ nor? ( ! equator1 daylight no long] ext5ds f pole to pole on ! e>?1 b ! s\? pole is 9 cont9ual d>kness1 :ile ! nor? pole is illum9at$4 ,y c illu/rate ?is1 & expla9 to y\rself :y ! relative l5g? ( day & night *anges1 & :y ! sun leaves "o pole 9 d>kness :ile ris+ high] ov] ! o!r1 by susp5d+ a small t]re/rial globe ) its axis incl9$ ab\t #bc f ! p]p5dicul>1 & pass+ a lamp ar\nd x 9 a horizontal plane4 ,at two po9ts only 9 its circuit w ! lamp be directly ov] ! equator ( ! globe4 ,call "o ( ^! po9ts ! v]nal equ9ox4 ,y w !n see t1 :5 ! lamp is directly ov] ?is po9t1 its light illum9ates ! globe f pole to pole1 b :5 x has pass$ r\nd s z to be at a po9t high] ?an ! equator1 its light no long] rea*es ! l[] pole1 al?\gh x passes ov] ! upp] "o4 ,illu/ration3 ,fig4 #aa4 ,! ,seasons4 ,! e>? is repres5t$ at f\r successive po9ts 9 its orbit ab\t ! sun4 ,s9ce ! axis ( ! e>? is virtually un*angeable 9 its direction 9 space 7leav+ out ( acc\nt ! sl[ effects ( ! precession ( ! equ9oxes71 x results t at "o "t ( ! ye>1 ! nor? pole is incl9$ t[>d ! sun & at ! opposite "t ( ! ye> away f x4 ,x atta9s its greate/ incl9ation sunw>d at ! summ] sol/ice1 !n ! l9e betwe5 day & night lies #bc beyond ! nor? pole1 s t ! :ole area )9 ! arctic circle is 9 p]petual daylight4 ,! days are long] ?an ! nights ?r\gh\t ! nor!rn hemisph]e1 b ! day becomes long] 9 proportion to ! night z ! arctic circle is approa*$1 & beyond t ! sun is cont9ually above ! horizon4 ,9 ! s\!rn hemisph]e exactly ! rev]se occurs4 ,:5 ! e>? has advanc$ to ! autumn equ9ox1 ! axis is incl9$ nei!r t[>d nor away f ! sun4 ,! latt] is !n p]p5dicul> ov] ! equator & day & night are ( equal l5g? all ov] ! e>?4 ,:5 ! e>? rea*es ! w9t] sol/ice ! nor? pole is incl9$ away f ! sun1 & n[ x is summ] 9 ! s\!rn hemisph]e4 ,at ! v]nal equ9ox aga9 !re is no incl9ation ( ! axis ei!r t[>d or away f ! sun1 & once m day & night are ev]y:]e equal4 ,a little /udy ( ?is diagram w %[ :y on ! equator day & night are always ( equal l5g?4 ,n[1 ) ! lamp ?us elevat$ above ! equator1 set ! globe 9 rotation ab\t its axis4 ,y w p]ceive t all po9ts 9 ! upp] hemisph]e are long] 9 light ?an 9 d>kness1 because ! plane divid+ ! illum9at$ & ! unillum9at$ halves ( ! globe is incl9$ to ! globe's axis 9 su* a way t x lies beyond ! upp] pole z se5 f ! direction ( ! lamp4 ,consequ5tly1 ! upp] half ( ! globe above ! equator1 z x goes r\nd1 has m ( its surface illum9at$ ?an unillum9at$1 &1 z x turns on its axis1 any po9t 9 t upp] half1 mov+ r\nd p>allel to ! equator1 is long] 9 light ?an 9 d>kness4 ,y w also obs]ve t ! ratio ( l5g? ( ! light to ! d>kness is great] ! ne>] ! po9t lies to ! pole1 & t :5 x is )9 a c]ta9 di/ance ( ! pole1 correspond+ ) ! elevation ( ! lamp above ! equator1 x lies 9 cont9ual light--9 o!r words1 )9 t di/ance f ! pole night vani%es & daylight is unceas+4 ,at ! same "t y w p]ceive t r\nd ! l[] pole !re is a simil> space )9 :i* day has vani%$ & night is unceas+1 & t 9 ! :ole ( ! l[] hemisph]e night is long] ?an day4 ,exactly on ! equator1 day & night are always ( equal l5g?4 ,endeav\r to repres5t all ?is cle>ly to y\r imag9ation1 be=e actually try+ ! exp]im5t1 or consult+ a diagram4 ,if y try ! exp]im5t y may1 in/ead ( sett+ ! axis ( ! globe at a slant1 place x upright1 & !n gradually raise & l[] ! lamp z x is c>ri$ r\nd ! globe1 n[ above & n[ bel[ ! equator4 ,we return to our description ( ! actual movem5ts ( ! sun4 ,z x rises high] f ! equator1 n only does ! day increase 9 l5g? relatively to ! night1 b ! rays ( sunlight desc5d m ne>ly p]p5dicul> ^u ! nor!rn hemisph]e4 ,! consequ5ce is t _! heat+ effect ^u ! gr\nd & ! atmosph]e increases & ! temp]ature rises until1 :5 ! sun rea*es its greate/ nor!rn decl9ation1 ab\t ! #bb;d ( ,june 7:5 x is #bc nor? ( ! equator71 ! a/ronomical summ] beg9s4 ,?is po9t 9 ! sun's c\rse ?r\gh ! circle ( ! ecliptic is call$ ! summ] sol/ice 7see ,"p ,i1 ,sect4 #h74 ,hav+ pass$ ! sol/ice1 ! sun beg9s to decl9e aga9 t[>d ! equator4 ,= a %ort "t ! decl9ation dim9i%es sl[ly because ! c\rse ( ! ecliptic close to ! sol/ice is ne>ly p>allel to ! equator1 & 9 ! meantime ! temp]ature 9 ! nor!rn hemisph]e cont9ues to increase1 ! am\nt ( heat radiat$ away dur+ ! night be+ less ?an t receiv$ f ! sun dur+ ! day4 ,?is condition cont9ues = ab\t six weeks1 ! greate/ heats ( summ] fall+ at ! end ( ,july or ! beg9n+ ( ,augu/1 :5 ! sun has already decl9$ f> t[>d ! equator1 & ! nights h begun notably to l5g!n4 ,b ! accumulation ( heat dur+ ! e>li] "p ( ! summ] is suffici5t to c\nt]balance ! loss caus$ by ! decl5sion ( ! sun4 ,ab\t ! #bc;d ( ,septemb] ! sun aga9 crosses ! equator1 ?is "t at ! autumnal equ9ox1 ! beg9n+ ( ! a/ronomical autumn1 & aft] t x s9ks l[] & l[] 7:ile appe>+ to rise 9 ! s\!rn hemisph]e71 until ab\t ! #bb;d ( ,decemb]1 :5 x rea*es its greate/ s\!rn decl9ation1 #bc1 at ! w9t] sol/ice1 :i* m>ks ! beg9n+ ( ! a/ronomical w9t]4 ,x is h>dly necess>y to po9t out t ! s\!rn w9t] corresponds 9 "t ) ! nor!rn summ]1 & vice v]sa4 ,f ! w9t] sol/ice ! sun turns nor?w>d once m1 rea*+ ! v]nal equ9ox aga9 on ! #ba/ ( ,m>*4 ,?us we see t we owe ! succession ( ! seasons entirely to ! incl9ation ( ! e>?'s axis out ( a p]p5dicul> to ! plane ( ! ecliptic4 ,if !re 7 no su* incl9ation !re w\ld be climate b no seasons4 ,!re w\ld be no summ] heat1 except 9 ! neighb\rhood ( ! equator1 :ile ! middle latitudes w\ld h a mod]ate temp]ature ! ye> r\nd4 ,ow+ to ! effects ( refraction1 p]petual day w\ld prevail )9 a small region r\nd ea* ( ! poles4 ,! sun w\ld be always p]p5dicul> ov] ! equator4 ,two ?+s rema9 to be po9t$ out ) reg>d to ! effect ( ! sun's annual motion 9 ! ecliptic4 ,"o ( ^! is ! circles call$ ! tropics4 ,^! are drawn r\nd ! e>? p>allel to ! equator & at a di/ance ( #bc f x1 "o 9 ! nor!rn & ! o!r 9 ! s\!rn hemisph]e4 ,! nor!rn "o is call$ ! tropic ( ,canc]1 because its correspond+ circle on ! cele/ial sph]e runs ?r\gh ! zodiacal sign ,canc]1 & ! s\!rn "o is call$ ! tropic ( ,capricorn = a simil> reason4 ,! tropics run ?r\gh ! two sol/ices1 & m>k ! app>5t daily track ( ! sun 9 ! sky :5 x is at ei!r its greate/ nor!rn or its greate/ s\!rn decl9ation4 ,! sun is !n p]p5dicul> ov] "o or ! o!r ( ! tropics4 ,t "p ( ! e>? ly+ betwe5 ! tropics is call$ ! torrid zone1 because ! sun is always n f> f p]p5dicul> ov] x1 & ! heat is v great4 ,illu/ration3 ,! ,six-,tail$ ,comet ( #agdd ,f a contempor>y draw+4 ,! o!r ?+ to be m5tion$ is ! pol> circles4 ,^! are situat$ #bc f ea* pole1 j z ! tropics are situat$ a simil> di/ance on ea* side ( ! equator4 ,! nor!rn is call$ ! arctic1 & ! s\!rn ! ant>ctic circle4 ,^? p>ts ( ! e>? :i* lie betwe5 ! tropics & ! pol> circles are call$ respectively ! nor!rn & ! s\!rn temp]ate zone4 ,! pol> circles m>k ! limits ( ! region r\nd ea* pole :]e ! sun %9es cont9u\sly :5 x is at "o or ! o!r ( ! sol/ices4 ,if ! read] w recall ! exp]im5t ) ! globe & ! lamp1 he w p]ceive t ^! circles correspond ) ! bord]s ( ! circul> spaces at ea* pole ( ! globe :i* are alt]nately c>ri$ into & out ( ! full light z ! lamp is elevat$ to its greate/ height above ! equator or depress$ to its greate/ di/ance bel[ x4 ,at ea* pole1 9 turn1 !re are six mon?s ( cont9ual day foll[$ by six mon?s ( cont9ual night1 & :5 ! sun is at "o ( ! sol/ices x j t\*es ! horizon on ! correspond+ pol> circle at ! h\r t m>ks midnight on ! p>ts ( ! e>? :i* lie outside ! pol> circles4 ,?is is ! celebrat$ ph5om5on ( ! 8midnight sun40 ,at any po9t )9 ! pol> circle conc]n$1 ! sun1 at ! h\r ( midnight approa*es ! horizon b does n t\* x1 its midnight elevation increas+ ) ne>ness to ! pole1 :ile exactly at ! pole itself ! sun simply moves r\nd ! sky once 9 tw5ty-f\r h\rs 9 a circle practically p>allel to ! horizon4 ,x is by obs]vations on ! daily movem5t ( ! sun t an explor] seek+ "o ( ! e>?'s poles dur+ ! long pol> day is able to det]m9e :5 he has actually rea*$ 8 goal4 ,! read] w h rem>k$ 9 ^! descriptions h[ frequ5tly ! angle ( #bc turns up1 & he %\ld rememb] t x is1 9 e case1 due to ! same cause1 viz41 ! incl9ation ( ! e>?'s axis f a p]p5dicul> to ! ecliptic4 ,a v rem>kable fact mu/ n[ be ref]r$ to4 ,al?\gh ! angul> di/ance t ! sun has to travel 9 pass+ fir/ f ! v]nal equ9ox to ! autumnal equ9ox1 on ! nor!rn side ( ! equator1 & !n back aga9 f ! autumnal equ9ox to ! v]nal equ9ox1 on ! s\!rn side ( ! equator1 is ! same1 ! "t t x occupies 9 mak+ ^! two half /ages 9 its annual j\rney is n ! same4 ,beg9n+ f ! #ba/ ( ,m>* & c\nt+ ! numb] ( days to ! #bc;d ( ,septemb]1 & !n beg9n+ f ! #bc;d ( ,septemb] & c\nt+ ! numb] ( days to ! next #ba/ ( ,m>*1 y w f9d t 9 an ord9>y ye> ! fir/ p]iod is sev5 days long] ?an ! second4 ,9 o!r words1 ! sun is a week long] above ! equator ?an bel[ x4 ,! reason = ?is diff]5ce is f\nd 9 ! fact t ! orbit ( ! e>? ab\t ! sun is n a p]fect circle1 b is a slightly elongat$ ellipse1 & ! sun1 in/ead ( be+ situat$ 9 ! c5tre1 is situat$ 9 "o ( ! two foci ( ! ellipse1 #c1jjj1jjj miles ne>] to "o end ( x ?an to ! o!r4 ,n[ ?is elliptical orbit ( ! e>? is s situat$ t ! e>? is ne>e/ to ! focus occupi$ by ! sun1 or 9 p]ihelion1 ab\t ,decemb] #ca/1 only a few days aft] ! w9t] sol/ice1 & f>!/ f ! sun1 or 9 aphelion1 ab\t ,july #a/1 only a few days aft] ! summ] sol/ice4 ,?us ! e>? is ne>] ! sun dur+ ! w9t] half ( ! ye>1 :5 ! sun appe>s s\? ( ! equator1 ?an dur+ ! summ] half ( ! ye>1 :5 ! sun appe>s nor? ( ! equator4 ,n[ ! law ( gravitation tea*es t :5 ! e>? is ne>] ! sun x mu/ move m rapidly 9 its orbit ?an :5 x is m di/ant1 f :i* x foll[s t ! "t occupi$ by ! sun 9 its app>5t passage f ! v]nal equ9ox to ! autumnal equ9ox is long] ?an t occupi$ 9 ! passage back f ! autumnal to ! v]nal equ9ox4 ,b :ile ! summ] half ( ! ye> is long] ?an ! w9t] half 9 ! nor!rn hemisph]e1 ! rev]se is ! case 9 ! s\!rn hemisph]e4 ,!re ! w9t] is long] ?an ! summ]4 ,moreov]1 ! w9t] ( ! s\!rn hemisph]e occurs :5 ! e>? is f>!/ f ! sun1 :i* acc5tuates ! disadvantage4 ,x has be5 ?\ght t ! great] quantity ( ice ab\t ! s\? pole may be due to ?is increas$ l5g? & sev]ity ( ! s\!rn w9t]4 ,x is true t ! s\!rn summ]1 al?\gh %ort]1 is hott] ?an ! nor!rn1 b :ile1 !oretically1 ?is %\ld re/ore ! balance z a :ole1 yet x w\ld appe> t ! %ort hot summ] does n1 9 fact1 suffice to arre/ ! accumulation ( ice4 ,h[ev]1 ! pres5t condition ( ?+s z betwe5 ! two hemisph]es w n cont9ue1 b 9 ! c\rse ( "t w be rev]s$4 ,! read] w recall t ! precession ( ! equ9oxes causes ! axis ( ! e>? to turn sl[ly r\nd 9 space4 ,at pres5t ! nor!rn end ( ! e>?'s axis is incl9$ away f ! aphelion & 9 ! direction ( ! p]ihelion po9t ( ! orbit1 s t ! nor!rn summ] occurs :5 ! e>? is 9 ! m di/ant "p ( its orbit1 & ! w9t] :5 x is 9 ! ne>] "p4 ,b ! precession sw+s ! axis r\nd we/w>d f its pres5t position at ! rate ( #ej4b p] ye>1 :ile at ! same "t ! position ( ! orbit itself is %ift$ 7by ! effects ( ! attraction ( ! planets7 9 su* a mann] t ! aphelion & p]ihelion po9ts1 :i* are call$ ! apsides1 move r\nd ea/w>d at ! rate ( #aa4be p] ye>4 ,! comb9ation ( ! precession ) ! motion ( ! apsides produces a revolution at ! rate ( #fa4de p] ye>1 :i* 9 ! c\rse ( #aj1ejj ye>s w completely rev]se ! exi/+ incl9ation ( ! axis ) reg>d to ! major diamet] ( ! orbit1 s t !n ! nor!rn hemisph]e w h its summ] :5 ! e>? is ne> p]ihelion & its w9t] :5 x is ne> aphelion4 ,! w9t]1 !n1 w1 = u1 be long & sev]e & ! summ] %ort ?\gh hot4 ,x has be5 ?\ght possible t su* a /ate ( ?+s may cause1 9 our hemisph]e1 a p>tial r5ewal ( :at is kn[n 9 geology z a glacial p]iod4 ,a glacial p]iod 9 ! s\!rn hemisph]e w\ld probably always be less sev]e ?an 9 ! nor!rn1 because ( ! great prepond]ance ( sea ov] l& 9 ! s\!rn half ( ! globe4 ,an ocean climate is m equable ?an a l& climate4 #aj4 ,! ,ye>1 ! ,cal5d>1 & ! ,mon?4 ,a ye> is ! p]iod ( "t requir$ = ! e>? to make "o revolution 9 its orbit ab\t ! sun4 ,b1 z !re are ?ree k9ds1 or measures1 ( "t1 s !re are ?ree k9ds1 or measures1 ( ! ye>4 ,! fir/ ( ^! is call$ ! sid]eal ye>1 b al?\gh1 l sid]eal "t1 x measures ! true l5g? ( ! p]iod 9 "q1 x is n suitable = ord9>y use4 ,to und]/& :at is meant by a sid]eal ye>1 imag9e y\rself to be look+ at ! e>? f ! sun1 & suppose t at "s in/ant y %\ld see ! e>? exactly 9 conjunction ) a />4 ,:51 hav+ gone r\nd ! sun1 x _h come back aga9 to conjunction ) ! same />1 precisely "o revolution w\ld h be5 p]=m$ 9 its orbit1 & ! p]iod elaps$ w\ld be a sid]eal ye>4 ,practically1 ! l5g? ( ! sid]eal ye> is det]m9$ by obs]v+ :5 ! sun1 9 its app>5t annual j\rney r\nd ! sky1 has come back to conjunction ) "s giv5 />4 ,! second k9d ( ye> is call$ ! tropical ye>1 & x is measur$ by ! p]iod tak5 by ! sun to pass r\nd ! sky f "o conjunction ) ! v]nal equ9ox to ! next4 ,?is p]iod diff]s slightly f ! fir/1 because1 ow+ to ! precession ( ! equ9oxes1 ! v]nal equ9ox is sl[ly %ift+ we/w>d1 z if to meet ! sun 9 its annual c\rse1 f :i* x results t ! sun ov]takes ! equ9ox a little be=e x has complet$ a sid]eal ye>4 ,! tropical ye> is ab\t tw5ty m9utes %ort] ?an ! sid]eal ye>4 ,x is1 h[ev]1 m conv5i5t = ord9>y purposes1 because we naturally ref] ! progress ( ! ye> to t ( ! seasons1 &1 z we h se51 ! seasons dep5d ^u ! equ9oxes4 ,b yet ! tropical ye> is n entirely satisfactory z a measure ( "t1 because ! numb] ( days conta9$ 9 x is n an ev5 "o4 ,its l5g? is #cff days1 #e h\rs1 #dh m9utes1 #df seconds4 ,accord+ly1 z ! irregul>ities ( app>5t sol> "t 7 avoid$ by ! inv5tion ( mean sol> "t1 s ! difficulty pres5t$ by ! tropical ye> is gott5 rid (1 z f> z possible1 by means ( :at is call$ ! civil ye>1 or ! cal5d> ye>1 ! av]age l5g? ( :i* is almo/ exactly equal to t ( ! tropical ye>4 ,?is br+s u to ! consid]ation ( ! cal5d>1 :i* is z full ( compromises z a political treaty--b !re is no help = x s9ce nature did n see fit to make ! day an exact fraction ( ! ye>1 or1 9 o!r words1 to make ! day & ! ye> comm5surable quantities ( "t4 ,)\t go+ into a hi/ory ( ! re=ms t ! cal5d> has und]gone1 :i* w\ld dem& a great deal ( space1 we may simply say t ! basis ( ! cal5d> we use to-day 0 e/abli%$ by ,julius ,cs>1 ) ! aid ( ! ,greek a/ronom] ,sosig5es4 ,?is is ! ,julian cal5d>1 & ! re=m$ %ape 9 :i* x exi/s at pres5t is call$ ! ,gregorian cal5d>4 ,cs> assum$ #cfe days z ! true l5g? ( ! ye>1 &1 9 ord] to get rid ( ! qu>t] day1 ord]$ t x %\ld be left out ( acc\nt = ?ree ye>s out ( e f\r4 ,9 ! f\r? ye> ! f\r qu>t] days 7 add$ toge!r to make "o additional day1 :i* 0 add$ to t p>ticul> ye>4 ,?us ! ord9>y ye>s 7 ea* #cfe days long & e f\r? ye> 0 #cff days long4 ,?is f\r? ye> 0 call$ ! bissextile ye>4 ,x 0 id5tical ) our leap ye>4 ,! days ( bo? ! ord9>y & ! leap ye>s 7 di/ribut$ among ! twelve mon?s v mu* z we di/ribute !m n[4 ,b ,cs>'s assumption ( #cfe days z ! l5g? ( ! ye> 0 errone\s1 be+ ab\t #aa m94 #ad sec4 long] ?an ! real tropical ye>4 ,9 ! sixte5? c5tury ?is error _h accumulat$ to su* a degree t ! mon?s 7 becom+ s]i\sly disjo9t$ f ! seasons ) :i* !y _h be5 cu/om>ily associat$4 ,9 consequ5ce1 ,pope ,gregory ,,xiii1 assi/$ by ! a/ronom] ,clavius1 introduc$ a slight re=m ( ! ,julian cal5d>4 ,! accumulat$ days 7 dropp$1 & a new />t tak51 & ! rule = leap ye> 0 *ang$ s z to read t 8all ye>s1 ^: date-numb] is divisible by f\r )\t a rema9d] are leap ye>s1 unless !y are c5tury ye>s 7su* z #ahjj1 #aijj1 etc474 ,! c5tury ye>s are n leap ye>s1 unless _! date numb] is divisible by #djj1 9 :i* case !y are40 ,& ?is is ! rule z x prevails to-day1 al?\gh !re is n[ 7#aiab7 s]i\s talk ( und]tak+ a new revision4 ,b ! ,gregorian cal5d> is s ne>ly correct t m ?an #cjjj ye>s mu/ elapse be=e ! l5g? ( ! ye> z det]m9$ by x w diff] by "o day f ! true tropical ye>4 ,! subject ( ! re=m ( ! cal5d> is a v int]e/+ "o1 b1 toge!r ) t ( ! rules = det]m9+ ! date ( ,ea/]1 its discussion mu/ be s\ght 9 m ext5sive works4 ,!re is "o o!r measure ( "t1 dep5d+ ^u ! motion ( a heav5ly body1 :i* mu/ be m5tion$4 ,?is is ! mon?1 or ! p]iod requir$ = ! moon to make a revolution r\nd ! e>?4 ,"h we enc\nt] aga9 ! same difficulty1 = ! mon? also is incomm5surable ) ! ye>4 ,!n1 too1 ! l5g? ( ! mon? v>ies accord+ to ! way 9 :i* x is reckon$4 ,we h1 fir/1 a sid]eal revolution ( ! moon1 :i* is measur$ by ! "t tak5 to pass r\nd ! e>? f "o conjunction ) a /> to ! next4 ,?is is1 on ! av]age1 #bg days1 #g h\rs1 #dc m9utes1 #ab seconds4 ,next we h a synodical revolution ( ! moon1 :i* is measur$ by ! "t x takes 9 pass+ f ! phase ( new moon r\nd to ! same phase aga94 ,?is seems ! mo/ natural measure ( a mon?1 because ! *ang+ phases ( ! moon are its mo/ conspicu\s peculi>ity4 7,^! w be expla9$ 9 ,"p ,,iii47 ,! l5g? ( ! mon?1 z ?us measur$1 is1 on ! av]age1 #bi days1 #ab h\rs1 #dd m9utes1 #c seconds4 ,! reason :y ! synodical mon? is s mu* long] ?an ! sid]eal mon? is because new moon c occur only :5 ! moon is 9 conjunction ) ! sun1 i4e4 exactly betwe5 ! e>? & ! sun1 & 9 ! int]val betwe5 two new moons ! sun moves onw>d1 s t = ! second conjunction ! moon mu/ g f>!r to ov]take ! sun4 ,x w be obs]v$ t bo? ( ! mon? measures are giv5 9 av]age figures4 ,?is is because ! moon's motion is n q regul>1 ow+ p>tly to ! ecc5tricity ( its orbit & p>tly to ! di/urb+ effects ( ! sun's attraction4 ,! l5g? ( ! sid]eal revolution v>ies to ! ext5t ( ?ree h\rs1 & t ( ! synodical revolution to ! ext5t ( ?irte5 h\rs4 ,b1 :i*ev] measure ( ! mon? we take1 x is incomm5surate ) ! ye>1 i4e4 !re is n an ev5 numb] ( mon?s 9 a ye>4 ,9 anci5t times ceaseless ef=ts 7 made to adju/ ! mon?s to ! measure ( ! ye>1 b we h practically giv5 up ! attempt1 & 9 our cal5d> ! lun> mon?s %ift along z !y w1 :ile ! ord9>y mon?s are p]iods ( a c]ta9 numb] ( days1 hav+ no relation to ! movem5ts ( ! moon4 ,x has be5 ?\ght t ! p]iod call$ a week1 :i* has be5 us$ f "t immemorial1 may h orig9at$ f ! fact t ! int]val f new moon to ! fir/ qu>t] & f fir/ qu>t] to full moon1 etc41 is v ne>ly sev5 days4 ,b ! week is z incorrigible z all its si/]s 9 ! discordant family ( "t1 & !re is no m difficult problem = human +5uity ?an t ( inv5t+ a sy/em ( reckon+1 9 :i* ! days1 ! weeks1 ! mon?s1 & ! ye>s %all be adju/$ to ! close/ possible h>mony4 ,illu/ration3 ,spiral ,nebula 9 ,ursa ,major 7,m #aja7 ,photograph$ at ! ,lick ,obs]vatory by ,j4 ,e4 ,keel]1 ) ! ,crossley reflector4 ,exposure f\r h\rs4 ,note ! appe>ance ( swift revolution1 z if ! nebula 7 ?r[+ itself to pieces l a sp9n+ p9-:eel4 ,illu/ration3 ,! ,:irlpool ,nebula 9 ,canes ,v5atici ,photograph$ at ! ,lick ,obs]vatory by ,j4 ,e4 ,keel]1 ) ! ,crossley reflector4 ,exposure f\r h\rs4 ,note ! 8bead+0 ( ! arms ( ! :irl+ nebula4 ---------------------------------------- -------------------------------- ,,"p ,,iii4 ,,! ,,sol> ,,sy/em4 ---------------------------------------- -------------------------------- ,,"p ,,iii4 ,,! ,,sol> ,,sy/em4 #a4 ,! ,sun4 ,by ! t]m sol> sy/em is meant ! sun toge!r ) ! sy/em ( bodies 7planets1 a/]oids1 comets & meteors7 revolv+ r\nd x4 ,! sun1 be+ a />1 e o!r />1 = all t we c tell1 may be ! rul] ( a simil> sy/em4 ,9 fact1 we "k t a few />s h huge d>k bodies revolv+ r\nd !m1 :i* may be lik5$ to gigantic planets4 ,! reason :y ! sun is ! common c5tre r\nd :i* ! o!r memb]s ( ! sol> sy/em move1 is because x va/ly exce$s all ( !m put toge!r 9 mass1 or quantity ( matt]1 & ! p[] ( any body to set ano!r body 9 motion by its attractive =ce dep5ds ^u mass4 ,if a great body & a small body attract ea* o!r1 bo? w move1 b ! motion ( ! small body w be s mu* m ?an t ( ! great "o t ! latt] w seem1 relatively1 to /& fa/ :ile ! small "o moves4 ,!n1 if ! small body _h orig9ally a motion across ! direction 9 :i* ! great body attracts x1 ! result ( ! comb9ation w be to cause ! small body to revolve 9 an orbit 7m or less elliptical accord+ to ! direction & velocity ( its orig9al motion7 ab\t ! great body4 ,if ! diff]5ce ( mass is v great1 ! l>ge body w rema9 virtually immovable4 ,?is is ! case ) ! sun & its planets4 ,! sun has #ccb1jjj times z mu* mass 7or1 we may say1 is #ccb1jjj times z heavy7 z ! e>?4 ,x has a little m ?an a ?\s& times z mu* mass z its l>ge/ planet1 ,jupit]4 ,x has millions ( times z mu* z ! greate/ comet4 ,! consequ5ce is t all ( ^! bodies revolve ar\nd ! sun1 9 orbits ( v>i\s degrees ( ecc5tricity1 :ile ! sun itself rema9s practically immovable1 or j sway+ a little ?is way & t1 l a huntsman hold+ 8 dogs 9 lea%4 ,! di/ance ( ! sun f ! e>?--ab\t #ic1jjj1jjj miles--has be5 det]m9$ by me?ods :i* w be briefly expla9$ 9 ! next section4 ,kn[+ its di/ance1 x is easy to calculate its size1 s9ce ! app>5t diamet] ( all objects v>ies directly ) _! di/ance4 ,! diamet] ( ! sun is ?us f\nd to be ab\t #hff1djj miles1 or ne>ly #aaj times t ( ! e>?4 ,9 bulk x exce$s ! e>? ab\t #a1cjj1jjj times1 b its mass1 or quantity ( matt]1 is only #ccb1jjj times ! e>?'s1 because its av]age d5sity is b "o qu>t] t ( ! e>?4 ,?is arises f ! fact t ! e>? is a solid1 compact body1 :ile ! sun is a body compos$ ( gases & vap\rs 7?\gh 9 a v compress$ /ate74 ,x is ! high temp]ature ( ! sun :i* ma9ta9s x 9 ?is /ate4 ,its temp]ature has be5 calculat$ at ab\t #af1jjj ,fahr5heit1 b v>i\s e/imates diff] r widely4 ,at any rate1 x is s hot t ! mo/ refractory sub/ances kn[n to u w\ld be r$uc$ to ! /ate ( vap\r1 if remov$ to ! sun4 ,! quantity ( heat receiv$ ^u ! e>? f ! sun c only be express$ 9 t]ms ( ! me*anical equival5t ( heat4 ,! unit ( heat 9 eng9e]+ is ! calorie1 :i* means ! am\nt ( heat requir$ to raise ! temp]ature ( "o kilogram ( wat] 7#b4b p\nds7 "o degree ,c5tigrade 7#a4h ,fahr5heit74 ,n[ obs]vation %[s t ! sun furni%es #cj ( ^! calories p] m9ute ^u e squ>e metre 7ab\t #a4b squ>e y>d7 ( ! e>?'s surface4 ,p]haps !re is no bett] illu/ration ( :at ?is means ?an ,pr(4 ,"y's /atem5t1 t 8! heat annually receiv$ on ea* squ>e foot ( ! e>?'s surface1 if employ$ 9 a p]fect heat eng9e1 w\ld be able to hoi/ ab\t a hundr$ tons to ! height ( a mile40 ,or take ,pr(4 ,todd's illu/ration ( ! me*anical p[] ( ! sunbeams3 8,if we measure (f a space five feet squ>e1 ! en]gy ( ! sun's rays :5 fall+ v]tically ^u x is equival5t to "o horse p[]40 ,a/ronom]s ord9>ily reckon ! sol> con/ant 9 8small calories10 :i* are b ! ?\s&? "p ( ! eng9e]'s calorie1 & ! late/ results ( ! ,smi?sonian ,in/itution obs]vations indicate t ! sol> con/ant is ab\t #a4ie ( ^! small calories p] squ>e c5timet] p] second4 ,ab\t #cj p] c5t4 mu/ be d$uct$ = atmosph]ic absorption4 ,heat1 l gravitation & l light1 v>ies inv]sely 9 int5sity ) ! squ>e ( ! di/ance2 h5ce1 if ! e>? 7 twice z ne> z x is to ! sun x w\ld receive f\r times z mu* heat & f\r times z mu* light1 & if x 7 twice z f> away x w\ld receive only "o qu>t] z mu*4 ,?is %[s h[ important x is = a planet n to be too ne>1 or too f> f1 ! sun4 ,! e>? w\ld be vap\ris$ if x 7 c>ri$ )9 a qu>t] ( a million miles ( ! sun4 ,! sun rotates on an axis incl9$ ab\t #g f a p]p5dicul> to ! plane ( ! ecliptic4 ,! av]age p]iod ( its rotation is ab\t #be days--we say 8av]age0 because1 n be+ a solid body1 diff]5t p>ts ( its surface turn at diff]5t rates4 ,x rotates fa/] at ! equator ?an at latitudes nor?-&-s\? ( ! equator1 ! velocity decreas+ t[>d ! poles4 ,! p]iod ( rotation at ! equator is ab\t #be days1 & at #dj nor? or s\? ( ! equator x is ab\t #bg days4 ,! direction ( rotation is ! same z t ( ! e>?'s4 ,! surface ( ! sun1 :5 view$ ) a telescope1 is (t5 se5 m or less spott$4 ,! spots are black1 or dusky1 & frequ5tly ( v irregul> %apes1 al?\gh _m ( !m are ne>ly circul>4 ,g5]ally !y appe> 9 gr\ps drawn out 9 ! direction ( ! sol> rotation4 ,"s ( ^! gr\ps cov] areas ( _m millions ( squ>e miles1 al?\gh ! sun is s imm5se t ev5 !n !y appe> to ! nak$ eye 7gu>d$ by a d>k glass7 only z small d>k spots on its surface4 ,! c5tres ( sun-spots1 are ! d>ke/ p>ts4 ,g5]ally ar\nd ! bord]s ( ! spots ! surface seems to be m or less heap$ up4 ,(t51 9 l>ge sun-spots1 imm5se promontories1 v brilliant1 project ov] ! d>k int]ior1 & _m ( ^! are prolong$ into bridges ( light1 app>5tly trav]s+ ! *asms b5ea?4 ,con/ant *anges ( %ape & arrangem5t take place1 & !re are few m a/oni%+ telescopic objects ?an a great sun-spot4 ,illu/ration3 8,tress ,nebula0 7,n4 ,g4 ,c4 #fiib7 9 ,cygnus ,photograph$ at ! ,y]kes ,obs]vatory by ,g4 ,w4 ,rit*ey1 ) ! two-foot reflector4 ,obs]ve ! /rangely twi/$ look ( ?is long curv$ nebula2 also ! curi\s curves compos$ ( m9ute />s ne> x4 ,! spots are n always visible 9 equal numb]s1 & 9 "s ye>s b few are se51 & !y are small4 ,x has be5 f\nd t !y occur 9 p]iods1 av]ag+ ab\t elev5 ye>s f maximum to m9imum1 al?\gh ! l5g? ( ! p]iod is v irregul>4 ,x has also be5 obs]v$ t :5 ! fir/ spots ( a new p]iod appe>1 !y are g5]ally se5 "s #cj f ! equator1 ei!r t[>d ! nor? or t[>d ! s\?1 & t z ! p]iod progresses ! spots increase 9 size1 & seem to draw t[>d ! equator1 ! la/ spots ( ! p]iod be+ se5 q close to ! equator1 on "o side or ! o!r4 ,! duration ( individual spots is v>iable2 "s la/ b a day or two1 & o!rs cont9ue = weeks1 sometimes be+ c>ri$ out ( view by ! rotation ( ! sun & br\ght into view aga9 f ! o!r side4 ,! surface ( ! sun 9 ! neighb\rhood ( gr\ps ( spots is frequ5tly m>k$ by l>ge areas cov]$ ) cr9kl$ bright l9es & pat*es1 :i* are call$ facul4 ,^!1 :i* are ! brighte/ p>ts ( ! sun1 appe> to be elevat$ above ! g5]al level4 ,z to ! cause & nature ( sun-spots mu* rema9s to be le>n$4 ,9 #aijh1 ,pr(4 ,george ,e4 ,hale1 by means ( an in/rum5t call$ ! spectro-heliograph1 :i* selects out ( ! total radiation ( ! sol> disk light peculi> to c]ta9 elem5ts1 & ?us p]mits ! use ( t light alone 9 photograph+ ! sun1 demon/rat$ t sun-spots probably arise f vortices1 or :irl+ /orms1 & t ^! vortices produce /rong magnetic fields 9 ! sun-spots4 ,! ph5om5on may be reg>d$1 says ,pr(4 ,hale1 z some:at analog\s to a tornado or wat]sp\t on ! e>?4 ,! :irl+ trombe becomes wid] at ! top1 c>ry+ ! gases f bel[ upw>d4 ,at ! c5tre ( ! /orm ! rapid rotation produces an expansion :i* cools ! gases & causes ! appe>ance ( a comp>atively d>k cl\d1 :i* we see z ! sun-spot4 ,! vortices :irl 9 opposite directions on opposite sides ( ! sun's equator1 ?us obey+ ! same law t gov]ns ! rotation ( cyclones on ! e>?4 ,x has long be5 a "q :e!r ! condition ( ! sun z manife/$ by ! spots ^u its surface has an influ5ce ^u ! meteorology ( ! e>?4 ,x is kn[n t ! sun-spot p]iod co9cides closely ) p]iodical *anges 9 ! e>?'s magnetism1 & great outbur/s on ! sun h frequ5tly be5 imm$iately foll[$ by viol5t magnetic /orms & brilliant displays ( ! aurora borealis on ! e>?4 ,! sun und\bt$ly ex]cises o!r influ5ces ^u ! e>? ?an ^? famili> to u "u ! names ( gravitation1 light1 & heat2 b ! nature ( ^! o!r influ5ces is n yet fully und]/ood4 ,! brilliant :ite surface ( ! sun is call$ ! photosph]e4 ,x has be5 lik5$ to a %ell ( int5sely hot cl\ds1 consi/+ ( sub/ances :i* are entirely vapor\s )9 ! body ( ! sun4 ,above ! photosph]e lies an envelope1 e/imat$ to be f #ejjj to #aj1jjj miles ?ick1 kn[n z ! *romosph]e4 ,x consi/s ma9ly ( hydrog5 & helium1 & :5 se5 dur+ a total eclipse1 :5 ! globe ( ! sun is conceal$ beh9d ! moon1 x pres5ts a brilliant sc>let col\r4 ,above ?is are frequ5tly se5 spl5did r$ flame-l objects1 nam$ prom95ces4 ,!y are ( two v>ieties--"o cl\d-l 9 appe>ance1 & ! o!r resembl+ spikes1 or trees ) spread+ tops1--b (t5 _! =ms are inf9itely v>i$4 ,! latt]1 ! s-call$ eruptive prom95ces1 exhibit rapid motion away f ! sun's surface1 z if !y consi/$ ( matt] :i* has be5 eject$ by explosion4 ,occasionally ^! objects h be5 se5 to gr[ to a height ( sev]al hundr$ ?\s& miles1 ) velocities ( two or ?ree hundr$ miles p] second4 ,! sun has /ill ano!r envelope1 ( *ang+ =m1--! corona4 ,?is app>5tly consi/s ( r>e gase\s matt]1 ^: *>act]i/ic con/itu5t is an elem5t unkn[n on ! e>?1 call$ coronium4 ,! corona appe>s 9 ! =m ( a lum9\s halo1 surr\nd+ ! hidd5 sun dur+ a total eclipse1 & x (t5 ext5ds outw>d sev]al million miles4 ,its %ape v>ies 9 accordance ) ! sun-spot p]iod4 ,x has a diff]5t appe>ance & outl9e at a "t ( maximum sun-spots f ^? :i* x pres5ts at a m9imum4 ,!re are _m ?+s ab\t ! corona :i* sugge/ ! play ( electric & magnetic =ces4 ,! corona1 al?\gh evid5tly always exi/+1 is nev] se5 except dur+ ! few m9utes ( complete obscuration ( ! sun t occurs 9 a total eclipse4 ,?is is because its light is n suffici5tly int5se to r5d] x visible1 :5 ! atmosph]e ar\nd ! obs]v] is illum9at$ by ! direct rays ( sunlight4 #b4 ,p>allax4 ,we n[ return to ! "q ( ! sun's di/ance f ! e>?1 :i* we treat 9 a sep>ate section1 because ?us x is possible to pres5t1 at a s+le view1 ! entire subject ( ! measurem5t ( ! di/ances ( ! heav5ly bodies4 ,! common basis ( all su* measurem5ts is furni%$ by :at is call$ p>allax1 :i* may be def9$ z ! diff]5ce ( direction ( an object :5 view$ alt]nately f two sep>ate po9ts4 ,! simple/ example ( p>allax is f\nd 9 look+ at an object fir/ ) "o eye & !n ) ! o!r )\t1 9 ! meantime1 alt]+ ! position ( ! head4 ,suppose y sit 9 front ( a w9d[ ?r\gh :i* y c see ! wall ( a h\se on ! opposite side ( ! /reet4 ,*oose "o ( ! v]tical b>s ( ! w9d[-sa%1 &1 clos+ ! left eye1 look at ! b> ) ! "r & note :]e x seems to be project$ aga9/ ! wall4 ,!n close ! "r eye & op5 ! left1 & y w obs]ve t ! place ( projection ( ! b> has %ift$ t[>d ! "r4 ,?is *ange ( direction is due to p>allax & its am\nt dep5ds bo? ^u ! di/ance betwe5 ! eyes & ^u ! di/ance ( ! w9d[ f ! obs]v]4 ,to see h[ ?is pr9ciple is appli$ by ! a/ronom]1 let u suppose t we wi% to asc]ta9 ! di/ance ( ! moon4 ,! moon is s f> away t ! di/ance betwe5 ! eyes is inf9itesimal 9 comp>ison1 s t no p>allactic %ift 9 its direction is app>5t on view+ x alt]nately ) ! two eyes4 ,b by mak+ ! obs]vations f widely sep>at$ po9ts on ! e>? we c produce a p>allactic %ift+ ( ! moon's position :i* w be easily measurable4 ,let "o ( ! po9ts ( obs]vation be 9 ! nor!rn hemisph]e & ! o!r 9 ! s\!rn1 ?\s&s ( miles ap>t4 ,! two obs]v]s might !n be comp>$ to ! eyes ( an enorm\s head1 ea* ( :i* sees ! moon 9 a measurably diff]5t direction4 ,if ! nor!rn obs]v] c>efully asc]ta9s ! angul> di/ance ( ! moon f 8 z5i?1 & ! s\!rn obs]v] does ! same ) reg>d to 8 z5i?1 z indicat$ 9 ,fig4 #ab1 !y c1 by a comb9ation ( _! measurem5ts1 con/ruct a quadrilat]al ,a ,c ,b ,m1 ( :i* all ! angles may be asc]ta9$ f ! two measurem5ts1 :ile ! l5g? ( ! sides ,a ,c & ,b ,c is already kn[n1 s9ce !y are ea* equal to ! radius ( ! e>?4 ,) ^! data x is easy1 by ! rules ( plane trigonometry1 to calculate ! l5g? ( ! o!r sides1 & also ! l5g? ( ! /raight l9e f ! c5tre ( ! e>? to ! moon4 ,9 all su* cases ! di/ance betwe5 ! po9ts ( obs]vation is call$ ! base-l9e1 ^: l5g? is kn[n to />t )1 :ile ! angles =m$ by ! l9es ( direction at ! opposite ends ( ! base-l9e are asc]ta9$ by measurem5t4 ,illu/ration3 ,fig4 #ab4 ,p>allax ( ! ,moon4 ,let ,c be ! c5tre ( ! e>?1 ,a & ,b ! /ations ( two obs]v]s1 "o 9 ! nor!rn1 ! o!r 9 ! s\!rn hemisph]e1 & ,m ! moon4 ,! l9es ,c ,a ,z & ,c ,b ,z indicate ! direction ( ! z5i? at ,a & ,b respectively4 ,subtract+ ! measur$ angles at ,a & ,b ea* f #ahj gives ! inside angles at ^? po9ts4 ,! angle at ,c is equal to ! sum ( ! latitudes ( ,a & ,b s9ce !y are on opposite sides ( ! equator4 ,) ?ree angles kn[n1 ! f\r?1 at ,m1 is f\nd by simply subtract+ _! sum f #cfj4 ,illu/ration3 ,! ,great ,&rom$a ,nebula ,photograph$ at ! ,y]kes ,obs]vatory by ,g4 ,w4 ,rit*ey1 ) ! two-foot reflector4 ,obs]ve ! va/ spiral1 or elliptic1 r+s surr\nd+ ! c5tral cond5sation & ! appe>ance ( break+ up & re-%ap+ into small] masses :i* "s ( ! r+s pres5t4 ,9 ! case ( ! sun ! di/ance conc]n$ is s great 7ab\t #djj times t ( ! moon7 t ! p>allax produc$ by view+ x f diff]5t po9ts on ! e>? is too small to be c]ta9ly measur$1 & a modification ( ! me?od has to be employ$4 ,"o su* modification1 :i* has be5 mu* us$1 dep5ds ^u ! fact t ! planet ,v5us1 be+ ne>] ! sun ?an ! e>? is1 appe>s1 at c]ta9 times1 pass+ directly ov] ! face ( ! sun4 ,?is is call$ a transit ( ,v5us4 ,dur+ a transit1 ,v5us is betwe5 ?ree & f\r times ne>] ! e>? ?an ! sun is1 & consequ5tly its p>allactic displacem5t1 :5 view$ f widely sep>at$ po9ts on ! e>?1 is mu* great] ?an t ( ! sun4 ,"o ( ! ways 9 :i* ! a/ronom] takes advantage ( ?is fact is %[n 9 ,fig4 #ac4 ,let ,a & ,b be two po9ts on opposite sides ( ! e>?1 b bo? some:]e ne> ! equator4 ,z ,v5us sw+s along 9 its orbit to pass betwe5 ! e>? & ! sun1 x w manife/ly be se5 j t\*+ ! sun's edge soon] f ,a ?an f ,b4 ,! obs]v] at ,a notes ) extreme accuracy ! exact mom5t :5 he sees ,v5us app>5tly t\* ! sun4 ,sev]al m9utes lat]1 ! obs]v] at ,b w see ! same ph5om5on1 & he also notes accurately ! "t ( ! app>5t contact4 ,n[1 s9ce we "k f ord9>y obs]vation ! "t t ,v5us requires to make "o complete circuit ( its orbit1 we c1 by simple proportion1 calculate1 f ! "t t x takes to pass f v to v#a1 ! angul> di/ance betwe5 ! l9es ,a ,s & ,b ,s1 or 9 o!r words ! size ( ! angle at ,s1 :i* is equal to ! p>allactic displacem5t ( ! sun1 z se5 f opposite ends ( ! e>?'s diamet]4 ,kn[+1 to beg9 )1 ! di/ance betwe5 ,a & ,b1 we h ! means ( det]m9+ ! l5g? ( all ! o!r l9es 9 ! triangle1 & h5ce ! di/ance ( ! sun4 ,?is process is kn[n z ,delisle's me?od4 ,!re is ano!r me?od1 call$ ,halley's1 b 9 a brief treatise ( ?is k9d we _c ent] into a description ( x4 ,x suffices to say t bo? dep5d ^u ! same fundam5tal pr9ciples4 ,illu/ration3 ,fig4 #ac4 ,p>allax ( ! ,sun f ,transit ( ,v5us4 7,= description see text47 ,x mu/ be add$1 h[ev]1 t o!r ways ( measur+ ! sun's di/ance ?an are af=d$ by transits ( ,v5us h be5 develop$4 ,"o ( ^! dep5ds ^u obs]vation ( ! a/]oid ,eros1 :i* p]iodically approa*es mu* ne>] to ! e>? ?an ,v5us ev] does4 ,by obs]v+ ! p>allax ( ,eros1 :5 x is ne>e/ ! e>?1 its di/ance c be asc]ta9$1 & t be+ kn[n ! di/ance ( ! sun is imm$iately d$ucible f x1 because1 by ! ?ird law ( ,kepl] 7to be expla9$ lat]71 ! relative di/ances ( all ! planets f ! sun are proportional to _! p]iods ( revolution1 s t if we "k any "o ( ! di/ances 9 miles we c calculate all ! o!rs4 ,x is important "h to /ate ! angul> am\nt ( ! sun's p>allax1 s9ce x is a quantity :i* is cont9ually ref]r$ to 9 books on a/ronomy4 ,accord+ to ! late/ det]m9ation1 bas$ on obs]vations ( ,eros1 ! sol> p>allax is #h4hjg1 :i* corresponds1 9 r\nd numb]s1 to a di/ance ( #ib1hjj1jjj miles4 ,a mean p>allax ( #h4gif is giv5 by ,mr4 ,c4 ,g4 ,abbot1 bas$ on a comb9ation ( results f a numb] ( diff]5t me?ods1 & ?is corresponds to a di/ance ( #ib1icj1jjj miles4 ,to ! a/ronom]1 :o seeks extreme exactness1 ! slighte/ diff]5ce is important4 ,x %\ld be not$ t ! figures #h4hjg or #h4gif repres5t ! p>allactic displacem5t ( ! sun1 z se5 n f ! opposite ends ( ! e>?'s entire diamet]1 b f opposite ends ( its radius1 or semi-diamet]4 ,accord+ly x is equal to half ( ! angle at ,s 9 ,fig4 #ac4 ,x is = conv5i5ce ( calculation t1 9 su* cases1 ! a/ronom] employs ! semi-diamet]1 in/ead ( ! :ole diamet] = 8 base-l9e4 ,! case ( ! />s mu/ next be consid]$1 & n[ we f9d t ! di/ances involv$ are s enorm\s t ! diamet]1 or semi-diamet]1 ( ! e>? is altoge!r too insignificant a quantity to af=d an available base-l9e = ! measurem5t4 ,we %\ld h rema9$ =ev] ignorant ( /> di/ances b = ! effects produc$ by ! e>?'s *ange ( place due to its annual revolution r\nd ! sun4 ,! mean diamet] ( ! e>?'s orbit is ab\t #ahf1jjj1jjj miles1 & we are able to make use ( ?is imm5se di/ance z a base-l9e = asc]ta9+ ! p>allax ( a />4 ,suppose1 = in/ance1 t ! direction ( a /> 9 ! sky is obs]v$ on ! #a/ ( ,janu>y1 & aga9 on ! #a/ ( ,july4 ,9 ! meantime1 ! e>? w h pass$ f "o end ( ! base-l9e j describ$ to ! o!r1 & unless ! /> obs]v$ is extremely remote1 a c>eful comp>ison ( ! two measurem5ts ( direction w reveal a p]ceptible p>allax1 f :i* ! actual di/ance ( ! /> 9 "q c be d$uc$4 ,x is to be obs]v$ t if all ! />s 7 equally di/ant ?is me?od w\ld fail1 because !n !re w\ld be no 8backgr\nd0 aga9/ :i* ! %ift ( place c\ld be obs]v$2 all ( ! />s w\ld %ift toge!r4 ,b1 9 fact1 ! va/ majority ( ! />s are s remote t ev5 a base-l9e ( #ahf1jjj1jjj miles is insuffici5t to produce a measurable %ift 9 _! direction4 ,x is only ! di/ances ( ! ne>] />s :i* we c measure1 & = !m ! multitude ( m remote ones s]ves1 l ! wall ( ! h\se 9 ! exp]im5t ) ! w9d[-b>1 z a backgr\nd on :i* ! %ift ( place c be not$4 ,j z 9 calculations ( ! sun's p>allax ! semi-diamet] ( ! e>? is *os5 = a base-l9e1 s 9 ! case ( ! />s ! semi-diamet] ( ! e>?'s orbit1 am\nt+ to #ic1jjj1jjj miles1 =ms ! basis4 ,measur$ 9 ?is way ! p>allaxes ( ! ne>e/ />s come out 9 t5?s1 or hundr$?s1 ( a second ( arc1 or angul> measurem5t4 ,?us ! p>allax ( ,alpha ,c5tauri1 ! ne>e/ kn[n />1 is ab\t #j4ge1 correspond+ to a di/ance ( ab\t #bf1jjj1jjj1jjj1jjj miles4 ,n[ #j4ge is a quantity inappreciable to ! nak$ eye1 & only to be measur$ ) delicate in/rum5ts1 & yet ?is almo/ invisible %ift ( direction is all t is produc$ by view+ ! ne>e/ /> 9 ! sky f ! opposite ends ( a base-l9e #ic1jjj1jjj miles long6 #c4 ,spectroscopic ,analysis4 ,we h next to deal ) ! con/itution ( ! sun1 or ! nature ( ! sub/ances ( :i* x consi/s1 & = ?is purpose we mu/ fir/ und]/& ! op]ation ( ! spectroscope1 9 _m respects ! mo/ wond]ful in/rum5t t man has inv5t$4 ,x has giv5 bir? to ! 8*emi/ry ( ! sun0 & ! 8*emi/ry ( ! />s10 = by its aid we c be z c]ta9 ( ! nature ( _m ( ! sub/ances ( :i* !y are made z we c\ld be by actually visit+ !m4 ,illu/ration3 ,fig4 #ad4 ,spectrum ,analysis4 ,! r$ is lea/ turn$ by ! prism f its orig9al c\rse & ! violet mo/4 ,if betwe5 ! prism & ! scre5 on :i* ! spectrum falls !re 7 int]pos$ a gas ( any k9d t gas w\ld absorb f ! col\r$ rays pass+ ?r\gh x ! exact waves ( light ) :i* x w\ld itself %9e if x 7 made lum9\s by heat4 ,x w\ld n take out an entire section1 or col\r1 f ! spectrum1 b only a small "p ( "o or m ( ! col\rs1 & ! abs5ce ( ^! p>ts w\ld be indicat$ on ! scre5 by n>r[ black l9es situat$ 9 v>i\s places2 & ^! l9es1 9 numb] & 9 situation1 w\ld diff] ) e diff]5t k9d ( gas t 0 int]pos$4 ,if sev]al k9ds 7 int]pos$ simultane\sly !y w\ld all pick out _! own peculi> rays f ! light1 & ?us ! spectrum w\ld be cross$ by a l>ge numb] ( d>k l9es1 by ! aid ( :i* ! nature ( ! v>i\s gases t produc$ !m c\ld be told4 ,! effect w\ld be ! same if ! gases 7 int]pos$ 9 ! pa? ( ! :ite light be=e x ent]s ! prism2--& ?is1 9 fact1 is :at happ5s :5 ! spectrum ( ! sun1 or a />1 is exam9$--! absorption has already occurr$ at ! surface ( ! lum9\s body be=e ! light comes to ! e>?4 ,fundam5tally1 spectroscopic analysis dep5ds ^u ! pr9ciple ( refraction1 ( :i* we h spok5 9 connection ) ! atmosph]e4 ,al?\gh ! mo/ p[]ful spectroscopes are n[ made on a diff]5t plan1 ! work+ ( ! in/rum5t c be/ be compreh5d$ by consid]+ x 9 ! =m 9 :i* x 0 fir/ inv5t$1 & 9 :i* x is /ill mo/ (t5 us$4 ,9 its simple/ =m ! spectroscope consi/s ( a ?ree-angle prism ( glass1 ?r\gh :i* a ray ( light is s5t f ! sun1 />1 or o!r lum9\s object to be exam9$4 ,glass1 l air or wat]1 has ! prop]ty ( refract+1 or b5d+1 all rays ( light t ent] x 9 an incl9$ direction4 ,9 pass+ ?r\gh two ( ! opposite-slop+ sides ( a prism1 ! ray is twice b5t1 once on ent]+ & aga9 on leav+1 9 accordance ) ! pr9ciple t we h already m5tion$ 7see ,"p ,,ii1 ,sect4 #d74 ,/ill1 m]ely b5d+ ! ray out ( its orig9al c\rse w\ld h no important result b = ano!r associat$ ph5om5on1 kn[n z disp]sion4 ,to expla9 disp]sion we mu/ recall ! famili> fact t :ite light consi/s ( a numb] ( col\r$ compon5ts :i*1 :5 unit$1 make :ite4 ,x is usual to speak ( ^! prim>y1 or prismatic1 col\rs1 z sev5 9 numb]4 ,^! are r$1 orange1 yell[1 gre51 blue1 indigo1 & violet4 ,physici/s n[ assign a diff]5t li/ ( prim>y col\rs1 b ^!1 be+ g5]ally famili>1 w be/ s]ve our purpose4 ,)\t go+ into an explanation ( ! reasons1 x w suffice to say t ! waves ( light produc+ ^! fundam5tal col\rs are n all equally affect$ by refraction4 ,! r$ is lea/1 & ! violet mo/1 b5t out ( its c\rse 9 pass+ ?r\gh ! prism1 ! o!r col\rs be+ b5t m & m 9 proportion to _! di/ance f ! r$4 ,x foll[s t ! ray1 or beam1 ( light1 :i* 0 :ite :5 x ent]$ ! prism1 becomes divid$ or disp]s$ dur+ its passage into a bru% ( sev5 diff]5t hues4 ,?us ! prism may be said to analyse ! light into its fundam5tal col\rs1 mak+ !m sep>ately visible4 ,?is1 z a sci5tific fact1 dates f ! "t ( ,newton4 ,b ,newton did n dream ( ! fur!r magic t lay 9 ! prism4 ,x 0 notic$ z e>ly z #ahja t1 :5 ! light ( ! sun 0 disp]s$ 9 ! way we h describ$1 n only did ! sev5 prim>y col\rs make _! appe>ance1 b across ! ribbon-l b&1 call$ ! spectrum1 t 0 ?us =m$1 ran a numb] ( ?9 black l9es1 l n>r[ gaps 9 ! b&4 ,! position ( ^! l9es 0 c>efully /udi$ by a ,g]man a/ronom]1 ,fraunh(]1 9 #ahad1 & !y /ill be> ! "n ( ,fraunh(] l9es4 ,b ! full explanation ( !m did n come until #aheh :51 ) _! aid1 ,kirs*(f laid ! f\ndations ( spectrum analysis4 ,?is analysis is bas$ ^u ! fact t ! ,fraunh(] l9es are visual indications ( ! exi/5ce ( c]ta9 sub/ances 9 ! sun4 ,to expla9 ?is we mu/ "k ?ree fundam5tal facts3 #a/3 ,e inc&esc5t body t is ei!r solid or liquid 7or1 if x consi/s ( gases1 is "u high pressure7 %9es ) comp\nd :ite light1 :i*1 :5 disp]s$ by prisms1 gives a cont9u\s col\r$ b&1 or spectrum4 #b;d3 ,e elem5t>y sub/ance :5 9 ! gase\s /ate1 & "u l[ pressure1 if br\ght to inc&esc5ce by heat1 %9es ) light :i*1 :5 disp]s$1 gives a discont9u\s spectrum1 made up ( sep>ate bright l9es2 & ea* diff]5t elem5t possesses its own peculi> spectral l9es1 nev] co9cid+ 9 position ) ! l9es ( any o!r elem5t4 #c;d3 ,if ! light f a body giv+ a cont9u\s spectrum is caus$ to pass ?r\gh a gas :i* is at a l[] temp]ature1 ! gas w absorb precisely ^? light waves1 ( :i* its own spectrum is compos$ & w leave 9 ! spectrum ( ! body a s]ies ( d>k l9es1 or gaps1 ^: numb] & position indicate ! nature ( ! gas ^: absorptive action has produc$ !m4 ,n[1 to apply ^! pr9ciples to ! sun we h only to rememb] t x is a globe ( gase\s sub/ances1 :i* are "u great pressure1 ow+ to ! imm5se =ce ( ! sun's gravitation4 ,consequ5tly x gives a cont9u\s spectrum4 ,b1 at ! same "t1 x is surr\nd$ ) gase\s envelopes1 :i* are n z mu* compress$ z ! int]nal gases are1 & :i* are at a l[] temp]ature because !y come 9 contact ) ! cold ( surr\nd+ space4 ,! light f ! body ( ! sun mu/ necess>ily pass ?r\gh ^! envelopes1 & ea* ( ! gases ( :i* !y consi/ absorbs f ! pass+ sunlight its own peculi> rays1 ) ! result t ! spectrum ( ! sun is se5 cross$ ) a great numb] ( black l9es--! ,fraunh(] l9es4 ,x w be rem>k$ t ! evid5ce :i* ! ,fraunh(] l9es af=d conc]n+ ! composition ( ! sun applies1 /rictly1 only to ! out] portion1 or to ! envelopes ( gase\s matt] t surr\nd ! int]ior globe4 ,b s9ce !re is e reason to believe t ! entire body ( ! sun is 9 a gase\s /ate1 not)/&+ ! int]nal pressure1 & s9ce we see t !re is a cont9ual circulation go+ on betwe5 ! inn] & out] portions1 x is logical to conclude t ess5tially ! same elem5ts exi/ "u v>y+ conditions 9 all p>ts ( ! sun4 ,9 ?is way1 !n1 we h le>n$ ! composition ( ! sun1 & we f9d t x consi/s ( virtually ! same elem5t>y sub/ances f\nd ^u ! e>?1 b exi/+ !re 9 a gase\s or vapor\s /ate4 ,among ! elem5ts :i* h be5 positively id5tifi$ 9 ! sun by means ( _! *>act]i/ic spectral l9es are iron1 calcium1 sodium1 alum9um1 copp]1 z9c1 silv]1 lead1 potassium1 nickel1 t91 silicon1 manganese1 magnesium1 cobalt1 hydrog51 & at lea/ tw5ty o!rs :i* are likewise f\nd ^u ! e>?4 ,"s elem5t>y sub/ances kn[n on ! e>?1 su* z gold & oxyg51 h n yet be5 c]ta9ly f\nd 9 ! sun1 b !re is e reason to believe t !y all exi/ !re1 ?\gh p]haps "u conditions :i* r5d] _! detection difficult or impossible4 ,helium 0 recognis$ z an elem5t 9 ! sun1 by giv+ spectral l9es diff]5t f any kn[n sub/ance1 & x receiv$ its "n 8sun-metal10 long be=e x 0 discov]$ on ! e>?4 ,we h se5 t !re is at lea/ "o elem5t 9 ! sun1 coronium1 :i*1 z f> z we "k1 does n exi/ at all ^u ! e>?1 & x is n improbable t !re may be o!rs :i* h no c\nt]p>ts on ! e>?4 ,! same k9d ( analysis applies to ! />s1 no matt] h[ f> away !y may be1 s long z !y give suffici5t light to =m a spectrum4 ,& 9 ?is way x has be5 f\nd t ! />s diff] some:at f ! sun & f "o ano!r 9 _! composition1 & ?us a classification ( ! />s has be5 made1 & x has be5 possible to draw conclusions conc]n+ _! relative age1 :i* %[ t "s />s are comp>atively y\ng] ?an ! sun1 o!rs old]1 & o!rs s f> advanc$ 9 age1 or evolution1 t !y are draw+ ne> ext9ction4 ,_m d>k bodies also exi/ among ! />s1 :i* appe> to be completely ext+ui%$ suns4 ,x only rema9s to add on ?is subject t1 accord+ to prevail+ !ories1 ! e>? itself 0 once an inc&esc5t body1 %9+ ) its own light1 & at t "t x1 too1 w\ld h yield$ a spectrum %[+ ( :at sub/ances x consi/$4 #d4 ,! ,moon4 ,! e>? is a satellite ( ! sun1 & ! moon is a satellite ( ! e>?4 ,! mean1 or av]age1 di/ance ( ! sun f ! e>? is ab\t #ic1jjj1jjj miles2 ! mean di/ance ( ! moon is a little less ?an #bci1jjj miles4 ,?is di/ance is v>iable to ! ext5t ( ab\t #ca1jjj miles1 ow+ to ! ecc5tricity ( ! moon's orbit ab\t ! e>?4 ,t is1 ! moon is sometimes ne>ly #bec1jjj miles away1 & sometimes only ab\t #bba1fjj4 ,! diamet] ( ! moon is #bafc miles4 ,its bulk is "o-=ty-n9? t ( ! e>?1 b its mass is only "o-eightie?1 because its mean d5sity is only ab\t six-t5?s z great z ! e>?'s4 ,! moon appe>s to travel 9 an orbit r\nd ! e>?1 b 9 fact ! orbit is always concave t[>d ! sun1 & ! di/urb+ attraction ( ! e>?1 z ! two move toge!r r\nd ! sun1 causes x to appe> n[ on "o side & n[ on ! o!r4 ,b we may treat ! moon's orbit z if ! e>? 7 ! true c5tre ( =ce1 ! attraction ( ! sun be+ reg>d$ z ! di/urb+ elem5t4 ,accord+ to a ma!matical !ory1 :i* has be5 l>gely accept$ z probably true1 b into :i* we _c ent] "h 7see ,pr(4 ,george ,d>w9's ,! ,tides1 or ,pr(4 ,r4 ,ball's ,"t & ,tide71 ! moon 0 ?r[n (f f ! e>? _m ages ago1 z a consequ5ce ( tidal 8friction40 ,z x moves r\nd 9 its orbit ! moon keeps ! same face t[>d ! e>?4 ,?is fact is also ascrib$ to tidal influ5ce4 ,illu/ration3 ,spiral ,nebula 9 ,cepheus 7,h ,,iv #gf7 ,photograph$ at ! ,lick ,obs]vatory by ,j4 ,e4 ,keel]1 ) ! ,crossley reflector4 ,exposure f\r h\rs4 ,obs]ve t ! c5tral portion is only ( /ell> magnitude4 ,illu/ration3 ,nebul\s ,gr\ndwork 9 ,taurus ,photograph$ at ! ,y]kes ,obs]vatory by ,e4 ,e4 ,b>n>d ) #aj-;in* ,bruce telescope4 ,exposure six h\rs tw5ty-eight m9utes4 ,pr(4 ,b>n>d has sugge/$ t "s ( ^! d>k lanes 9 ri* regions ( />s are non-lum9\s nebul4 ,app>5tly ! moon has no atmosph]e1 or if x has any x is too r>e to be c]ta9ly detect$4 ,on its surface1 !re is no appe>ance ( wat]4 ,consequ5tly we _c suppose x to be inhabit$1 at lea/ by any =ms ( life famili> to u on ! e>?4 ,b :5 ! moon is view$ ) a telescope l>ge relatively flat areas are se51 :i* "s ?9k may h be5 ! b$s ( seas 9 anci5t times4 ,!y are /ill call$ m>ia1 or 8seas10 & are visible to ! nak$ eye 9 ! =m ( great irregul> dusky regions4 ,ne>ly two-?irds ( ! surface ( ! moon1 z we see x1 consi/s ( bright regions1 :i* are v brok5 & m\nta9\s4 ,mo/ ( ! m\nta9s ( ! moon are r\ghly circul>1 surr\nd+ enorm\s depressions1 :i* look l gigantic pits4 ,= ?is reason !y are call$ lun> volcanoes1 b1 to say no?+ ( _! imm5se size--= _m are fifty or sixty miles across--!y diff] 9 _m ways f ! volcanoes ( ! e>?4 ,x suffices to po9t out t :at resemble volcanic crat]s are n situat$1 z is ! case on ! e>?1 at ! summits ( m\nta9s1 b are va/ s9k-holes1 desc5d+ ?\s&s ( feet bel[ ! g5]al surface ( ! moon4 ,_! real orig9 is unkn[n1 b x is possible t volcanic =ces may h produc$ !m4 7,= a description1 ) photographs1 ( ^! gigantic =mations 9 ! lun> _w1 see ! pres5t au?or's ,! ,moon47 ,9 addition to ! circul> m\nta9s1 or crat]s1 !re are sev]al long & l(ty *a9s ( lun> m\nta9s mu* resembl+ t]re/rial m\nta9 ranges4 ,z to ! abs5ce ( air & wat] f ! moon1 "s h suppos$ t !y once exi/$1 b1 9 ! c\rse ( ages1 h disappe>$1 ei!r by absorption1 p>tly me*anical & p>tly *emical1 into ! int]ior rocks1 or by escap+ into space on acc\nt ( ! slight =ce ( gravity on ! moon1 :i* appe>s to be insuffici5t to enable x to reta91 p]man5tly1 su* volatile gases z oxyg51 hydrog51 & nitrog54 ,?is leads u to consid] ! =ce ( ! moon's attraction at its surface4 ,we h se5 t sph]ical bodies attract z if _! :ole mass 7 collect$ at _! c5tres4 ,we also "k t ! =ce ( attraction v>ies directly z ! mass ( ! attract+ body & inv]sely z ! squ>e ( ! di/ance f its c5tre4 ,n[ ! mass ( ! moon is "o-eightie? t ( ! e>?1 s t1 ^u bodies situat$ at an equal di/ance f ! c5tres ( bo?1 ! moon's attraction w\ld be only "o-eightie? ( ! e>?'s4 ,b ! diamet] ( ! moon is n v mu* m ?an "o qu>t] t ( ! e>?1 & = ! sake ( r\nd numb]s let u call x "o qu>t]4 ,x foll[s t an object on ! surface ( ! moon is f\r times ne>] ! c5tre ( attraction ?an is an object on ! surface ( ! e>?1 & s9ce ! =ce v>ies inv]sely z ! squ>e ( ! di/ance ! moon's attraction ^u bodies on its surface is relatively sixte5 times z great z ! e>?'s4 ,b ! total =ce ( ! e>?'s attraction is eighty times great] ?an ! moon's4 ,9 ord]1 !n1 to f9d ! real relative attraction ( ! moon at its surface we mu/ divide #hj by #af1 ! quoti5t1 #e1 %[+ ! ratio ( ! e>?'s =ce ( attraction at its surface to t ( ! moon at its surface4 ,9 o!r words1 ?is calculation %[s t ! moon draws bodies on its surface ) only "o-fif? ! =ce ) :i* ! same bodies w\ld be drawn on ! e>?'s surface4 ,! weight ( bodies ( equal mass w\ld1 !re=e1 be only "o-fif? z great on ! moon z on ! e>?4 ,b ! real diff]5ce is great] ?an ?is1 = we h us$ r\nd numb]s1 :i* exagg]at$ ! size ( ! e>? z comp>$ ) t ( ! moon4 ,if we employ ! fractional numb]s :i* %[ ! actual ratio ( ! moon's radius 7half-diamet]7 to t ( ! e>?1 we %all f9d t ! weight ( ! same body w\ld be only ab\t "o-six? z great on ! moon z on ! e>?4 ,x has be5 ?\ght t ?is relative lack ( weight on ! moon may acc\nt = ! gigantic proportions assum$ by its crat]s1 s9ce ! same elective =ce w\ld ?r[ volcanic matt] to a mu* great] height & di/ance !re ?an on our planet4 ,! connection ( ! slight =ce ( gravity on ! moon ) its ability to reta9 an atmosph]e is %[n by ! foll[+ consid]ations4 ,x is possible to calculate = any planet ( kn[n mass ! velocity ) :i* a p>ticle w\ld h to move 9 ord] to escape f ! control ( t planet4 ,9 ! case ( ! e>? ?is critical velocity1 z x is call$1 am\nts to ab\t #g miles p] second1 & 9 ! case ( ! moon to only #a miles p] second4 ,n[ ! k9etic !ory ( gases in=ms u t _! molecules are cont9ually fly+ 9 all directions ) velocities v>y+ ) ! nature & ! temp]ature ( ! gas4 ,! maximum velocity ( ! molecules ( oxyg5 is #a4h miles p] second1 ( hydrog5 #g4d miles1 ( nitrog5 #b miles1 ( wat] vap\r #b4e miles4 ,x is evid5t1 !n1 t ! =ce ( ! e>?'s attraction is suffici5t p]man5tly to reta9 all ^! gases except hydrog51 & 9 fact !re is no gase\s hydrog5 9 ! atmosph]e1 t elem5t be+ f\nd on ! e>? only 9 comb9ation ) o!r sub/ances4 ,b oxyg5 & nitrog51 :i* con/itute ! bulk ( ! atmosph]e1 h maximum molecul> velocities mu* less ?an ! critical velocity above describ$4 ,9 ! case ( ! moon1 h[ev]1 ! critical velocity is less ?an ^? ( ! molecules ( oxyg51 nitrog51 & wat] vap\r1 to say no?+ ( hydrog52 !re=e ! moon _c p]man5tly reta9 !m4 ,we say 8p]man5tly10 because !y might be reta9$ = a "t = ! reason t ! molecules ( a gas fly 9 all directions1 & cont9ual collisions occur among !m 9 ! int]ior ( ! gase\s mass1 s t x w\ld be only ^? at ! ext]ior ( ! atmosph]e :i* w\ld escape2 b gradually all t rema9$ free f comb9ation w\ld get away4 ,illu/ration3 ,fig4 #ae4 ,! ,phases ( ! ,moon4 ,z ! moon goes r\nd ! e>? 9 ! direction indicat$ by ! arr[s1 ! sun rema9+ always on ! left-h& side1 x is evid5t t ! illum9at$ half ( ! moon w be turn$ away f ! e>? at new moon1 & t[>d x at full moon1 :ile betwe5 ^! positions m or less w be se5 accord+ to ! direction ( ! moon ) reg>d to ! sun4 ,z ! moon travels r\nd ! e>? x %[s itself 9 diff]5t =ms1 gradually *ang+ f "o into ano!r1 :i* are kn[n z phases4 ,if ! moon %one ) light ( its own its outl9e w\ld always be circul>1 l ! sun's4 ,! app>5t *ange ( =m is due1 fir/1 to its be+ an opaque globe1 reflect+ ! sunlight t falls ^u x1 & necess>ily illum9at$ on only "o side at a "t2 & second1 to ! fact t z x travels r\nd ! e>? ! half illum9at$ by ! sun is sometimes turn$ directly t[>d u1 at o!r times only p>tly t[>d u1 & at /ill o!r times directly away f u4 ,:5 x is 9 t "p ( its orbit :i* passes betwe5 ! sun & ! e>?1 ! moon1 s to speak1 has its back turn$ to u1 ! illum9at$ side be+1 ( c\rse1 t[>d ! sun4 ,x is !n invisible1 & ?is unse5 phase is ! true 8new moon40 ,x is cu/om>y1 h[ev]1 to give ! "n new moon to ! n>r[1 sickle-%ap$ figure1 :i* x %[s 9 ! we/1 aft] sunset1 a few days aft] ! date ( ! real new moon4 ,! sickle gradually enl>ges into a half circle z ! moon passes away f ! sun1 & ! half circle phase1 :i* occurs :5 ! moon arrives at a position 9 ! sky at "r angles to ! direction ( ! sun1 is call$ fir/ qu>t]4 ,aft] fir/ qu>t] ! moon beg9s to move r\nd beh9d ! e>?1 ) respect to ! sun1 & :5 x has arriv$ j beh9d ! e>?1 its :ole illum9at$ face is turn$ t[>d ! e>?1 because ! sun1 :i* causes ! illum9ation1 is on t same side4 ,?is phase is call$ full moon4 ,aft]w>d ! moon returns r\nd ! o!r "p ( its orbit t[>d its orig9al position betwe5 ! e>? & ! sun1 & z x does s1 x aga9 assumes1 fir/1 ! =m ( a half circle1 :i* 9 ?is case is call$ ?ird1 or la/1 qu>t]1 !n t ( a sickle1 kn[n z 8old moon10 & f9ally disappe>s once m to become new moon aga94 ,a p]fectly evid5t explanation ( ^! *anges ( =m1 cle>] ?an any description1 c be graphically obta9$ 9 ?is way4 ,take a billi>d ball1 a croquet ball1 or a p]fectly r\nd1 smoo?1 & tightly roll$ ball ( :ite y>n1 &1 plac+ y\rself n too ne> a brightly burn+ lamp & sitt+ on a piano /ool 79 ord] to turn m easily71 hold ! ball up 9 ! light1 & cause x to revolve r\nd y by turn+ ^u ! /ool4 ,z x passes f a position betwe5 y & ! lamp to "o on ! opposite side f ! lamp1 & s on r\nd to its orig9al position1 y w see its illum9at$ half g ?r\gh all ! *anges ( =m exhibit$ by ! moon1 & y w ne$ no fur!r explanation ( ! lun> phases4 ,illu/ration3 ,nebula 9 ,sagitt>ius 7,m #h7 ,photograph$ at ! ,lick ,obs]vatory by ,j4 ,e4 ,keel]1 ) ! ,crossley reflector4 ,exposure ?ree h\rs4 ,note ! clu/]+ ( />s ov] ! :ole field1 ! intricate =ms ( ! nebula1 & p>ticul>ly ! curi\s black spots1 or 8holes10 resembl+ drops ( ink4 ,! ,h>ve/ ,moon & ! ,hunt]'s ,moon1 :i* are popul>ly celebrat$ n only on acc\nt ( _! romantic associations1 b also because 9 "s p>ts ( ! _w !y af=d a useful prolongation ( light aft] sunset1 occur only ne> ! "t ( ! autumnal equ9ox1 & !y are always full moons4 ,! full moon ne>e/ ! date ( ! equ9ox1 ,septemb] #bc;d1 is ! ,h>ve/ ,moon1 & ! full moon next foll[+ is ! ,hunt]'s ,moon4 ,_! peculi>ity is t !y rise1 = sev]al successive ev5+s1 almo/ at ! same h\r1 imm$iately aft] sunset4 ,?is is due to ! fact t at t "t ( ! ye> ! ecliptic1 f :i* ! moon's pa? does n v widely dep>t1 is1 9 high latitudes1 ne>ly p>allel ) ! horizon4 ,! full moon 9 w9t] runs high] 9 ! sky1 & consequ5tly gives a bright] light1 ?an 9 summ]4 ,! reason is because1 s9ce ! full moon mu/ always be opposite to ! sun1 & s9ce toe sun 9 w9t] runs l[1 be+ s\? ( ! equator1 ! full moon rides proportionally high4 #e4 ,eclipses4 ,we h m5tion$ ! connection ( ! moon ) ! tides1 b !re is ano!r ph5om5on 9 :i* ! moon plays ! mo/ conspicu\s "p--t ( eclipses4 ,!re are two k9ds ( eclipses--sol> & lun>4 ,9 ! =m] x is ! moon t causes ! eclipse1 by hid+ ! sun f view2 & 9 ! latt] x is ! moon t suff]s ! eclipse1 by pass+ ?r\gh ! %ad[ :i* ! e>? ca/s into space on ! side away f ! sun4 ,9 bo? cases1 9 ord] t !re may be an eclipse x is necess>y t ! ?ree bodies1 ! moon1 ! sun1 & ! e>?1 %all be ne>ly on a /raight l9e1 drawn ?r\gh _! c5tres4 ,s9ce ! moon occupies ab\t a mon? 9 go+ r\nd ! e>? !re w\ld be two eclipses 9 e su* p]iod 7"o ( ! sun & ! o!r ( ! moon71 if ! moon's orbit lay exactly 9 ! plane ( ! ecliptic1 or ( ! e>?'s orbit4 ,b1 9 fact1 ! orbit ( ! moon is incl9$ to t plane at an angle ( some?+ ov] #e4 ,ev5 s1 !re w\ld be eclipses e mon? if ! two opposite po9ts1 call$ nodes1 :]e ! moon crosses ! plane ( ! ecliptic1 always lay 9 a direct l9e ) ! e>? & ! sun2 b !y d n lie ?us4 ,if1 !n1 ! moon comes betwe5 ! e>? & ! sun :5 %e is 9 a "p ( h] orbit sev]al degrees above or bel[ ! plane ( ! ecliptic1 x is evid5t t %e w pass ei!r above or bel[ ! /raight l9e jo9+ ! c5tres ( ! e>? & ! sun1 & consequ5tly _c hide ! latt]4 ,b1 s9ce eclipses d occur 9 "s mon?s & d n occur 9 o!rs1 we mu/ conclude t ! situation ( ! nodes *anges1 & su* is ! fact4 ,9 consequ5ce ( ! conflict+ attractions ( ! sun & ! e>?1 ! orbit ( ! moon1 al?\gh1 l t ( ! e>?1 x always reta9s ne>ly ! same %ape & ! same incl9ation1 sw+s r\nd 9 space1 s t ! nodes1 or cross+ po9ts on ! ecliptic1 cont9ually *ange _! position1 revolv+ r\nd ! e>?4 ,?is motion may be comp>$ to t ( ! precession ( ! equ9oxes1 b x is mu* m rapid1 a complete revolution occurr+ 9 a p]iod ( ab\t n9ete5 ye>s4 ,f ?is x foll[s t sometimes ! moon 9 pass+ its nodes w be 9 a l9e ) ! sun1 & sometimes w n4 ,b1 ow+ to ! fact t ! sun & moon are n m]e po9ts1 b on ! contr>y pres5t to our view circul> disks1 ea* ab\t half a degree 9 diamet]1 an eclipse may occur ev5 if ! moon is n 9 an exact l9e ) ! c5tres ( ! sun & ! e>?4 ,! edge ( ! moon w ov]lap ! sun1 & !re w be a p>tial eclipse1 if ! c5tres ( ! two bodies are )9 "o degree ap>t4 ,n[1 ! incl9ation ( ! moon's orbit to ! ecliptic be+ only a little ov] #e1 x is app>5t t 9 approa*+ "o ( its nodes1 along s g5tle a slope1 x w come )9 less ?an a degree ( ! ecliptic :ile /ill q f> f ! node4 ,?us1 eclipses occur = a consid]able "t be=e & aft] ! moon passes a node4 ,! di/ances on ea* side ( ! node1 )9 :i* an eclipse ( ! sun may occur1 are call$ ! sol> ecliptic limits1 & !y am\nt to #ah 9 ei!r direction1 or #cf 9 sum4 ,)9 ^! limits ! sun may be :olly or p>tially eclips$ accord+ z ! moon is ne>] to1 or f>!r f1 ! node4 ,if %e is exactly at1 or v close to1 ! node ! eclipse w be total4 ,sol> eclipses v>y 9 ano!r way4 ,:at w\ld be a total eclipse1 "u o!r circum/ances1 may be only an annul> eclipse if ! moon happ5s to be ne> h] greate/ di/ance f ! e>?4 ,we h describ$ ! v>iations 9 h] di/ance due to ! ecc5tricity ( h] orbit1 & we h said t ! orbit itself sw+s r\nd ! e>? 9 su* a way z to cause ! nodes cont9ually to *ange _! places on ! ecliptic4 ,! motion ( ! orbit also causes ! lun> apsides1 or ! po9ts :]e %e is at h] greate/ & lea/ di/ances f ! e>?1 to *ange _! places1 b _! revolution is opposite 9 direction to t ( ! nodes1 z ! revolution ( ! apsides ( ! e>?'s orbit is opposite to t ( ! equ9oxes4 ,! moon's apsides sometimes move ea/w>d & sometimes we/w>d1 b ^u ! :ole ! ea/w>d motion prevails & ! apsides complete "o revolution 9 t direction once 9 ab\t n9e & "o-half ye>s4 ,9 consequ5ce ( ! comb9$ effects ( ! revolution ( ! nodes & t ( ! apsides1 ! moon is sometimes at h] greate/ di/ance f ! e>? at ! mom5t :5 %e passes c5trally ov] ! sun1 & sometimes at h] lea/ di/ance1 or %e may be at any int]v5+ di/ance4 ,if %e is 9 ! ne>] "p ( h] orbit1 h] disk j cov]s t ( ! sun1 & ! eclipse is total2 if %e is 9 ! f>!r "p 7s9ce ! app>5t size ( bodies dim9i%es ) increase ( di/ance71 h] disk does n entirely cov] ! sun1 & a rim ( ! latt] is visible all ar\nd ! moon4 ,?is is call$ an annul> eclipse1 because ( ! r+ %ape ( ! "p ( ! sun rema9+ visible4 ,! l5g? ( ! %ad[ :i* ! moon ca/s t[>d ! e>? dur+ a sol> eclipse also plays an important "p 9 ^! ph5om5a4 ,?is l5g? v>ies ) ! di/ance f ! sun4 ,s9ce ! moon accompanies ! e>?1 x foll[s t :5 ! latt] is 9 aphelion1 or at its greate/ di/ance f ! sun1 ! moon is also at its greate/ mean di/ance f ! sun1 & ! l5g? ( ! lun> %ad[ may1 9 su* circum/ances1 be z mu* z #bcf1jjj miles4 ,:5 ! e>?1 att5d$ by ! moon1 is 9 p]ihelion1 ! l5g? ( ! moon's %ad[ may be only ab\t #bbh1jjj miles4 ,! av]age l5g? ( ! %ad[ is ab\t #bcb1jjj miles4 ,?is is ne>ly #gjjj miles less ?an ! av]age di/ance ( ! moon f ! e>?1 s x is evid5t t g5]ally ! %ad[ is too %ort to rea* ! e>?1 & x w\ld nev] rea* x1 & !re w\ld nev] be a total eclipse ( ! sun1 b = ! v>y+ di/ance ( ! moon f ! e>?4 ,:5 ! moon is ne>e/ ! e>?1 or 9 p]igee1 its di/ance may be z small z #bba1fjj miles1 & 9 all cases :5 ne> p]igee x is ne> en\gh = ! %ad[ to rea* ! e>?4 ,inasmu*1 z ! moon's %ad[1 ev5 "u ! mo/ fav\rable circum/ances1 is dim9i%$ almo/ to a po9t be=e t\*+ ! e>?1 x h>dly ne$ be said t x c cov] b a v small area on ! e>?'s surface4 ,its greate/ possible diamet] _c exce$ ab\t #afg miles1 b ord9>ily x is mu* small]4 ,if bo? ! e>? & ! moon 7 motionless1 ?is %ad[ w\ld be a r\nd or oblong dot on ! e>?1 its %ape v>y+ accord+ z x fell squ>e or slop+ to ! surface2 b s9ce ! moon is cont9ually advanc+ 9 its orbit1 & ! e>? is cont9ually rotat+ on its axis1 ! %ad[ moves across ! e>?1 9 a g5]al we/ to ea/ direction4 ,b ! precise direction v>ies ) circum/ances1 z does also ! spe$4 ,! latt] c nev] be less ?an ab\t a ?\s& miles p] h\r1 & t1 only 9 ! neighb\rhood ( ! equator4 ,! moon advances ea/w>d ab\t #bajj miles p] h\r1 & ! e>?'s surface turns 9 ! same direction ) a velocity dim9i%+ f ab\t a ?\s& miles an h\r at ! equator to #j at ! poles4 ,x is ! diff]5ce betwe5 ! velocity ( ! e>?'s rotation & t ( ! moon's orbital revolution :i* det]m9es ! spe$ ( ! %ad[4 ,! greate/ "t1 :i* ! %ad[ c occupy 9 pass+ a p>ticul> po9t on ! e>? is only eight m9utes1 b ord9>ily ?is is r$uc$ to "o1 two1 or ?ree m9utes4 ,! true %ad[ only la/s dur+ ! "t t ! moon cov]s ! :ole face ( ! sun1 b be=e & aft] ?is total obscuration ( ! sol> disk ! sun is se5 p>tially cov]$ by ! moon1 & ^! p>tial phases ( ! eclipse may be se5 f places f> aside f ! track :i* ! c5tral %ad[ pursues4 ,x is only dur+ a total eclipse1 & only f po9ts situat$ )9 ! %ad[ track1 t ! sol> corona is visible4 ,9 a lun> eclipse x is ! e>? t is ! int]v5+ body1 & its %ad[ falls ^u ! moon4 ,a sol> eclipse c only occur at ! "t ( new moon1 & a lun> eclipse only at ! "t ( full moon4 ,! %ad[ ( ! e>? is mu* long] & broad] ?an t ( ! moon1 & x nev] fails to rea* ! moon1 s t x is n necess>y "h to consid] its v>y+ l5g?4 ,! wid?1 or diamet]1 ( ! %ad[ at ! av]age di/ance ( ! moon f ! e>? is ab\t #egjj miles4 ,! moon may pass ?r\gh ! c5tre ( ! %ad[1 or to "o side ( ! c5tre1 or m]ely dip into ! edge ( x4 ,:5 x goes deep en\gh into ! %ad[ to be entirely cov]$1 ! eclipse is total2 o!rwise x is p>tial4 ,s9ce 9 a total lun> eclipse ! entire moon is cov]$ by ! %ad[1 x is evid5t t su* an eclipse1 unlike a sol> "o1 may be visible simultane\sly f all p>ts ( ! e>? :i*1 at ! "t1 lie on ! side fac+ ! moon4 ,9 o!r words1 ! e>?'s %ad[ does n make m]ely a n>r[ track across ! face ( ! moon1 b completely buries x4 ,:5 ! moon passes c5trally ?r\gh ! %ad[1 %e may rema9 totally obscur$ = ab\t two h\rs4 ,b ! moon does n completely disappe> at su* times1 because ! refraction ( ! e>?'s atmosph]e b5ds a little sunlight r\nd its edges & ca/s x into ! %ad[4 ,if ! atmosph]e r\nd ! edges ( ! e>? happ5s to be ?ickly *>g$ ) cl\ds1 b little light is ?us refract$ into ! %ad[1 & ! moon appe>s v fa9t1 or almo/ entirely disappe>s4 ,b ?is is r>e1 & ord9>ily ! eclips$ moon %9es ) a pale copp]i% light4 ,! occurr5ce ( a lun> eclipse is gov]n$ by simil> circum/ances to ^? affect+ sol> eclipses4 ,! lun> ecliptic limits1 or ! di/ance on ea* side ( ! node )9 :i* an eclipse may occur1 v>y f #i to #ab 9 ei!r direction4 ,tak+ all ! v>i\s circum/ances into acc\nt1 x is f\nd t !re may be1 ?\gh r>ely1 sev5 eclipses 9 a ye>1 two be+ ( ! moon & five ( ! sun1 & t ! lea/ possible numb] ( eclipses 9 a ye> is two1 9 :i* case bo? w be ( ! sun4 ,tak+ into acc\nt also all ! v>i\s positions :i* ! sun & moon occupy ) reg>d to ! e>?1 x is f\nd t !re exi/s a p]iod ( #ah ye>s1 #aa days1 #h h\rs1 at ! return ( :i* eclipses ( bo? k9ds beg9 to recur aga9 9 ! same ord] t !y occur 9 ! next prec$+ p]iod4 ,?is is call$ ! s>os1 & x 0 kn[n to ! ,*aldeans #bfjj ye>s ago4 #f4 ,! ,planets4 ,we h sev]al times m5tion$ ! fact t1 beside ! e>?1 !re are sev5 o!r pr9cipal planets revolv+ r\nd ! sun1 9 ! same direction z ! e>?1 b at v>i\s di/ances4 ,we %all consid] ea* ( ^! 9 ! ord] ( its di/ance f ! sun4 ,b fir/ x is desirable to expla9 briefly c]ta9 s-call$ 8laws0 :i* gov]n ! motions ( all ! planets4 ,^! are kn[n z ,kepl]'s laws ( planet>y motion1 & are ?ree 9 numb]4 ,! demon/ration ( _! tru? w\ld c>ry u beyond ! scope ( ?is book1 & consequ5tly we %all m]ely /ate !m z !y are recognis$ by a/ronom]s4 #a/ ,law3 ,! orbit ( e planet is an ellipse1 hav+ ! sun situat$ 9 "o ( ! foci4 #b;d ,law3 ,! radius vector ( a planet describes equal areas 9 equal times4 ,by ! radius vector is meant ! /raight l9e jo9+ ! planet to ! sun1 & ! law decl>es t z ! planet moves r\nd ! sun1 ! area ( space swept ov] by ?is l9e 9 any giv5 "t1 say "o day1 is equal to ! area :i* x w sweep ov] 9 any o!r equal l5g? ( "t4 ,if ! orbit 7 a circle x is evid5t at a glance t ! law mu/ be true1 because !n ! sun w\ld be situat$ 9 ! c5tre ( ! circle1 ! l5g? ( ! radius vector1 no matt] :]e ! planet might be 9 ! orbit1 w\ld nev] v>y1 & ! area swept ov] by x 9 "o day w\ld be equal to ! area swept ov] 9 any o!r day1 because all ^! areas w\ld be precisely simil> & equal triangles4 ,b ,kepl] discov]$ t ! same ?+ is true :5 ! orbit is an ellipse1 & :51 9 consequ5ce ( ! ecc5tricity ( ! orbit1 ! planet is sometimes f>!r f ! sun ?an at o!r times4 ,z ! triangul> area swept ov] 9 a giv5 "t increases 9 l5g? ) ! planet's recession f ! sun1 x dim9i%es 9 bread? j en\gh to make up ! diff]5ce :i* w\ld o!rwise exi/ betwe5 ! diff]5t areas4 ,?is law gr[s out ( ! fact t ! =ce ( gravitation v>ies inv]sely ) ! squ>e ( ! di/ance4 #c;d ,law3 ,! squ>es ( ! p]iods 7i4 e4 times ( revolution 9 _! orbits7 ( ! diff]5t planets are proportional to ! cubes ( _! mean 7av]age7 di/ances f ! sun4 ,! mean+ ( ?is w be be/ expla9$ by an example4 ,suppose "o planet1 ^: di/ance we "k1 has a p]iod only "o-eigh? z long z t ( ano!r planet1 ^: di/ance we d n "k4 ,!n ,kepl]'s ?ird law enables u to calculate ! di/ance ( ! second planet4 ,call ! p]iod ( ! fir/ planet #a1 & t ( ! second #h1 & also call ! di/ance ( ! fir/ #a1 s9ce all we really ne$ to "k is ! relative di/ance ( ! second1 f :i* its di/ance 9 miles is readily d$uc$ by comp>ison ) ! di/ance ( ! fir/4 ,!n1 by ! law1 #ab 3 #hb 3 #ac 3 x#c 78x0 repres5t+ ! unkn[n quantity74 ,n[1 ?is is simply a problem 9 proportion :]e ! product ( ! means is equal to ! product ( ! extremes4 ,b #ab #a1 & also #ac #a2 !re=e x#c #hb1 & x 7#hb7 7! cube root ( ! squ>e ( #h71 :i* is #d4 ,?us we see t ! di/ance ( ! second planet mu/ be f\r times t ( ! fir/4 ,?is ?ird law ( ,kepl] is appli$ to asc]ta9 ! di/ances ( newly discov]$ planets1 ^: p]iods are easily asc]ta9$ by simple obs]vation4 ,if we "k ! di/ance ( any "o planet by measurem5t1 we c calculate ! di/ances ( all ! o!rs aft] obs]v+ _! p]iods4 ,! law also works conv]sely1 i4 e4 f ! di/ances ! p]iods c be calculat$4 99 99 99 99 99 ,we n[ take up ! v>i\s planets s+ly4 ,! ne>e/ to ! sun1 z f> z kn[n1 is ,m]cury1 its av]age di/ance be+ only #cf1jjj1jjj miles4 ,b its orbit is s ecc5tric t ! di/ance v>ies f #bh1ejj1jjj miles at p]ihelion to #dc1ejj1jjj at aphelion4 ,9 consequ5ce its spe$ 9 its orbit is v v>iable1 & likewise ! am\nt ( heat & light receiv$ by x f ! sun4 ,on ! av]age x gets m ?an #f times z mu* sol> light & heat z ! e>? gets4 ,b at p]ihelion x gets #b times z mu* z at aphelion1 & ! "t :i* x occupies 9 pass+ f p]ihelion to aphelion is only six weeks1 its entire ye> be+ equal to #hh ( our days4 ,be+ situat$ s mu* ne>] ! sun ?an ! e>? is1 ,m]cury is nev] visible to u except 9 ! morn+ or ! ev5+ sky1 & !n n v f> f ! sun4 ,its diamet] is ab\t #cjjj miles1 b its mass is n c]ta9ly kn[n f lack ( k ( its mean d5sity4 ,?is lack ( k is due to ! fact t ,m]cury has no satellite4 ,:5 a planet has a satellite x is easy to calculate its d5sity f its measur$ diamet] comb9$ ) ! orbital spe$ ( its satellite4 ,c]ta9 consid]ations h l$ "s to believe t ! mean d5sity ( ,m]cury may be v great1 p]haps z great z t ( lead1 or ( ! metal m]cury itself4 ,n kn[+ ! mass1 we _c say exactly :at ! weight ( bodies on ,m]cury is4 ,we are also virtually ignorant ( ! condition ( ! surface ( ?is planet1 ! telescope reveal+ v little detail1 b x is g5]ally ?\ght t x be>s a consid]able resemblance to ! surface ( ! moon4 ,!re is ano!r way 9 :i* ,m]cury is rem>kably l ! moon4 ,! latt]1 z we h se51 always keeps ! same side turn$ t[>d ! e>?1 :i* is ! same ?+ z say+ t x turns once on its axis1 :ile go+ once ar\nd ! e>?4 ,s ,m]cury keeps always ! same side t[>d ! sun1 mak+ "o rotation on its axis 9 ! c\rse ( "o revolution 9 its orbit4 ,consequ5tly1 "o side ( ,m]cury is cont9ually 9 ! sunlight1 :ile ! opposite side is cont9ually buri$ 9 night4 ,!re mu/1 h[ev]1 be regions along ! bord] betwe5 ^! two sides1 :]e ! sun does rise & set once 9 ! c\rse ( "o ( ,m]cury's ye>s4 ,?is arises f ! ecc5tricity ( ! orbit1 & ! consequ5t v>iations 9 ! orbital velocity ( ! planet1 :i* cause n[ a little ( "o edge & n[ a little ( ! o!r edge ( ! d>k hemisph]e to come )9 ! l9e ( sunlight4 7,! same ?+ occurs ) ! moon1 ?\gh to a less degree ow+ to ! small] ecc5tricity ( ! moon's orbit1 :i*1 h[ev]1 is suffici5t to enable u to see at "o "t a %ort di/ance r\nd "o side ( ! moon & at ano!r "t a %ort di/ance r\nd ! opposite side47 ,?is ph5om5on is kn[n z libration4 ,m]cury app>5tly possesses an atmosph]e1 b we "k no?+ c]ta9 conc]n+ its d5sity4 99 99 99 99 99 ,! next planet1 9 ! ord] ( di/ance f ! sun1 is ,v5us1 ^: av]age di/ance is #fg1bjj1jjj miles4 ,! orbit ( ,v5us is rem>kable = its small ecc5tricity1 s t ! diff]5ce betwe5 its greate/ & lea/ di/ances f ! sun is less ?an a million miles4 ,! p]iod1 or ye>1 ( ,v5us is #bbe ( our days4 ,ow+ to h] situation clos] to ! sun1 %e gets ne>ly twice z mu* light & heat z ! e>? gets4 ,9 size ,v5us is rem>kably l ! e>?1 h] diamet] be+ #ggac miles1 :i* diff]s by only #bje miles f ! mean diamet] ( ! e>?4 ,h] axis is ne>ly p]p5dicul> to ! plane ( h] orbit4 ,h] globe is a m p]fect sph]e ?an t ( ! e>?1 be+ v little flatt5$ at ! poles or swoll5 at ! equator4 ,al?\gh ,v5us1 l ,m]cury1 has no satellite1 h] mean d5sity has be5 calculat$ by o!r means1 & is f\nd to be #j4hi t ( ! e>?4 ,f ?is1 9 connection ) h] measur$ diamet]1 x is easy to d$uce h] mass1 & ! =ce ( gravity on h] surface4 ,! latt] comes out at ab\t #j4he t ( ! e>?1 i4 e4 a body weigh+ #ajj p\nds on ! e>? w\ld weigh #he p\nds if remov$ to ,v5us4 ,%e possesses an atmosph]e d5s] & m ext5sive ?an w\ld !oretically h be5 expect$--indicat+1 p]haps1 a diff]5ce ( con/itution4 ,h] atmosph]e has be5 e/imat$ to be twice z d5se z ours1 a great advantage1 x may be rem>k$1 f ! po9t ( view ( aronautics4 ,b ?is d5se & abundant atmosph]e r5d]s ,v5us a v difficult object = ! telescope on acc\nt ( ! brilliance ( its reflection4 ,9 consequ5ce1 we "k b little ( ! surface ( ! planet4 ,"o important result ( ?is is t ! "q rema9s undecid$ :e!r ,v5us rotates on h] axis at a rate closely correspond+ ) t ( ! e>?1 z "s obs]v]s ?9k1 or :e!r1 z o!rs ?9k1 %e1 l ,m]cury1 turns only once on h] axis 9 go+ once r\nd ! sun4 ,! importance ( ! "q 9 its be>+ on ! habitability ( ,v5us is app>5t1 = if %e keeps "o face always sunw>d1 !n on "o side !re is p]petual day & on ! o!r p]petual night4 ,on ! o!r h&1 if %e has days & nights approximately equal 9 l5g? to ^? ( ! e>?1 x may well be ?\ght t %e is habitable by be+s n altoge!r unlike ourselves1 because ! =ce ( gravity on h] surface is n mu* less ?an on ! e>?1 & h] d5se atmosph]e1 fill$ ) cl\ds1 might t5d to %ield h] inhabitants f ! effects ( ! great] am\nt ( heat p\r$ ^u h] by ! sun4 ,z h] orbit is inside t ( ! e>?1 ,v5us1 l ,m]cury1 is only visible ei!r 9 ! ev5+ or 9 ! morn+ sky1 b ow+ to h] great] actual di/ance f ! sun1 h] app>5t di/ance f x 9 ! sky is great] ?an t ( ,m]cury4 ,illu/ration3 ,! ,great ,nebula 9 ,orion ,photograph$ at ! ,lick ,obs]vatory by ,j4 ,e4 ,keel]1 ) ! ,crossley reflector4 ,exposure "o h\r4 ,bo? ( ^! planets1 9 consequ5ce ( pass+ alt]nately betwe5 ! sun & ! e>? & r\nd ! opposite side ( ! sun1 pres5t phases resembl+ ^? ( ! moon4 ,! read] c expla9 ^! to himself by means ( ! exp]im5t1 be=e m5tion$1 ) a billi>d ball & a lamp4 ,9 ?is case let ! obs]v] rema9 seat$ 9 8 *air :ile ano!r p]son c>ries ! ball r\nd ! lamp 9 su* a mann] t x %all alt]nately pass betwe5 ! lamp & ! obs]v] & r\nd ! o!r side ( ! lamp4 ,:5 ,v5us comes ne>ly 9 l9e betwe5 ! e>? & ! sun1 %e becomes an exce$+ly brilliant object 9 ei!r ! ev5+ or ! morn+ sky1 al?\gh at su* times we see1 9 ! =m ( a cresc5t1 only a "p ( t half ( h] surface :i* is illum9at$4 ,h] increase ( brightness at su* times is due to h] great] ne>ness to ! e>?4 ,:5 betwe5 ! e>? & ! sun %e may be only ab\t #bf1jjj1jjj miles away1 :ile :5 %e is on ! o!r side ( ! sun %e may be ov] #afj1jjj1jjj miles away4 ,bo? ,v5us & ,m]cury :5 pass+ exactly betwe5 ! sun & ! e>? are se51 9 ! =m ( small black circles1 mov+ sl[ly across ! sun's disk4 ,^! occurr5ces are call$ transits1 & 9 ! case ( ,v5us h be5 be=e ref]r$ to4 ,!y are m frequ5t ) ,m]cury ?an ) ,v5us1 b ,m]cury's transits are n utilisable = p>allax obs]vations4 ,! late/ transit ( ,v5us occurr$ 9 #ahhb1 & !re w n be ano!r until #bjjd4 ,! late/ transit ( ,m]cury occurr$ 9 #aijg1 & !re w be ano!r 9 #aiad4 99 99 99 99 99 ,! e>? is ! ?ird planet 9 ord] ( di/ance1 & !n comes ,m>s1 ^: av]age di/ance f ! sun is #ada1ejj1jjj miles4 ,! orbit ( ,m>s is s ecc5tric t ! di/ance v>ies betwe5 #adh1jjj1jjj & #ace1jjj1jjj miles4 ,its p]iod or ye> is ab\t #fhg ( our days4 ,9 consequ5ce ( its di/ance1 ,m>s gets1 on ! av]age1 a little less ?an half z mu* light & heat z ! e>? gets4 ,:5 x is on ! same side ( ! sun ) ! e>?1 & ne>ly 9 l9e ) !m1 x is said to be 9 opposition4 ,at su* times x is manife/ly z ne> ! e>? z x c come1 & ?us an opposition ( ,m>s (f]s a good opportunity = ! telescopic /udy ( its surface4 ,^! oppositions occur once 9 ab\t #ghj days1 b !y are n all ( equal importance1 because ! di/ance betwe5 ! two planets is n ! same at diff]5t oppositions4 ,! cause ( ! diff]5ce ( di/ance is ! ecc5tricity ( ! orbit4 ,if an opposition occurs :5 ,m>s is 9 aphelion its di/ance f ! e>? w be ab\t #fa1jjj1jjj miles1 b if ! opposition occurs :5 ,m>s is 9 p]ihelion ! di/ance w be only ab\t #ce1jjj1jjj miles4 ,! av]age di/ance at an opposition is ab\t #dh1ejj1jjj miles4 ,! mo/ fav\rable oppositions always occur 9 ,augu/ or ,septemb]1 & are repeat$ at an int]val ( f fifte5 to sev5te5 ye>s4 ,b at "s ( ! int]v5+ oppositions ! di/ance ( ! planet is n too great to af=d good views ( its surface4 ,! diamet] ( ,m>s is ab\t #dccj miles1 ) a simil> pol> flatt5+ to t ( ! e>?4 ,its d5sity is #j4ga t ( ! e>?1 & ! =ce ( gravity on its surface #j4ch4 ,a body weigh+ #ajj p\nds on ! e>? w\ld weigh #ch p\nds on ,m>s4 ,! evid5ce 9 reg>d to its atmosph]e is conflict+1 b ! probability is t x has an atmosph]e n d5s] ?an t exi/+ on our highe/ m\nta9 peaks4 ,op9ions conc]n+ ! exi/5ce ( wat] vap\r on ,m>s are also conflict+4 ,"o fact t5d+ to %[ t its atmosph]e mu/ be v r>e & cl\dless is t its surface features are v pla9ly disc]nible ) telescopes4 ,illu/ration3 ,fig4 #af4 ,orbits ( ,m>s & ! ,e>?4 ,inspection %[s at once :y ! oppositions ( ,m>s :i* occur 9 ,augu/ & ,septemb] are ! mo/ fav\rable because ,m>s be+ !n ne> ! p]ihelion po9t ( its elongat$ orbit is comp>atively ne> ! e>?1 :ile oppositions :i* occur 9 ,febru>y & ,m>* are v unfav\rable because !n ,m>s is ne> ! aphelion po9t ( its orbit1 & its di/ance f ! e>? is mu* great]4 ,! oppositions occur along ! m fav\rable "p ( ! orbit ab\t two ye>s & two mon?s ap>t4 ,?us ! figure %[s t ! opposition ( ,septemb] #bd1 #aiji 0 foll[$ by "o on ,novemb] #be1 #aiaa4 ,ab\t ea* pole1 z x happ5s to be turn$ e>?w>d1 is to be se5 a r\nd :ite pat* 7suppos$ to be sn[71 & ?is gradually disappe>s z ! summ] advances 9 t hemisph]e ( ! planet--= ,m>s has seasons v closely resembl+ our seasons1 except t !y are ab\t twice z long4 ,! incl9ation ( ! axis ( ,m>s to ! plane ( its orbit is ab\t #bd #ej1 :i* is n v diff]5t f ! incl9ation ( ! e>?'s axis4 ,moreov]1 ,m>s rotates 9 a p]iod ( #bd h\rs1 #cg m941 #bb sec41 s t ! l5g? ( day & night ^u its surface is v ne>ly ! same z ^u ! e>?4 ,! surface ( ! planet is m>k$ by broad irregul> areas ( contra/+ col\r1 or tone1 "s ( !m be+ ( a slightly r$di%1 or yell[i%1 hue1 & o!rs ( a neutral dusky t9t4 ,! g5]al resemblance to a globe ( ! e>?1 ) diff]5tly %ap$ seas & oceans1 is /rik+4 ,on acc\nt ( ! _m lik5esses betwe5 ,m>s & ! e>?1 "s a/ronom]s are dispos$ to ?9k t ,m>s may be a habitable planet4 ,! t]ms 8seas0 & 8cont95ts0 7 =m]ly appli$ to ! contra/$ areas j spok5 (1 b n[ x is believ$ t !re are no l>ge bodies ( wat] on ,m>s4 ,cross+ ! light1 or r$di%-col\r$1 areas !re are sometimes se5 great numb]s ( int]sect+ l9es1 v n>r[ & fa9t1 :i* h receiv$ ! "n ( 8canals40 ,"s speculative m9ds f9d 9 ^! gr\nd = believ+ t !y are ( artificial orig91 & a !ory has be5 built up1 accord+ to :i* ! s-call$ canals are 8irrigat$ b&s10 ! result ( ! lab\rs ( ! inhabitants4 ,! argum5t ( ! advocates ( ?is !ory is put ab\t z foll[s3 ,m>s is evid5tly a ne>ly dri$-up planet1 & mo/ ( ! wat] left ^u x is p]iodically lock$ up 9 ! pol> sn[s4 ,z ^! sn[s melt away 9 ! summ] "t1 n[ 9 "o hemisph]e & n[ 9 ! o!r1 ! wat] ?us =m$ is conduct$ (f t[>d ! tropical & equatorial zones by innum]able canals1 too small to be se5 f ! e>?4 ,! l&s irrigat$ by ^! canals are n>r[ /rips1 ^: situation is det]m9$ by local circum/ances1 & :i* cross "o ano!r 9 all directions4 ,)9 ^! b&s1 :i* enl>ge into r\nd$ 8oases0 :]e _m ( !m int]sect1 vegetation pu%es1 & its col\r causes !m to appe> z d>k l9es & pat*es on ! surface ( ! planet4 ,! fact t ! l9es make _! appe>ance gradually1 aft] ! pol> caps beg9 to disappe>1 is reg>d$ z /rongly corroborative ( ! !ory4 ,9 answ] to ! objection t works s ext5sive z ?is !ory ( irrigation calls = w\ld be practically impossible1 x is repli$ t ! relatively small =ce ( gravity on ,m>s n only imm5sely dim9i%es ! weight ( all bodies !re1 b also r5d]s x possible = animal =ms to atta9 a great] size1 ) correspond+ increase ( muscul> p[]4 ,x is likewise argu$ t ,m>s may h be5 long] inhabit$ ?an ! e>?1 & t its inhabitants may consequ5tly h develop$ a m complete ma/]y ov] ! p[]s ( nature ?an we z yet possess4 ,_m a/ronom]s reject ^! speculations1 & ev5 av] t ! l9es call$ 8canals0 7& x mu/ be admitt$ t _m p[]ful telescopes %[ few or none ( !m7 h no real exi/5ce1 :at is se51 or imag9$ to be se51 be+ due to "s peculi>ity ( ! soil1 rocks1 or atmosph]e4 ,m>s has two small satellites1 revolv+ r\nd x ) great spe$ at close qu>t]s4 ,! m di/ant satellite1 ,deimos1 is #ad1fjj miles f ! c5tre ( ,m>s & goes r\nd x 9 #cj h\rs1 #ah m94 ,! ne>] "o1 ,phobos1 is only #ehjj miles f ! planet's c5tre1 & its p]iod ( revolution is only #g h\rs1 #ci m941 s t x makes m ?an ?ree circuits :ile ! planet is rotat+ once on its axis4 ,bo? ( ! satellites are m9ute 9 size1 probably "u t5 miles 9 diamet]4 99 99 99 99 99 ,beyond ,m>s1 at an av]age di/ance ( ab\t #bdf1jjj1jjj miles f ! sun1 is a sy/em ( little planets call$ a/]oids4 ,m ?an #fjj are n[ kn[n1 & new ones are discov]$ e ye>1 pr9cipally by means ( photography4 ,only f\r ( ^! bodies are ( any consid]able size1 & !y 71 naturally1 ! fir/ to be discov]$4 ,!y are ,c]es1 diamet] #dgg miles2 ,pallas1 #cjd miles2 ,ve/a1 #bci miles2 & ,juno1 #abj miles4 ,_m ( ! o!rs h a diamet] ( only ab\t t51 or ev51 p]haps1 z little z five1 miles4 ,_! orbits are m ecc5tric ?an ^? ( any ( ! l>ge planets1 & "o ( !m1 ,eros1 has a mean di/ance ( #ace1jjj1jjj miles1 & a lea/ di/ance ( only #aje1jjj1jjj1 s t x is ne>] to ! sun ?an ,m>s is4 ,eros may1 "u fav\rable circum/ances1 approa* )9 #ad1jjj1jjj miles ( ! e>?4 ,?is fact1 z already m5tion$1 has be5 tak5 advantage ( = measur+ its di/ance f ! e>?1 f :i* ! di/ance ( ! sun may be calculat$ ) increas$ accuracy4 ,eros & "s o!rs ( ! a/]oids seem to be ( an irregul> or fragm5t>y =m1 & ?is has be5 us$ to support a !ory1 :i* is n1 h[ev]1 g5]ally accept$1 t ! a/]oids are ! result ( an explosion1 by :i* a l>g] planet 0 bl[n to pieces4 99 99 99 99 99 ,six? 9 ord] ( di/ance f ! sun 7c\nt+ ! a/]oids z repres5t+ a s+le body7 is ! greate/ ( all ! planets1 ,jupit]4 ,8 av]age di/ance f ! sun is #dhc1jjj1jjj miles1 b ! ecc5tricity ( 8 orbit causes him to approa* )9 #dgb1ejj1jjj miles at p]ihelion1 & to rec$e to #dic1ejj1jjj miles at aphelion4 ,:5 9 opposition1 ,jupit]'s mean di/ance f ! e>? is #cij1jjj1jjj miles4 ,?is gigantic planet has a mean diamet] ( #hg1chj miles1 b is s flatt5$ at ! poles & bulg$ r\nd ! equator t ! pol> diamet] is only #hd1egj miles1 :ile ! equatorial diamet] is #ij1aij miles1 a diff]5ce ( #efhj miles4 ,?is peculi> =m is d\btless due to ! planet's swift rotation4 ,! axis1 l t ( ,v5us1 is ne>ly p]p5dicul> to ! plane ( ! orbit4 ,he makes a complete turn on 8 axis 9 a mean p]iod ( #i h\rs1 #ee m9utes4 ,! reason = say+ 8a mean p]iod0 w appe> 9 a mom5t4 ,jupit]'s ye> is equal to #aa4hf ( our ye>s1 b x comes into opposition to ! sun1 z se5 f ! e>?1 once 9 e #cii days4 ,! volume ( ,jupit] is ab\t #acjj times t ( ! e>?1 i4e4 x w\ld take #acjj e>?s roll$ into "o to equal ,jupit] 9 size4 ,b its mean d5sity is slightly less ?an "o qu>t] ( ! e>?'s1 s t its mass is only #caf times great] ?an ! e>?'s4 ,! =ce ( gravity on its surface is #b4fd times ! e>?'s4 ,a body weigh+ #ajj p\nds on ! e>? w\ld weigh #bfd p\nds on ,jupit]4 ,x w be obs]v$ t ,jupit]'s mean d5sity is v ne>ly ! same z t ( ! sun1 & we conclude t x _c be a solid1 rigid globe l ! e>?4 ,?is conclusion is made c]ta9 by ! fact t its p]iod ( rotation on its axis is v>iable1 ano!r resemblance to ! sun4 ,! equatorial p>ts g r\nd 9 a %ort] p]iod ?an p>ts situat$ "s di/ance nor? or s\? ( ! equator4 ,x may be suppos$ t !re is a solid nucleus )91 b if s1 no direct evid5ce ( its exi/5ce has be5 f\nd4 ,nev]!less1 al?\gh ,jupit] appe>s to be 9 a cl\d-l /ate1 x does n %9e ) light ( its own1 s t its temp]ature1 :ile no d\bt high] ?an t ( ! e>?1 _c approa* any:]e ne> t ( ! sun4 ,we d n "k ( :at mat]ials ,jupit] is compos$1 = spectroscopic analysis applies especially to bodies :i* %9e ) _! own light4 ,:5 !y %9e only by reflect$ light receiv$ f ! sun1 _! spectra resemble ! regul> sol> spectrum1 except = ! pres5ce ( fa9t b&s due to absorption 9 ! planet's atmosph]e4 ,x may be t !re are no elem5ts ( great atomic d5sity1 su* z iron or lead1 9 ! globe ( ,jupit]4 ,yet 9 ! c\rse ( long ages ! planet may become small] & m cond5s$1 9 consequ5ce ( ! escape ( its int]nal heat4 ,9 ?is way ,jupit] may be reg>d$ z repres5t+ an int]m$iate /age ( evolution betwe5 an altoge!r vapor\s & v hot body l ! sun1 & a cool & solid "o l ! e>?4 ,jupit] pres5ts a magnific5t appe>ance 9 a good telescope4 ,its oblong disk is se5 cross$ 9 an ea/ & we/ direction1 & p>allel to its equator1 by broad1 v>i-col\r$ b&s1 call$ belts4 ,^! frequ5tly *ange 9 =m &1 to "s ext5t1 9 situation1 z well z 9 numb]4 ,b !re are always at lea/ two wide belts1 "o on ea* side ( ! equator4 ,9 #ahgh a v rem>kable feature 0 notic$ j s\? ( ! pr9cipal s\? belt ( ,jupit]1 :i* has become celebrat$ "u ! "n ( ! ,great ,r$ ,spot4 ,9 a few ye>s aft] its discov]y its col\r fad$1 b x /ill rema9s visible1 ) v>y+ degrees ( di/9ctness1 z an oblong m>k+1 ab\t #cj1jjj miles long & #gjjj miles broad4 ,! out] bord] ( ! great s\? belt b5ds away f ! spot1 z if "s =ce ( repulsion act$ betwe5 !m1 or z if ! spot 7 an elevation r\nd :i* ! cl\ds ( ! belt fl[$ l a riv] r\nd a project+ headl&4 ,! nature ( ?is curi\s spot is unkn[n4 ,o!r small] spots1 sometimes :ite1 sometimes dusky1 occasionally make _! appe>ance1 b !y d n exhibit ! durability ( ! ,great ,r$ ,spot4 ,jupit] has eight satellites1 f\r ( :i*1 kn[n s9ce ! "t ( ,galileo1 are conspicu\s objects 9 ! smalle/ telescope4 ,all b "o ( ^! f\r are l>g] ?an our moon1 :ile ! o!r f\r are extremely insignificant 9 size4 ,! f\r pr9cipal satellites are designat$ by ,roman num]als1 ,i1 ,,ii1 ,,iii1 ,,iv1 arrang$ 9 ! ord] ( di/ance f ! planet4 ,!y also h names :i* are seldom us$4 ,satellite ,i 7,io7 has a diamet] ( #bdeb miles1 & revolves 9 a p]iod ( #a day1 #ah h\rs1 #bg m941 #ce4e sec41 at a mean di/ance ( #bfa1jjj miles2 ,,ii 7,europa7 is #bjde miles 9 diamet]1 & revolves 9 #c days1 #ac h\rs1 #ac m941 #db4a sec41 at a mean di/ance ( #dae1jjj miles2 ,,iii 7,ganym$e7 has a diamet] ( #ceeh miles1 a p]iod ( sev5 days1 #c h\rs1 #db m941 #cc4d sec41 & a mean di/ance ( #ffd1jjj miles2 ,,iv 7,calli/o7 is #ccde miles 9 diamet]1 has a p]iod ( #af days1 #af h\rs1 #cb m941 #aa4b sec41 & a mean di/ance ( #a1afg1jjj miles4 ,! object ( giv+ ! p]iods ) extreme accuracy w appe> :5 we speak ( ! use made ( obs]vations ( ,jupit]'s satellites4 ,! fir/ ( ! f\r small satellites1 discov]$ by ,b>n>d 9 #ahib1 is probably less ?an #ajj miles 9 diamet]1 & has a mean di/ance ( #aab1ejj miles1 & a p]iod ( only #aa h\rs1 #eg m941 #bb4f sec4 ,! o!r small satellites are mu* m di/ant ?an any ( ! l>ge ones1 ! late/ to be discov]$1 ! eigh?1 be+ situat$ at a mean di/ance ( ab\t #ae1jjj1jjj miles1 b travell+ 9 an orbit s ecc5tric t ! di/ance ranges betwe5 #aj1jjj1jjj & #bj1jjj1jjj miles4 ,! p]iod is ab\t two & a fif? ye>s4 ,b ! mo/ rem>kable fact is t ?is satellite revolves r\nd ,jupit] f ea/ to we/1 a direction contr>y to t pursu$ by all ! o!rs1 & contr>y to ! direction :i* is almo/ univ]sal among ! rotat+ & revolv+ bodies ( ! sol> sy/em4 ,! l>ge satellites are v int]e/+ objects = ! telescope4 ,:5 !y come betwe5 ! sun & ,jupit] _! r\nd black %ad[s c be pla9ly se5 mov+ across 8 disk1 & :5 !y pass r\nd into 8 %ad[ !y are sudd5ly eclips$1 em]g+ aft] a "t out ( ! o!r side ( ! %ad[4 ,^! ph5om5a are kn[n z transits & eclipses1 & _! times ( occurr5ce are c>efully pr$ict$ 9 ! ,am]ican ,ephem]is & ,nautical ,almanac1 publi%$ at ,wa%+ton = ! b5efit ( a/ronom]s & navigators1 because ^! eclipses c be employ$ 9 comp>+ local "t ) /&>d m]idian "t4 ,!y 7 =m]ly utilis$ to det]m9e ! velocity ( light1 9 ?is way3 ,z ! e>? goes r\nd its orbit inside t ( ,jupit] ! latt] is se5 9 opposition to ! sun at int]vals ( #cii days4 ,:5 x is ?us se5 ! e>? mu/ be betwe5 ! sun & ,jupit]1 & ! di/ance betwe5 ! two planets is ! lea/ possible4 ,b :5 ! e>? has pass$ r\nd to ! o!r side ( ! sun f ,jupit] ?is di/ance becomes ! greate/ possible4 ,! increase ( di/ance betwe5 ! two planets1 z ! e>? goes f ! ne>e/ to ! f>!/ side ( its orbit1 is ab\t #ahf1jjj1jjj miles4 ,n[ x 0 notic$ by ! ,dani% a/ronom]1 ,roem]1 t z ! e>? mov$ f>!r & f>!r f ,jupit] ! times ( occurr5ce ( ! eclipses kept gett+ lat] & lat]1 until :5 ! e>? arriv$ at its greate/ di/ance ! eclipses 7 ab\t #af m9utes beh9d "t4 ,he correctly inf]r$ t ! ret>dation ( ! "t 0 due to ! increase ( ! di/ance1 & t ! #af m9utes by :i* ! eclipses 7 beh9dh& :5 ! di/ance 0 greate/ repres5t$ ! "t tak5 by light to cross ! #ahf1jjj1jjj miles ( space by :i* ! e>? _h increas$ its di/ance f ,jupit]4 ,9 o!r words1 light mu/ travel #ahf1jjj1jjj miles 9 ab\t sixte5 m9utes1 f :i* x 0 easy to calculate its spe$ p] second--:i* we n[ "k to be #ahf1ccj miles4 ,our k ( ! velocity ( light furni%es "o ( ! means ( calculat+ ! di/ance ( ! sun4 ,we come next to ! beautiful planet ,saturn1 ^: mean di/ance f ! sun is #hhf1jjj1jjj miles4 ,! di/ance v>ies betwe5 #iaa1jjj1jjj & #hfa1jjj1jjj miles4 ,saturn's ye> is equal to #bi4df ( our ye>s4 ,x comes into opposition e #cgh days4 ,! mo/ surpris+ feature ( ,saturn is ! sy/em ( imm5se r+s surr\nd+ x above ! equator4 ,! globe ( ! planet is #gf1dgj miles 9 equatorial diamet]1 & #fi1ghj miles 9 pol> diamet]1 a diff]5ce ( #ffij miles1 s t ,saturn is ev5 m compress$ at ! poles & swoll5 at ! equator ?an ,jupit]4 ,! axis ( rotation is incl9$ #bg f a p]p5dicul>4 ,! r+s are ?ree 9 numb]1 v ?9 9 proportion to _! va/ size1 & plac$ "o )9 ano!r 9 ! same plane4 ,! out] diamet] ( ! out] r+1 call$ ,r+ ,a1 is ab\t #afh1jjj miles4 ,its bread? is ab\t #aj1jjj miles4 ,!n comes a gap1 ab\t #afjj miles across1 sep>at+ x f ,r+ ,b1 ! brighte/ ( ! set4 ,?is is ab\t #af1ejj miles broad1 & at its inn] edge x gradually fades out1 bl5d+ ) ,r+ ,c1 :i* is call$ ! crape1 or gauze1 r+1 because x has a dusky appe>ance1 & is s transluc5t t ! globe ( ! planet c be se5 ?r\gh x4 ,?is r+ is ab\t #aj1jjj miles broad1 & its inn] edge comes )9 a di/ance ( betwe5 #ijjj & #aj1jjj miles ( ! surface ( ! planet4 ,r+ ,a app>5tly has a v n>r[ gap runn+ r\nd at ab\t a ?ird ( its bread? f ! out] edge4 ,?is1 kn[n z ,encke's ,division1 is n equally pla9 at all times4 ,occasionally obs]v]s report ! tempor>y appe>ance ( o!r ?9 gaps4 ,! mean d5sity ( ,saturn is less ?an t ( any o!r planet1 be+ b #j4ac t ( ! e>?1 or #j4gb t ( wat]4 ,x foll[s t ?is great planet w\ld float 9 wat]4 ,! weight ( bodies at its surface w\ld be a little less ?an ?ree-qu>t]s ( _! weight on ! surface ( ! e>?4 ,! globe ( ,saturn1 l t ( ,jupit]1 is m>k$ by belts p>allel ) ! equator1 b !y are less def9ite 9 outl9e & less conspicu\s ?an ! belts ( ,jupit]4 ,! equatorial zone (t5 %[s a beautiful pale salmon t9t1 :ile ! regions r\nd ! poles are fa9tly blui%4 ,light spots are occasionally se5 ^u ! planet1 & x appe>s to rotate m rapidly at ! equator ?an 9 ! high] latitudes4 ,!re seems to be e reason to ?9k t ,saturn1 also1 is ( a vapor\s con/itution1 al?\gh x may h a relatively cond5s$ nucleus4 ,b :ile ! globe ( ! planet appe>s to be vapor\s1 ! same is n true ( ! r+s4 ,we h already m5tion$ ! fact t !y are exce$+ly ?9 9 proportion to _! great size & wid?4 ,! ?ickness has n be5 det]m9$ ) exactness1 b x probably does n exce$1 on ! av]age1 "o hundr$ miles4 ,!re appe> to be portions ( ! r+s :i* are ?ick] ?an ! av]age1 z if ! matt] ( :i* !y are compos$ 7 heap$ up !re4 ,?is matt] evid5tly consi/s ( an innum]able multitude ( small bodies4 ,9 o!r words1 ! r+s are compos$ ( sw>ms ( :at may be call$ meteors4 ,t _! composition mu/ be ( ?is nature1 al?\gh ! telescope does n reveal x1 has be5 prov$ 9 two ways3 fir/1 by ma!matical calculation1 :i* %[s t if ! r+s 7 all ( a piece1 :e!r solid or liquid1 !y w\ld be de/roy$ by ! cont5d+ =ces ( attraction to :i* !y are subject2 &1 second1 by spectroscopic obs]vation1 :i* proves1 9 a way t w be %[n :5 we come to deal ) ! />s1 t ! r+s rotate ) velocities proportional to ! di/ances ( _! v>i\s p>ts f ! c5tre ( ! planet4 ,h5ce x is inf]r$ t !y mu/ consi/ ( a va/ numb] ( small bodies or p>ticles4 ,saturn has t5 satellites1 all revolv+ outside ! r+s4 ,! names ( n9e ( ^! 9 ! ord] ( increas+ di/ance are3 ,mimas1 di/ance #aag1jjj miles2 ,enceladus1 di/ance #aeg1jjj miles2 ,te?ys1 di/ance #ahf1jjj miles2 ,dione1 di/ance #bch1jjj miles2 ,rhea1 di/ance #ccb1jjj miles2 ,titan1 di/ance #gga1jjj miles2 ,hyp]ion1 di/ance #icd1jjj miles2 ,japetus1 di/ance #b1bbe1jjj miles2 & ,phbe1 di/ance #h1jjj1jjj miles4 ,! la/1 l ! eigh? satellite ( ,jupit]1 revolves 9 a retrograde direction4 ,only ,titan & ,japetus are conspicu\s objects4 ,! p]iod ( ,mimas is only ab\t #bb h\rs2 t ( ,titan is #ae days1 #bb h\rs1 #da m941 & t ( ,japetus ab\t #gi days1 #h h\rs4 ,b>n>d's measurem5ts indicate = ,titan a diamet] ( #bgbj miles4 ,japetus is probably ab\t two-?irds z great 9 diamet] z ,titan4 ,illu/ration3 ,photographs ( ,m>s ,made at ! ,y]kes ,obs]vatory by ,e4 ,e4 ,b>n>d1 ) ! =ty-in* refractor1 ,septemb] #bh1 #aiji4 ,beyond ,saturn1 9 ! ord] nam$1 are ,uranus & ,neptune4 ,! mean di/ance ( ! =m] f ! sun is #a1ghb1jjj1jjj miles1 & t ( ! latt] #b1gia1ejj1jjj miles4 ,! orbit ( ,uranus is m ecc5tric ?an t ( ,neptune4 ,! diamet] ( ,uranus is ab\t #cb1jjj miles & t ( ,neptune ab\t #ce1jjj miles4 ,! ye> ( ,uranus is equal to #hd ( our ye>s1 & t ( ,neptune to #afd4gh4 ,^! planets are s remote1 & s poorly illum9at$ by ! sun1 t ! telescope reveals v little detail on _! surfaces4 ,_! d5sity is some:at less ?an t ( ,jupit]4 ,uranus has f\r satellites1 ,ariel1 ,umbriel1 ,titania1 & ,ob]on1 situat$ at ! respective di/ances ( #abj1jjj1 #afg1jjj1 #bgc1jjj1 & #cfe1jjj miles4 ,neptune has "o1 nameless1 satellite1 at a di/ance ( #bbe1jjj miles4 ,! mo/ rem>kable ?+ ab\t ^! two planets is t _! axes ( rotation1 z comp>$ ) ^? ( all ! o!r planets1 are tipp$ ov] into a diff]5t plane1 s t !y rotate 9 a retrograde or backw>d direction1 & _! satellites1 9 l mann]1 revolve f ea/ to we/4 ,! axis ( ,uranus is n f> f upright to ! plane ( ! ecliptic1 s t ! motion ( its satellites c>ries !m alt]nately f> nor?w>d & f> s\?w>d ( t plane1 b ! axis ( ,neptune is tipp$ s f> ov] t ! retrograde1 or ea/ to we/1 motion is v pron\nc$4 ,neptune is celebrat$ = hav+ be5 discov]$ by means ( ma!matical calculations1 bas$ on its di/urb+ attraction on ,uranus4 ,^! calculations %[$ :]e x ought to be at a c]ta9 "t1 & :5 telescopes 7 po9t$ at ! indicat$ spot ! planet 0 f\nd4 ,simil> di/urbances ( ! motions ( ,neptune lead "s a/ronom]s to ?9k t !re is ano!r1 yet undiscov]$1 planet /ill m di/ant4 #g4 ,comets4 ,comets are ! mo/ extraord9>y 9 appe>ance ( all cele/ial objects visible to ! nak$ eye4 ,great comets h be5 reg>d$ ) t]ror & sup]/iti\s dread 9 all ages ( ! _w1 :]ev] ignorance ( _! nature has prevail$4 ,!y h be5 tak5 = progno/icators ( w>s1 fam9es1 plagues1 ! dea? ( rul]s1 ! outbreak ( revolutions1 & ! subv]sion ( empires4 ,"o reason = ?is1 aside f _! /range & m5ac+ appe>ance1 is1 no d\bt1 ! r>ity ( v great & conspicu\s comets4 ,x 0 n until ,newton _h demon/rat$ ! law ( gravitation t ! fact began to be recognis$ t comets are controll$ 9 _! motions by ! sun4 ,we n[ "k t !y travel 9 orbits1 frequ5tly1 & p]haps always1 elliptical1 hav+ ! sun 9 "o ( ! foci4 ,comets are habitually divid$ into two classes3 fir/1 p]iodical comets1 mean+ ^? :i* h be5 obs]v$ at m ?an "o return to p]ihelion2 &1 second1 non-p]iodical comets1 mean+ ^? :i* h be5 se5 b once1 b :i*1 nev]!less1 may return to p]ihelion 9 a p]iod s long t a second return has n be5 obs]v$4 ,a bett] division is into comets ( %ort p]iod1 & comets ( long1 or unkn[n1 p]iods4 ,illu/ration3 ,fig4 #ag4 ,ellipse1 ,p>abola1 & ,hyp]bola4 ,! figure %[s graphically :y x is s difficult to tell exactly ! =m ( a comet's orbit4 ,! ?ree k9ds ( curves are ne>ly ( ! same =m ne> ! focus 7! ,sun71 & x is only 9 t "p ( its orbit t ! comet c be se54 ,moreov] a comet is1 at be/1 a mi/y & indef9ite object1 :i* r5d]s x s mu* ! m difficult to obta9 good obs]vations ( its precise position & movem5t4 ,/ill1 _m a/ronom]s are dispos$ to ?9k t ! majority ( comets d n travel 9 elliptical b 9 p>abolic1 & a few 9 hyp]bolic1 orbits4 ,?is calls = a few words ( explanation4 ,ellipses1 p>abolas1 & hyp]bolas are all conic-section curves1 b ! ellipse alone returns into itself1 or =ms a clos$ circuit4 ,9 ea* case ! sun is situat$ at ! focus :]e ! p]ihelion1 or ne>e/ approa*1 ( ! comet occurs1 b only comets travell+ 9 elliptical orbits return aga9 aft] hav+ once be5 se54 ,a comet mov+ 9 a p>abola w\ld g back into ! dep?s ( space ne>ly 9 ! direction f :i* x _h come1 & w\ld nev] be se5 aga92 & if x mov$ 9 a hyp]bola x w\ld g (f t[>d ano!r qu>t] ( ! cele/ial sph]e1 & likewise w\ld nev] return4 ,n[ x is true t ! =ms calculat$ = ! orbits ( ! majority ( comets t h be5 obs]v$ appe> to be p>abolic 7a v few seem to be hyp]bolic71 & if ?is is ! fact su* comets _c be p]man5t memb]s ( ! sol> sy/em1 b mu/ ent] x f f>-(f regions ( space1 & hav+ visit$ ! sun mu/ return to su* regions )\t any t5d5cy to come back aga94 ,9 t case !y may pay simil> visits to o!r suns4 ,b x is q possible t :at appe> to be p>abolic orbits may1 9 reality1 be ellipses ( v great ecc5tricity4 ,! difficulty 9 det]m9+ ! precise %ape ( a comet's orbit arises f ! fact t all ?ree ( ! curves j m5tion$ closely approximate to "o ano!r 9 ! neighb\rhood ( _! common focus1 ! sun1 & x is only 9 t "p ( _! orbits t comets are visible4 ,! :ole "q is yet 9 abeyance1 b1 z we h said1 x seems likely t all comets really move 9 elliptical orbits1 & consequ5tly nev] get entirely beyond ! control ( ! sun's attraction4 ,b 9 all cases ! orbits ( comets are mu* m ecc5tric ?an ^? ( ! l>ge planets4 ,! fam\s comet ( ,halley1 = in/ance1 :i* has ! longe/ p]iod ( any ( ! kn[n p]iodical class1 ab\t sev5ty-five ye>s1 is #c1bic1jjj1jjj miles f ! sun :5 9 aphelion1 & only #ed1ggj1jjj miles :5 9 p]ihelion4 ,comets1 :5 ne> ! sun1 are greatly affect$ by ! di/urb+ attraction ( l>ge planets1 & especially ( ! mo/ massive ( !m all1 ,jupit]4 ,! effect ( ?is di/urbance is to *ange ! =m ( _! orbits1 ) ! n infrequ5t result t ! latt] are alt]$ f app>5t p>abolas into unque/ionable ellipses1 & ?us ! comets conc]n$ are said to be 8captur$10 or made prison]s to ! sun1 by ! influ5ce ( ! di/urb+ planet4 ,ab\t tw5ty small comets are kn[n z 8,jupit]'s ,comet ,family10 because !y appe> to h be5 8captur$0 9 ?is way by him4 ,a few o!rs are believ$ to h be5 simil>ly captur$ by ,saturn1 ,uranus1 & ,neptune4 ,! orbits ( comets diff] f ^? ( ! planets 9 o!r ways beside _! great] ecc5tricity4 ,! planets all move r\nd ! sun f we/ to ea/1 b comets move 9 bo? directions4 ,! orbits ( ! planets1 ) ! exception ( "s ( ! a/]oids1 all lie ne> "o common plane1 b ^? ( comets are incl9$ at all angles to ?is plane1 "s ( !m com+ d[n f ! nor? side ( ! ecliptic & o!rs up f ! s\? side4 ,a comet consi/s ( two di/9ct portions3 fir/1 ! head1 or nucleus2 &1 second1 ! tail4 ,! latt] only makes its appe>ance :5 ! comet is draw+ ne> ! sun1 &1 z a :ole1 x is always direct$ away f ! sun1 b usually m or less curv$ backw>d along ! comet's c\rse1 z if ! head t5d$ to run away f x4 ,! appe>ance ( a comet's tail at once sugge/s t x is produc$ by "s repulsive =ce emanat+ f ! sun4 ,rec5tly !re has be5 a t5d5cy to expla9 ?is on ! pr9ciple ( :at is kn[n z ! pressure ( light4 ,?is dem&s a brief explanation4 ,light is believ$ to be a di/urbance ( ! univ]sal e!r 9 ! =m ( waves :i* proce$ f ! lum9\s body4 ,^! waves possess a c]ta9 me*anical en]gy t5d+ to drive away bodies ^u :i* !y imp+e4 ,! en]gy is relatively slight1 & 9 ord9>y circum/ances produces no p]ceptible effect1 b :5 ! body act$ ^u by ! light is extremely small ! pressure may become s great relatively to gravitation z to prevail ov] ! latt]4 ,to illu/rate ?is1 let u recall two facts--fir/1 t gravitation acts ^u ! mass1 i4e4 all ! p>ticles ( a body ?r\gh\t its entire volume2 &1 second1 t pressure acts only ^u ! ext]ior surface4 ,consequ5tly gravitation is proportional to ! volume1 :ile ! pressure ( light is proportional to ! surface ( ! body act$ ^u4 ,n[ ! mass1 or volume1 ( any body v>ies z ! cube ( its diamet]1 & ! surface only z ! squ>e4 ,if1 !n1 we h two bodies1 "o ( :i* has twice ! diamet] ( ! o!r1 ! mass ( ! second w be eight times less ?an t ( ! fir/1 b ! surface w be only f\r times less4 ,if ! second has only "o-?ird ! diamet] ( ! fir/1 !n its mass w be tw5ty-sev5 times less1 b its surface only n9e times less4 ,?us we see t z we dim9i% ! size ( ! body1 ! mass falls (f m rapidly ?an ! area ( ! surface1 & consequ5tly ! pressure ga9s relatively to ! gravitation4 ,exp]im5t has corroborat$ ! conclusions ( ma!matics on ?is subject1 & has %[n t :5 a p>ticle ( matt] is only ab\t "o "o-hundr$-?\s&? ( an in* 9 diamet] ! pressure ( light ^u x becomes great] ?an ! =ce ( gravitation1 & su* a p>ticle1 situat$ 9 op5 space1 w\ld be driv5 away f ! sun by ! light waves4 ,?is critical size w\ld v>y ) ! d5sity ( ! matt] compos+ ! p>ticle1 b :at we h said w s]ve to convey an idea ( its m9ut5ess4 ,n[ 9 apply+ ?is to comets' tails x is only necess>y to rem>k t !y are compos$ ( ei!r gase\s or du/y p>ticles1 or bo?1 ris+ f ! nucleus1 probably "u ! influ5ce ( ! heat or ! electrical action ( ! sun1 & ^! p>ticles1 be+ bel[ ! critical size1 are driv5 away f ! sun1 & appe> 9 ! =m ( a tail foll[+ ! comet4 ,x may be add$ t ! same pr9ciple has be5 evok$ to expla9 ! corona ( ! sun1 :i* may be compos$ ( cl\ds ( gas or du/ kept 9 susp5sion by ! pressure ( light4 ,! nuclei ( comets conta9 ne>ly _! :ole mass4 ,! actual mass ( no comet is kn[n1 b x c 9 no case be v great4 ,moreov]1 x is probable t ! nucleus ( a comet does n consi/ ( a s+le body1 ei!r solid or liquid1 b is compos$ ( a l>ge numb] ( sep>ate small bodies1 l a flock ( meteors1 cr[d$ toge!r & con/antly imp++ ^u "o ano!r4 ,z ! comet approa*es ! sun ! nucleus becomes viol5tly agitat$1 & !n ! tail beg9s to make its appe>ance4 ,! possibility exi/s ( an enc\nt] betwe5 ! e>? & ! head ( a comet1 b no su* occurr5ce is kn[n4 ,two or ?ree times1 h[ev]1 ! e>? is believ$ to h gone ?r\gh ! tail ( a comet1 ! la/ "t 9 #aiaj1 :5 ,halley's comet pass$ betwe5 ! e>? & ! sun1 b no c]ta9 effects h be5 obs]v$ f su* enc\nt]s4 ,! spectroscope %[s t comets conta9 v>i\s hydroc>bons1 sodium1 nitrog51 magnesium1 & possibly iron1 b we "k1 z yet1 v little ab\t _! composition4 ,! pres5ce ( cyanog5 gas 0 report$ 9 ,halley's comet at its la/ appe>ance4 ,we are /ill m ignorant ( ! orig9 ( comets4 ,we d "k1 h[ev]1 t !y t5d to g to pieces1 especially ^? :i* approa* v close to ! sun4 ,! great comet ( #ahhb1 :i* almo/ graz$ ! sun1 0 aft]w>d se5 retreat+ into space scatt]$ into sev]al p>ts1 ea* provid$ ) a tail4 ,9 at lea/ "o case1 sev]al comets h be5 f\nd travell+ 9 ! same track1 an indication t "o l>ge orig9al comet has be5 sep>at$ into ?ree or f\r small] ones4 ,?is appe>s to be true ( ! comets ( #ahdc1 #ahhj1 & #ahhb1--;& p]haps ! comet ( #aegf %\ld be add$4 ,b ! mo/ rem>kable case ( disruption is t ( ,biela's ,comet1 :i* fir/ divid$ into two p>ts 9 #ahdf & !n app>5tly became scatt]$ into a sw>m ( meteors :i* 0 enc\nt]$ by ! e>? 9 #ahgb1 :5 x pass$ ne> ! old track ( ! comet4 ,?is leads u to our next subject4 #h4 ,meteors4 ,ev]ybody mu/1 at "s "t1 h beheld ! ph5om5on kn[n z a fall+1 or %oot+1 />4 ,a few ( ^! objects c be se5 d>t+ across ! sky on almo/ any cle> night 9 ! c\rse ( an h\r or two ( wat*+4 ,sometimes !y appe> m num]\sly1 & at int]vals !y are se5 9 8%[]s40 ,!y are call$ meteors1 & x is believ$ t !y are m9ute solid bodies1 p]haps av]ag+ b a small fraction ( an ounce 9 weight1 :i* plunge into ! atmosph]e ) velocities v>y+ f tw5ty to ?irty or m miles p] second1 & are set afire & consum$ by ! heat ( friction develop$ by _! ru% ?r\gh ! air4 ,anybody :o has se5 a bullet melt$ by ! heat sudd5ly develop$ :5 x /rikes a /eel t>get has _h a graphic illu/ration ( ! trans=mation ( motion into heat4 ,b if we c\ld make ! bullet move fa/ en\gh x w\ld melt 9 ! air1 ! heat be+ develop$ by ! con/ant friction4 ,! connection ( meteors ) comets is v int]e/+4 ,9 ! ye> #ahcc1 a magnific5t & impos+ display ( meteors1 :i*1 = h\rs1 on ! night betwe5 ! #ac? & #ad? ( ,novemb]1 fill$ ! sky ) fire-balls & flam+ /reaks1 a/oni%$ all behold]s & fill$ _m ) t]ror4 ,x 0 f\nd t ^! meteors travell$ 9 an orbit int]sect+ t ( ! e>? at ! po9t :]e ! latt] arriv$ 9 ! middle ( ,novemb]1 & also t !y _h a p]iod ( revolution ab\t ! sun ( #cc ye>s1 & 7 s f> scatt]$ along _! orbit t !y requir$ ne>ly ?ree ye>s to pass ! po9t ( int]section ) ! orbit ( ! e>?4 ,?us x 0 conclud$ t = ?ree ye>s 9 succession1 9 mid-,novemb]1 !re %\ld be a display ( ! meteors plung+ into ! e>?'s atmosph]e4 ,b only 9 ! ye> :5 ! ?icke/ "p ( ! sw>m 0 enc\nt]$ by ! e>? w\ld ! display be v impos+4 ,^u ?is x 0 pr$ict$ t !re w\ld be a recurr5ce ( ! ph5om5on ( #ahcc 9 ! ye> #ahff4 ,x happ5$ z pr$ict$1 except t ! numb] ( meteors 0 n q s great z be=e4 ,9 ! meantime1 x _h be5 discov]$ t ^! meteors foll[$ 9 ! track ( a comet kn[n z ,temple's ,comet1 & also t c]ta9 o!r meteors1 :i* appe> e ye> 9 consid]able numb]s ab\t ! #aj? ( ,augu/1 foll[$ ! track ( ano!r comet call$ ,tuttle's ,comet4 ,!n 9 #ahgb came ! display1 m5tion$ 9 ! la/ section1 ( meteors :i* 7 evid5tly ! debris ( ! vani%$ comet ( ,biela4 ,! inf]5ce f s _m simil> cases 0 irresi/ible t ! meteors mu/ be fragm5ts ( de/roy$ or p>tially de/roy$ comets4 ,sev]al o!r cases ( id5tity ( orbits betwe5 meteors & comets h be5 discov]$4 ,x has be5 said t ! ,augu/ meteors appe> e ye>4 ,! explanation ( ?is is t !y h1 9 ! c\rse ( _m ages1 be5 scatt]$ ar\nd ! :ole circuit ( _! orbit1 s t ea* ye>1 ab\t ! #aj? ( ,augu/1 :5 ! e>? crosses _! track1 "s ( ! meteors are enc\nt]$4 ,!y are l an endless railroad tra9 travell+ ^u a circul> track4 ,! ,novemb] meteors also appe>1 9 small numb]s1 e ye>1 a fact indicat+ t "s ( !m1 too1 h be5 scatt]$ all ar\nd _! orbit1 al?\gh ! great mass ( !m is /ill conc5trat$ 9 an elongat$ sw>m1 & a notable display c only occur :5 ?is sw>m is at ! cross+ simultane\sly ) ! e>?4 ,^! meteors 7 eag]ly await$ 9 #ahii1 :5 x 0 hop$ t ! spl5did displays ( #ahcc & #ahff might be repeat$1 b1 un=tunately1 9 ! meantime ! planets ,jupit] & ,saturn1 by _! di/urb+ attractions1 _h s alt]$ ! position ( ! pa? ( ! meteors 9 space t ! pr9cipal sw>m miss$ ! connection4 ,!re are _m o!r p]iodical meteor %[]s1 g5]ally less brilliant ?an ^? already m5tion$1 & "s a/ronom]s ?9k t all ( !m _h _! orig9 f comets4 ,x is n kn[n t any meteor f any ( ^! sw>ms has ev] rea*$ ! surface ( ! e>?4 ,! meteors appe> to be s small t !y are entirely burnt up be=e !y c get ?r\gh ! atmosph]e1 :i* ?us acts z a %ield aga9/ ^! little missiles f out] space4 ,b !re is ano!r class ( meteoric bodies1 v>i\sly kn[n z meteorites1 arolites1 uranoli?s1 or bolides1 :i* consi/s ( l>g] masses1 & ^! sometimes fall ^u ! e>?1 aft] a fi]y passage ?r\gh ! air4 ,specim5s ( !m may be se5 9 _m museums4 ,!y are divid$ into two pr9cipal classes1 accord+ to _! composition3 fir/1 /ony meteorites1 :i* are by f> ! mo/ num]\s2 &1 second1 iron meteorites1 :i* consi/ ( almo/ pure iron1 g5]ally alloy$ ) a little nickel4 ,! /ony meteorites1 :i* usually conta9 "s comp\nd ( iron1 consi/ ( a great v>iety ( sub/ances1 includ+ betwe5 tw5ty & ?irty diff]5t *emical elem5ts4 ,al?\gh !y resemble 9 _m ways m9]als ( volcanic orig9 on ! e>?1 !y also possess c]ta9 *>act]i/ics by :i* !y c be recognis$ ev5 :5 !y h n be5 se5 to fall4 ,:5 a meteorite passes ?r\gh ! air x makes a brilliant display ( light1 & frequ5tly bur/s asund]1 ) a trem5d\s noise1 scatt]+ its fragm5ts ab\t4 ,! l>ge/ fragm5t ( a meteorite actually se5 to fall1 weighs ab\t a qu>t] ( a ton4 ,^u /rik+ ! gr\nd ! meteorite sometimes p5etrates to a dep? ( sev]al feet1 & "s h be5 pick$ up :i* 7 yet hot on ! surface1 al?\gh v cold )94 ,x is n kn[n t meteorites h any connection ) comets1 & _! orig9 c only be conjectur$4 ,among ! v>i\s sugge/ions t h be5 made ! foll[+ may be m5tion$3 7#a7 t !y h be5 %ot out ( ! sun--p>ticul>ly ! iron meteorites2 7#b7 t !y 7 ca/ into space by lun> volcanoes :5 ! moon 0 /ill subject to volcanic action2 7#c7 t !y are ! products ( explosion 9 ! />s4 ,b "s a/ronom]s are dispos$ to ?9k t !y orig9at$ 9 a simil> mann] to o!r memb]s ( ! sol> sy/em1 al?\gh x is difficult1 on ?is hypo!sis1 to acc\nt = _! great d5sity4 ,! op9ion t ! iron meteorites h come f ! sun1 or "s o!r />1 is en=c$ by ! fact t !y conta9 hydrog51 c>bon1 & helium1 9 =ms sugge/+ t ^! gases 7 absorb$ :ile ! bodies 7 imm]s$ 9 a hot1 d5se atmosph]e4 ---------------------------------------- -------------------------------- ,,"p ,,iv4 ,,! ,,fix$ ,,/>s4 ---------------------------------------- -------------------------------- ,,"p ,,iv4 ,,! ,,fix$ ,,/>s4 #a4 ,! ,/>s4 ,! />s are di/ant suns1 v>y+ greatly 9 remot5ess1 9 magnitude1 & 9 condition4 ,_m ( !m are mu* small] ?an our sun1 & _m o!rs are z mu* l>g]4 ,!y v>y1 likewise 9 age1 or /ate ( developm5t4 ,"s are relatively "y1 o!rs 9 a middle /age1 & /ill o!rs 9 a condition t may be call$ sol> decrepitude4 ,^! pro(s ( evolution among ! />s1 ! k ( :i* we owe ma9ly to spectroscopic analysis1 s]ve to e/abli% m firmly ! conclusion1 to :i* ! simple aspect ( ! heav5s fir/ leads u1 t ! univ]se is a connect$ sy/em1 gov]n$ ev]y:]e by simil> laws & consi/+ ( l mat]ials4 ,! numb] ( />s visible to ! nak$ eye is ab\t six ?\s&1 b telescopes %[ t5s ( millions4 ,x is cu/om>y to divide ! />s into classes1 call$ magnitudes1 accord+ to _! app>5t brightness4 ,by a sy/em ( photometry1 or light-measurem5t1 !y are gr\p$ into />s ( ! fir/1 second1 ?ird1 etc41 magnitude4 ,) ! nak$ eye no />s fa9t] ?an ! six? magnitude are visible1 b v p[]ful telescopes may %[ !m d[n to ! eighte5? magnitude4 ,ea* magnitude is ab\t two & a half times bright] ?an ! next magnitude bel[ 9 ! scale4 ,a fir/-magnitude /> is ab\t "o hundr$ times bright] ?an "o ( ! six? magnitude4 ,b1 9 reality1 ! v>iation ( brightness is gradual1 & = v accurate e/imates fractions ( a magnitude h to be employ$4 ,!re are ab\t tw5ty fir/-magnitude />s1 b !y are n all ( equal brightness4 ,a m accurate photometry assumes a z]o magnitude1 v ne>ly1 repres5t$ by ! /> ,arcturus1 & makes ! ratio #b4eab4 ,?us a />1 ne>ly repres5t$ by ,aldeb>an or ,altair1 :i* is #b4eab times fa9t] ?an ! z]o magnitude1 is ( ! fir/ magnitude1 & a />1 ne>ly repres5t$ by ! ,nor? ,/>1 :i* is #b4eab times fa9t] ?an ! fir/ magnitude1 is ( ! second magnitude4 ,c\nt+ 9 ! o!r direction1 a />1 l ,sirius1 :i* is bright] ?an ! z]o magnitude1 is said to be ( a negative magnitude4 ,! magnitude ( ,sirius is--#a4f4 ,!re is only "o o!r /> ( negative magnitude1 ,canopus1 ^: magnitude is--#j4i4 ,b = ord9>y purposes "o ne$ n tr\ble himself ) ^! ref9em5ts4 ,illu/ration3 ,s*iap>elli's ,*>t ( ,m>tian 8,canals40 ,! />s are divid$ into five pr9cipal types1 accord+ to _! spectra4 ,^! are3 ,i4 ,:ite />s1 hav+ a blui% t+e1 9 :i* ! spectrum is *>act]is$ by broad d>k b&s1 due app>5tly to an ext5sive atmosph]e ( hydrog51 :ile !re are b few l9es indicat+ ! pres5ce ( metallic vap\rs4 ,ab\t half ! />s ^: spectra h be5 /udi$ belong to ,type ,i4 ,,ii4 ,yell[i%-:ite />s1 resembl+ ! sun 9 hav+ _! spectra cross$ ) a great numb] ( l9es produc$ by metallic vap\rs1 :ile ! hydrog5 l9es are less conspicu\s4 ,^! are (t5 call$ sol> />s1 & !y1 too1 are v num]\s4 ,,iii4 ,orange & slightly r$di% />s1 ^: spectra conta9 mo/ly broad b&s in/ead ( n>r[ l9es1 ! b&s be+ situat$ t[>d ! blue end ( ! spectrum1 :5ce ! prevail+ col\r1 s9ce ! blue light is ?us cut (f4 ,only a few hundr$ ( ^! />s are kn[n1 b !y include mo/ ( ! well-kn[n v>iable />s4 ,,iv4 ,small deep-r$ />s hav+ d>k b&s absorb+ ! light ( ! r$ end ( ! spectrum4 ,less ?an a hundr$ ( ^! />s are kn[n4 ,v4 ,/>s ^: spectra are *>act]is$ by bright in/ead ( d>k l9es1 al?\gh !y also %[ d>k b&s4 ,! bright l9es indicate t ! atmosph]ic vap\rs produc+ !m are at a high] temp]ature ?an ! body ( ! />4 ,/>s ( ?is type are sometimes call$ ,wolf-,rayet />s & !y are few 9 numb]4 ,v>i\s modifications ( ^! ma9 types exi/1 b we _c "h ent] into an acc\nt ( !m4 ,9 a g5]al way1 al?\gh !re are exceptions dep5d+ ^u ! precise nature ( ea* spectrum1 ! :ite />s are ?\ght to be y\ng] ?an ! yell[i% ones1 & ! r$ />s old]4 ,9 speak+ ( ! 8size0 ( ! />s we really mean _! lum9osity1 or ! am\nt ( light radiat$ f !m4 ,:5 a /> is said to be a ?\s& times great] ?an ! sun1 ! mean+ is t ! am\nt ( light t x gives w\ld1 if bo? 7 view$ f ! same di/ance1 be equal to a ?\s& times ! am\nt giv5 by ! sun4 ,we h no direct k ( ! actual size ( ! />s z globes1 because ! mo/ p[]ful telescope is unable to reveal ! real disk ( a />4 ,9 comp>+ ! lum9osity ( a /> ) t ( ! sun its di/ance mu/ be tak5 into acc\nt4 ,mo/ ( ! />s are s f> away t we really "k no?+ ( _! di/ances1 b !re are fifty or m :i* lie )9 a di/ance n too great to enable u to obta9 an approximate idea ( :at x is4 ,! ne>e/ /> 9 ! nor!rn sky is s f> f be+ ! brighte/ t x c b>ely be se5 ) ! nak$ eye4 ,x mu/ be v mu* less lum9\s ?an ! sun4 ,on ! o!r h&1 "s v bright />s lie at a di/ance s imm5se t x c h>dly be e/imat$1 & !y mu/ exce$ ! sun 9 lum9osity hundr$s & ev5 ?\s&s ( times4 ,! "q ( ! di/ance ( ! />s has already be5 treat$ 9 ! section on ,p>allax4 ,9 employ+ our k ( /> di/ances = ! purpose ( comp>+ _! lum9osity ) t ( ! sun1 we mu/ fir/ asc]ta91 z accurately z possible1 ! actual am\nt ( light t ! /> s5ds to ! e>? z comp>$ ) ! actual am\nt ( light t ! sun s5ds4 ,! /> ,arcturus gives to our eyes ab\t "o =ty-billion? z mu* light z ! sun does4 ,kn[+ ?is1 we mu/ rememb] t ! int5sity ( light v>ies1 l gravitation1 inv]sely z ! squ>e ( ! di/ance4 ,?us1 if ! sun 7 twice z f> away z x is1 ! am\nt ( its light receiv$ on ! e>? w\ld be r$uc$ to "o f\r?1 & if its di/ance 7 increas$ ?ree times1 ! am\nt w\ld be r$uc$ to "o n9?4 ,if ! sun 7 #bjj1jjj times z f> away1 its light w\ld be r$uc$ to "o =ty-billion?1 or ! same z t ( ,arcturus4 ,at ?is po9t ! actual di/ance ( ,arcturus ent]s into ! calculation4 ,if t di/ance 7 #bjj1jjj times ! sun's di/ance1 we %\ld h to conclude t ,arcturus 0 exactly equal to ! sun 9 lum9osity1 s9ce ! sun1 if remov$ to ! same di/ance1 w\ld give u ! same am\nt ( light4 ,b1 9 fact1 we f9d t ! di/ance ( ,arcturus1 in/ead ( be+ #bjj1jjj times t ( ! sun1 is ab\t #aj1jjj1jjj times4 ,9 o!r words1 x is fifty times z f> away z ! sun w\ld h to be 9 ord] t x %\ld appe> to our eyes no bright] ?an ,arcturus4 ,f ?is x foll[s t ! real lum9osity ( ,arcturus mu/ be ! squ>e ( #ej1 or #bejj1 times t ( ! sun4 ,9 ! same mann] we f9d t ,sirius1 :i* to ! eye appe>s to be ! brighte/ /> 9 ! sky 7mu* bright] ?an ,arcturus because mu* ne>]71 is ab\t ?irty times z lum9\s z ! sun4 ,_m ( ! />s are *angeable 9 brightness1 & ^? 9 :i* ! *anges occur to a notable ext5t1 & p]iodically1 are kn[n z v>iable />s4 ,x is probable t all ! />s1 includ+ ! sun1 are v>iable to a slight degree4 ,among ! mo/ rem>kable v>iables are ,mira1 or ,omicron ,ceti1 9 ! con/ellation ,cetus1 :i* 9 ! c\rse ( ab\t #cca days rises f ! n9? to ! ?ird magnitude & !n falls back aga9 7! maxima ( brightness are irregul>72 & ,algol1 or ,beta ,p]sei1 9 ! con/ellation ,p]seus1 :i*1 9 a p]iod ( #b days1 #bj h\rs1 #di m9utes1 *anges f ! ?ird to ! second magnitude & back aga94 ,9 ! case ( ,mira ! cause ( ! *anges is believ$ to lie 9 ! /> itself1 & !y may be connect$ ) its gradual ext9ction4 ,! majority ( ! v>iable />s belong to ?is class4 ,z to ,algol1 ! v>iability is app>5tly due to a huge d>k body circl+ close ar\nd ! /> ) great spe$1 & p]iodically produc+ p>tial eclipses ( its light4 ,!re are a few o!r />s ) %ort p]iods ( v>iability :i* belong to ! class ( ,algol4 ,:5 exam9$ ) telescopes _m ( ! />s are f\nd to be d\ble1 triple1 or multiple4 ,(t5 ?is arises simply f ! fact t two or m happ5 to lie 9 ne>ly ! same l9e ( sight f ! e>?1 b 9 _m o!r cases x is f\nd t !re is a real connection1 & t ! />s conc]n$ revolve1 "u ! influ5ce ( _! mutual gravitation1 r\nd a common c5tre ( =ce4 ,:5 two />s are ?us connect$ !y are call$ a b9>y4 ,! p]iods ( revolution range f fifty to sev]al hundr$ ye>s4 ,among ! mo/ celebrat$ b9>y />s are ,alpha ,c5tauri1 9 ! s\!rn hemisph]e1 ! ne>e/ kn[n /> to ! sol> sy/em1 ^: compon5ts revolve 9 a p]iod ( ab\t eighty ye>s2 ,gamma ,virg9is1 9 ! con/ellation ,virgo1 p]iod ab\t "o hundr$ & sev5ty ye>s2 & ,sirius1 p]iod ab\t fifty-?ree ye>s4 ,9 ! case ( ,sirius1 "o ( ! compon5ts1 al?\gh p]haps half z massive z its companion1 is t5 ?\s& times less bright4 ,!re is ano!r class ( b9>y />s1 9 :i* "o ( ! companions is invisible1 its pres5ce be+ indicat$ by ! effects ( its gravitational pull ^u ! o!r4 ,algol may be reg>d$ z an example ( ?is k9d ( /ell> association4 ,b !re are />s ( ?is class1 :]e ! companion causes no eclipses1 ei!r because x is n d>k1 or because x nev] passes ov] ! o!r1 z se5 f ! e>?1 b :]e its exi/5ce is prov$1 9 a v int]e/+ way1 by ! spectroscope4 ,9 ^! />s1 call$ spectroscopic b9>ies1 two bright compon5ts are s close toge!r t no telescope is able to make !m sep>ately visible1 b :5 _! plane ( revolution lies ne>ly 9 our l9e ( sight ! l9es 9 _! comb9$ spectrum are se5 p]iodically split asund]4 ,to und]/& ?is1 we mu/ recall ! pr9ciples und]ly+ spectroscopic analysis & add some?+ to :at 0 said be=e on t subject4 ,light consi/s ( waves 9 ! e!r ( diff]5t l5g?s & mak+ ^u ! eye diff]5t impressions ( col\r accord+ to ! l5g? ( ! waves4 ,! longe/ waves are at ! r$ end ( ! spectrum & ! %orte/ at ! blue1 or violet1 end4 ,b s9ce !y all move onw>d ) ! same spe$1 x is cle> t ! %ort blue waves mu/ fall 9 quick] succession on ! ret9a ( ! eye ?an ! long r$ waves4 ,n[ suppose t ! s\rce ( light f :i* ! waves come is approa*+ v swiftly2 x is easy to see t all ! waves w /rike ! eye ) great] rapidity1 & t ! :ole spectrum w be %ift$ t[>d ! blue1 or %ort-wave1 end4 ,! ,fraunh(] l9es w %>e 9 ?is %ift+ ( position4 ,next suppose t ! s\rce ( ! light is retreat+ f ! eye4 ,! same effect w occur 9 a rev]s$ s5se1 = n[ !re w be a g5]al %ift t[>d ! r$ end ( ! spectrum4 ,a suffici5tly cle> illu/ration1 by analogy1 is furni%$ by ! waves ( s\nd4 ,we "k t l[-pit*$ s\nds are produc$ by long waves1 & high-pit*$ ones by %ort waves2 !n if ! s\rce ( ! s\nd1 su* z a locomotive :i/le1 rapidly approa*es ! e> ! waves are cr[d$ toge!r1 or %ift$ z a :ole t[>d ! %ort end ( ! gamut1 :]eupon ! s\nd rises to a %rill scream4 ,if1 on ! contr>y1 ! s\rce ( s\nd is retreat+1 ! %ift is 9 ! o!r direction1 & ! s\nd drops to a l[] pit*4 ,?is is precisely :at happ5s 9 ! spectrum ( a /> :i* is ei!r approa*+ or rec$+ f ! eye4 ,if x is approa*+1 ! ,fraunh(] l9es are se5 %ift$ out ( _! normal position t[>d ! blue1 & if x is rec$+ !y are %ift$ t[>d ! r$4 ,! am\nt ( %ift+ w dep5d ^u ! spe$ ( ! />'s motion4 ,if t motion is across ! l9e ( sight !re w be no %ift+1 because !n ! s\rce ( light is nei!r approa*+ nor rec$+4 ,n[ take ! case ( a b9>y /> ^: compon5ts are too close to be sep>at$ by a telescope4 ,if !y happ5 to be revolv+ r\nd _! common c5tre 9 a plane ne>ly co9cid+ ) ! l9e ( sight f ! e>?1 "o ( !m mu/ be approa*+ ! eye at ! same "t t ! o!r is rec$+ f x1 & ! consequ5ce is t ! spectral l9es ( ! fir/ w be %ift$ t[>d ! blue1 :ile ^? ( ! second are %ift$ t[>d ! r$4 ,! col\rs ( ! two int]m+l$ spectra bl5d into ea* o!r too gradually to enable ?is effect to be detect$ by _! means1 b ! ,fraunh(] l9es are %>ply def9$1 & 9 !m ! %ift is cle>ly se52 & s9ce !re is a simultane\s %ift+ 9 opposite directions ! l9es appe> split4 ,b :5 ! two />s are 9 t "p ( _! orbit :]e _! common motion is across ! l9e ( sight ! l9es close up aga91 because !n !re is no %ift4 ,?is ph5om5on is beautifully exhibit$ by "o ( ! fir/ spectroscopic b9>ies to be discov]$1 ,beta ,aurig4 ,9 #ahhi1 ,pr(4 ,e4 ,c4 ,pick]+ notic$ t ! spectral l9es ( ?is /> appe>$ split e second night1 f :i* he inf]r$ t x consi/$ ( two />s revolv+ r\nd a common c5tre 9 a p]iod ( f\r days4 ,?is spectroscopic me?od has be5 appli$ to det]m9e ! spe$ ) :i* c]ta9 s+le />s are approa*+ or rec$+ f ! sol> sy/em4 ,x has also s]v$ to %[1 :at we h be=e rem>k$1 t ! inn] p>ts ( ,saturn's r+s travel fa/] ?an ! out] p>ts4 ,moreov]1 x has be5 us$ 9 measur+ ! rate ( ! sun's rotation on its axis1 = x is pla9 t "o edge ( ! sun approa*es u :ile ! opposite edge is rec$+4 ,ev5 ! effect ( ! rotation ( ,jupit] has be5 reveal$ 9 ?is way1 & ! same me?od w probably settle ! "q :e!r ,v5us rotates rapidly1 or keeps ! same face always t[>d ! sun4 ,n only d _m />s revolve 9 orbits ab\t ne>-by companions1 b all ! />s1 )\t exception1 are indep5d5tly 9 motion4 ,!y appe> to be travell+ ?r\gh space 9 _m diff]5t directions1 ea* foll[+ its own *os5 way )\t reg>d to ! o!rs1 & ea* mov+ at its own gait4 ,^! movem5ts ( ! />s are call$ prop] motions4 ,! direction ( ! sun's prop] motion is1 r\ghly speak+1 nor?w>d1 & x travels at ! rate ( twelve or f\rte5 miles p] second1 c>ry+ ! e>? & ! o!r planets along ) x4 ,"s />s h a mu* great] spe$ ?an ! sun1 & "s a less spe$4 ,z we h said1 ^! motions are 9 _m diff]5t directions1 & no attempt to discov] any common law und]ly+ !m has be5 entirely successful1 al?\gh x has be5 f\nd t 9 "s p>ts ( ! sky a c]ta9 numb] ( />s appe> to be travell+ along ne>ly p>allel pa?s1 l flocks ( migrat+ birds4 ,9 rec5t ye>s "s indications h be5 f\nd ( ! possible exi/5ce ( two great g5]al curr5ts ( movem5t1 almo/ directly oppos$ to ea* o!r1 "p ( ! />s foll[+ "o curr5t & "p ! o!r4 ,b no indication has be5 discov]$ ( ! exi/5ce ( any common c5tre ( motion4 ,sev]al relatively ne>-by />s appe> to be mov+ 9 ! same direction z ! sun4 ,/>s t are closely gr\p$ toge!r1 l ! clu/] ( ! ,pleiades1 seem to %>e a common motion ( translation ?r\gh space4 ,we h already rem>k$ t :5 />s are f\nd to be mov+ t[>d or away f ! sun1 spectroscopic obs]vation ( ! %ift+ ( _! l9es gives a means ( calculat+ _! velocity4 ,9 o!r cases1 ! velocity across ! l9e ( sight c be calculat$ if we "k ! di/ance ( ! />s conc]n$4 ,"o int]e/+ result ( ! fact t ! e>? goes along ) ! sun 9 its flight is t ! orbit ( ! e>? _c be a clos$ curve1 b mu/ h ! =m ( a spiral 9 space4 ,9 consequ5ce ( ?is we are cont9ually advanc+1 at ! rate ( at lea/ #djj1jjj1jjj miles p] ye>1 t[>d ! nor!rn qu>t] ( ! sky4 ,! pa? pursu$ by ! sun appe>s to be /raight1 al?\gh x may1 9 fact1 be a curve s l>ge t we are unable 9 ! c\rse ( a lifetime1 or _m lifetimes1 to detect its dep>ture f a direct l9e4 ,at any rate we "k t1 z ! e>? accompanies ! sun1 we are cont9ually mov+ into new regions ( space4 ,x has be5 /at$ t _m millions ( />s are visible ) telescopes--p]haps a hundr$ millions1 or ev5 m4 ,! great majority ( ^! are f\nd 9 a broad irregul> b&1 ext5d+ entirely r\nd ! sky1 & call$ ! ,milky ,way1 or ! ,galaxy4 ,to ! nak$ eye ! ,milky ,way appe>s z a s(tly %9+ baldric encircl+ ! heav5s1 b ! telescope %[s t x consi/s ( multitudes ( fa9t />s1 ^: m9ut5ess is probably ma9ly due to ! imm5sity ( _! di/ance1 al?\gh x may be p>tly a result ( _! relative lack ( actual size1 or lum9osity4 ,9 _m p>ts ( ! ,milky ,way ! />s appe> s cr[d$ t !y pres5t ! appe>ance ( sp>kl+ cl\ds4 ,! photographs ( ^! aggregations ( />s 9 ! ,milky ,way1 made by ,b>n>d1 are m>vell\s beyond description4 ,9 ! ,milky ,way1 & sometimes outside x1 !re exi/ globul> />-clu/]s1 9 :i* ! />s seem s cr[d$ t[>d ! c5tre t x is impossible to sep>ate !m ) a telescope1 & ! effect is t ( a gli/]+ ball made up ( ?\s&s ( silv]y p>ticles1 l a heap ( microscopic !rmomet] bulbs 9 ! sun%9e4 ,a fam\s clu/] ( ?is k9d is f\nd 9 ! con/ellation ,h]cules4 ,! ,milky ,way evid5tly has ! =m ( a va/ wrea?1 made up ( _m int]lac$ bran*es1 "s ( :i* ext5d consid]ably beyond its mean bord]s4 ,)91 ?is />ry wrea? space is relatively empty ( />s1 al?\gh "s ?\s&s d exi/ !re1 ( :i* ! sun is "o4 ,we are at pres5t situat$ n v f> f ! c5tre ( ! op5+ )9 ! r+ or wrea?1 b ! prop] motion ( ! sun is c>ry+ u across ?is comp>atively op5 space1 & 9 ! c\rse ( "t1 if ! direction ( our motion does n *ange1 we %all arrive at a po9t n f> f its nor!rn bord]4 ,! ,milky ,way probably indicates ! g5]al plan on :i* ! visible univ]se is con/ruct$1 or :at has be5 call$ ! ar*itecture ( ! heav5s1 b we /ill "k too little ( ?is plan to be able to say exactly :at x is4 ,! numb] ( />s 9 exi/5ce at any "t v>ies to a slight degree1 = occasionally a /> disappe>s1 or a new "o makes its appe>ance4 ,^!1 h[ev]1 are r>e ph5om5a1 & new />s usually disappe> or fade away aft] a %ort "t1 = :i* reason !y are (t5 call$ tempor>y />s4 ,! greate/ ( ^! ph5om5a ev] beheld 0 ,ty*o ,brahe's />1 :i* sudd5ly bur/ into view 9 ! con/ellation ,cassiopeia 9 #aegb1 & disappe>$ aft] a c\ple ( ye>s1 al?\gh at fir/ x 0 ! brighte/ /> 9 ! heav5s4 ,ano!r tempor>y />1 ne>ly z brilliant1 appe>$ 9 ! con/ellation ,p]seus1 9 #aija1 & ?is1 z x fad$1 gradually turn$ into a nebula1 or a /> surr\nd$ by a nebula4 ,x is g5]ally ?\ght t outbur/s ( ?is k9d are caus$ by ! collision ( two or m massive bodies1 :i* 7 invisible be=e _! disa/r\s enc\nt] 9 space4 ,! heat develop$ by su* a collision w\ld be suffici5t to vaporise !m1 & ?us to produce ! appe>ance ( a new blaz+ />4 ,x is possible t space conta9s an enorm\s numb] ( great obscure bodies1--ext+ui%$ suns1 p]haps--:i* are mov+ 9 all directions z rapidly z ! visible />s4 #b4 ,! ,nebul4 ,^! objects1 :i* get _! "n f _! cl\d-l appe>ance1 are among ! mo/ puzzl+ ph5om5a ( ! heav5s1 al?\gh !y seem to sugge/ a means ( expla9+ ! orig9 ( />s4 ,_m ?\s&s ( nebul are kn[n1 b !re are only two or ?ree bright en\gh to be visible to ! nak$ eye4 ,"o ( ^! is 9 ! 8sword0 ( ! imag9>y giant figure m>k+ ! con/ellation ,orion1 & ano!r is 9 ! con/ellation ,&rom$a4 ,!y look to ! unaid$ eye l mi/y specks1 & require consid]able att5tion to be se5 at all4 ,b 9 telescopes _! appe>ance is m>vell\s4 ,! ,orion nebula is a broad1 irregul> cl\d1 ) _m bright] po9ts1 & a consid]able numb] ( />s int]m+l$ ) x1 :ile ! ,&rom$a nebula has a long sp9dle %ape1 ) a bright] spot 9 ! c5tre4 ,x is cov]$ & surr\nd$ ) multitudes ( fa9t />s4 ,x 0 only aft] a/ronomical photography _h be5 p]fect$ t ! real %apes ( ! nebul 7 cle>ly reveal$4 ,?\s&s ( nebul h be5 discov]$ by photography1 :i* are b>ely if at all visible to ! eye1 ev5 :5 aid$ by p[]ful telescopes4 ,?is arises f ! fact t ! s5sitive photographic plate accumulates ! impression t ! light makes ^u x1 %[+ m & m ! long] x is expos$4 ,plates plac$ 9 ! focus ( telescopes1 arrang$ to utilise specially ! 8photographic rays10 are (t5 expos$ = _m h\rs on end 9 ord] to picture fa9t nebul & fa9t />s1 s t !y reveal ?+s t ! eye1 :i* sees all x c see at a glance1 is unable to p]ceive4 ,nebul are g5]ally divid$ into two classes--! 8:ite0 nebul & ! 8gre50 nebul4 ,! fir/1 ( :i* ! ,&rom$a nebula is a /rik+ example1 give a cont9u\s spectrum )\t d>k l9es1 z if !y consi/$ ei!r ( gas "u high pressure1 or ( some?+ 9 a solid or liquid /ate4 ,! second1 conspicu\sly repres5t$ by ! ,orion nebula1 give a spectrum consi/+ ( a few bright l9es1 *>act]i/ic ( su* gases z hydrog5 & helium1 toge!r ) o!r sub/ances n yet recognis$4 ,b !re is no cont9u\s spectrum l t %[n by ! :ite nebul1 f :i* x is inf]r$ t ! gre5 nebul1 at lea/1 are :olly gase\s 9 _! con/itution4 ,! precise con/itution ( ! :ite nebul rema9s to be det]m9$4 ,x is only 9 relatively rec5t ye>s t ! fact has become kn[n t ! majority ( nebul h a spiral =m4 ,!re is almo/ inv>iably a c5tral cond5s$ mass f :i* great spiral arms w9d away on all sides1 giv+ to _m ( !m ! appe>ance ( sp9n+ p9-:eels1 fl++ (f /reams ( fire & sp>ks on all sides4 ,! spirals look z if !y 7 gase\s1 b along & 9 !m are array$ _m cond5s$ knots1 & frequ5tly curv+ r[s ( fa9t />s are se5 app>5tly 9 cont9uation ( ! nebul\s spirals4 ,! sugge/ion convey$ is t ! />s h be5 =m$ by cond5sation f ! spirals4 ,^! nebul g5]ally give ! spectra ( ! :ite class1 b !re are also sometimes se5 bright l9es due to gl[+ gases4 ,! ,&rom$a nebula is sometimes describ$ z spiral1 b its aspect is r t ( a great c5tral mass surr\nd$ ) imm5se elliptical r+s1 "s ( :i* h brok5 up & are cond5s+ into sep>ate masses4 ,! ,orion nebula is a *aotic cl\d1 fill$ ) p>tial vacancies & ribb$ ) _m curv+1 wave-l =ms4 ,!re are o!r nebul :i* h ! =m ( elliptical r+s1 occasionally ) "o or m />s ne> ! c5tre4 ,a fam\s example ( ?is k9d is f\nd 9 ! con/ellation ,lyra4 ,/ill o!rs h be5 comp>$ 9 %ape to ! planet ,saturn ) its r+s1 & "s are altoge!r biz>re 9 =m1 occasionally look+ l gl[+ tresses float+ among ! />s4 ,! app>5t association ( nebul ) />s l$ to ! s-call$ nebul> hypo!sis1 accord+ to :i* />s are =m$1 z already sugge/$1 by ! cond5sation ( nebul\s matt]4 ,9 ! celebrat$ =m :i* ,laplace gave to ?is hypo!sis1 x 0 conc]n$ specially ) ! orig9 ( our sol> sy/em4 ,he assum$ t ! sun 0 once enorm\sly exp&$1 9 a nebul\s /ate1 or surr\nd$ ) a nebul\s cl\d1 & t z x contract$ r+s 7 left (f ar\nd ! p]iph]y ( ! va/ rotat+ mass4 ,^! r+s subsequ5tly break+ & cond5s+ into globes1 7 suppos$ to h giv5 rise to ! planets4 ,x is /ill believ$ t ! sun & ! o!r />s may h orig9at$ f ! cond5sation ( nebul1 b _m objections h be5 f\nd to ! =m 9 :i* ,laplace put 8 hypo!sis1 & ! discov]y ( ! spiral nebul has l$ to o!r conjectures conc]n+ ! way 9 :i* ! trans=mation is br\ght ab\t4 ,b we h n "h ! space to ent] into ?is discussion1 al?\gh x is ( fasc9at+ int]e/4 ,a ^w m %\ld be said ab\t ! use ( photography 9 a/ronomy4 ,x is h>dly go+ too f> to av] t ! photographic plate has tak5 ! place ( ! human ret9a 9 record+ cele/ial ph5om5a1 especially among ! />s & nebul4 ,n only are ! =ms ( su* objects n[ exclusively record$ by photography1 b ! spectra ( all k9ds ( cele/ial objects--sun1 />s1 nebul1 etc4--are photograph$ & aft]w>d /udi$ at leisure4 ,9 ?is way _m ( ! mo/ important discov]ies ( rec5t ye>s h be5 made1 includ+ ^? ( v>iable />s & new />s4 ,photographic *>ts ( ! heav5s exi/1 & by comp>+ ^! ) o!rs made lat]1 *anges :i* w\ld escape ! eye c be detect$4 ,comets are sometimes1 & new a/]oids almo/ inv>iably1 discov]$ by photography4 ,! *anges 9 ! spectra ( comets & new />s are ?us record$ ) an accuracy t w\ld be o!rwise unatta9able4 ,photographs ( ! moon excel 9 accuracy all t c be done by manual draw+1 & :ile photographs ( ! planets /ill fail to %[ _m ( ! f9e details visible ) telescopes1 cont9ual improvem5ts are be+ made4 ,_m ( ! great telescopes n[ 9 use or 9 c\rse ( con/ruction are int5d$ specially = photographic "w4 #c4 ,! ,con/ellations4 ,! division ( ! />s into con/ellations con/itutes ! uranography or ! 8geography ( ! heav5s40 ,! majority ( ! con/ellations are v anci5t1 & _! precise orig9 is unkn[n1 b ^? :i* are invisible f ! nor!rn hemisph]e h all be5 nam$ s9ce ! great explor+ exp$itions to ! s\? seas4 ,!re are m ?an sixty con/ellations n[ g5]ally recognis$4 ,twelve ( ^! belong to ! zodiac1 & be> ! same names z ! zodiacal signs1 al?\gh ! precession ( ! equ9oxes has drift$ !m out ( _! orig9al relation to ! signs4 ,_m ( ! con/ellations are memorials ( prehi/oric my?s1 & a l>ge numb] are connect$ ) ! /ory ( ! ,argonautic exp$ition & ) o!r fam\s ,greek leg5ds4 ,?us ! con/ellations =m a pictorial scroll ( leg5d>y hi/ory & my?ology1 & possess a deep int]e/ indep5d5t ( ! sci5ce ( a/ronomy4 ,= _! hi/ory & = ! leg5ds connect$ ) !m1 ! read] :o desires a n too detail$ rsum1 may consult ,a/ronomy ) ! ,nak$ ,eye1 & = guidance 9 f9d+ ! con/ellations1 ,a/ronomy ) an ,op]a-glass1 or ,r\nd ! ,ye> ) ! ,/>s4 ,! quicke/ way to le>n ! con/ellations is to engage ! aid ( "s "o :o kn[s !m already1 & c po9t !m out 9 ! sky4 ,! next be/ way is to use /> *>ts1 or a />-f9d] or planisph]e4 ,a consid]able numb] ( ! bright] & m important />s are kn[n by individual names1 su* z ,sirius1 ,canopus1 ,a*]n>1 ,arcturus1 ,vega1 ,rigel1 ,betelgeuse1 ,procyon1 ,spica1 ,aldeb>an1 ,regulus1 ,altair1 & ,fomalhaut4 ,a/ronom]s usually designate ! pr9cipal />s ( ea* con/ellation by ! lett]s ( ! ,greek alphabet1 1 1 1 etc41 ! brighte/ /> 9 ! con/ellation be>+ ! "n ( ! fir/ lett]1 ! next brighte/ t ( ! second lett]1 & s on4 ,! con/ellations are v irregul> 9 outl9e1 & _! bord]s are only fix$ ) suffici5t def9it5ess to avoid ! inclusion ( />s catalogu$ z belong+ to "o1 )9 ! limits ( ano!r4 ,9 all cases ! names come f "s fanci$ resemblance ( ! figures =m$ by ! pr9cipal />s ( ! con/ellation to a man1 woman1 animal1 or o!r object4 ,9 only a few cases are ^! resemblances v /rik+4 ,! mo/ useful con/ellations = ! beg9n] are ^? surr\nd+ ! nor? cele/ial pole1 & we give a little circul> *>t %[+ _! *>act]i/ic />s4 ,! names ( ! mon?s runn+ r\nd ! circle indicate ! times ( ! ye> :5 ^! con/ellations are to be se5 on or ne> ! m]idian 9 ! nor?4 ,turn ! *>t s t ! p>ticul> mon? is at ! bottom1 & suppose y\rself to be fac+ nor?w>d4 ,! h\r :5 ! obs]vation is suppos$ to be made is1 9 e case1 ab\t #i o'clock 9 ! ev5+1 & ! date is ab\t ! fir/ ( ! mon?4 ,! top ( ! *>t repres5ts ! sky a little bel[ ! z5i? 9 ! nor?1 & ! bottom repres5ts ! horizon 9 ! nor?4 ,illu/ration3 ,fig4 #ah4 ,! ,nor? ,circumpol> ,/>s4 ,! app>5t ye>ly revolution ( ! heav5s1 result+ f ! motion ( ! e>? 9 its orbit1 causes ! con/ellations to move we/w>d 9 a circle r\nd ! pole1 at ! rate ( ab\t #cj p] mon?4 ,b ! daily rotation ( ! e>? on its axis causes a simil> we/w>d motion ( ! heav5s1 at ! rate ( ab\t #cj = e two h\rs4 ,f ?is x results t on ! same night1 aft] an int]val ( two h\rs1 y w see ! con/ellations occupy+ ! place t !y w h1 at ! orig9al h\r ( obs]vation1 "o mon? lat]4 ,?us1 if y obs]ve _! positions at #i ,p4,m4 on ! fir/ ( ,janu>y1 & !n turn ! *>t s z to br+ ,febru>y at ! bottom1 y w see ! con/ellations ar\nd ! nor? pole ( ! heav5s plac$ z !y w be at #aa ,p4,m4 on ! fir/ ( ,janu>y4 ,illu/ration3 ,fig4 #ai4 ,key to ,nor? ,circumpol> ,/>s4 ,only ! conspicu\s />s h be5 repres5t$ 9 ! *>t1 j en\gh be+ includ$ to enable ! le>n] to recognise ! con/ellations by _! *>act]i/ic /> gr\ps1 f :i* !y h receiv$ _! names4 ,! *>t ext5ds to a di/ance ( #dj f ! pole1 s t1 = obs]v]s situat$ 9 ! mean latitude ( ! ,unit$ ,/ates1 none ( ! con/ellations repres5t$ ev] desc5ds bel[ ! horizon1 ^? t are at ! bord] ( ! *>t j skimm+ ! horizon :5 !y are bel[ ! pole4 ,on ! key to ! *>t ! ,greek-lett] names ( ! pr9cipal />s h be5 atta*$1 b "s ( !m h o!r names :i* are m picturesque4 ,^! are z foll[s3 ,9 ,ursa ,major 7! ,great ,be>1 :i* includes ! ,great ,dipp]71 is call$ ,dubhe1 ,m]ak1 ,pha$1 ,megrez1 ,alio?1 ,miz>1 & ,b5etna%4 ,! little /> close by ,miz> is ,alcor4 ,9 ,cassiopeia1 is call$ ,s*$>1 ,caph1 & ,ru*b>4 ,9 ,ursa ,m9or1 ! ,little ,be>1 is call$ ,pol>is1 or ! ,nor? ,/>1 & ,ko*ab4 ,9 ,draco1 is call$ ,?uban1 & ,eltan94 ,9 ,cepheus1 is call$ ,ald]am91 & ,alfirk4 ,^! names are ne>ly all ( ,arabic orig94 ,x w be obs]v$ t ,m]ak & ,dubhe are ! fam\s 8,po9t]s10 :i* s]ve to indicate ! position ( ! ,nor? ,/>1 :ile ,?uban is ! 8/> ( ! pyramid10 be=e m5tion$4 ,! nor? cele/ial pole is situat$ almo/ exactly on a /raight l9e drawn f ,miz> ?r\gh ! ,nor? ,/> to ,ru*b>1 & a little m ?an a degree f ! ,nor? ,/> 9 ! direction ( ,ru*b>4 ,?is furni%es a ready means = asc]ta9+ ! position ( ! m]idian4 ,= in/ance1 ab\t ! middle ( ,octob]1 ,miz> is v close to ! m]idian bel[ ! pole1 & ,ru*b> equally close to x above ! pole1 & !n1 s9ce ! ,nor? ,/> is 9 l9e ) ^! two1 x also mu/ be practically on ! m]idian1 & its direction indicates v ne>ly true nor?4 ,! same me?od is applicable :5ev]1 at any o!r "t ( ! ye> or ( ! night1 ,miz> & ,ru*b> are obs]v$ to lie ^u a v]tical l9e1 no matt] :i* is above & :i* bel[4 ,x is also possible to make a v good guess at ! "t ( night by kn[+ ! v>y+ position ( ! l9e jo9+ ^! />s4 ,! /> ,caph is an important l&m>k because x lies almo/ on ! great circle ( ! equ9octial colure1 :i* passes ?r\gh ! v]nal & autumnal equ9oxes4 ,on ! key1 ! location ( ! ,nor? ,pole ( ! ,ecliptic is %[n1 & ! great] "p ( ! circle describ$ by ! nor? cele/ial pole 9 ! p]iod ( #be1hjj ye>s4 ,:ile ! read] :o wi%es to pursue ! /udy ( ! con/ellations 9 detail mu/ be ref]r$ to "s ( ! works be=e m5tion$1 or o!rs ( l "*1 x is possible "h to aid him 9 mak+ a prelim9>y acqua9tance ) o!r con/ellations beside ^? includ$ 9 our little *>t1 by tak+ ea* ( ! mon?s 9 turn1 & describ+ ! con/ellations :i* he w see on or ne> ! m]idian s\? ( ! bord] ( ! *>t at ! same "t t ! pol> con/ellations correspond+ to ! mon? select$ are on or ne> ! m]idian 9 ! nor?4 ,?us1 at #i ,p4,m4 ab\t ! fir/ ( ,janu>y1 ! con/ellation ,p]seus1 ly+ 9 a ri* "p ( ! ,milky ,way1 is ne>ly ov]head & directly s\? ( ! ,nor? ,/>4 ,?is con/ellation is m>k$ by a curv$ r[ ( />s1 ! brighte/ ( :i*1 ( ! second magnitude1 is ,alg5ib1 or ,p]sei4 ,a few degrees s\?-we/ ( ,alg5ib is ! wond]ful v>iable ,algol4 ,ea/ ( ,p]seus is se5 ! v brilliant :ite /> ,capella 9 ! con/ellation ,auriga4 ,?is is "o ( ! brighte/ />s 9 ! sky4 ,almo/ directly s\? ( ,p]seus1 ! eye w be caught by ! glimm]+ clu/] ( ! ,pleiades 9 ! con/ellation ,taurus4 ,a %ort di/ance s\?-ea/ ( ! ,pleiades is ! gr\p ( ! ,hyades 9 ,taurus1 %ap$ l ! lett] ,v1 ) ! beautiful r$di% /> ,aldeb>an 9 ! upp] end ( ! s\!rn bran* ( ! lett]4 ,! ecliptic runs betwe5 ! ,pleiades & ! ,hyades4 ,/ill l[] 9 ! s\? w be se5 a "p ( ! long-w9d+ con/ellation ,eridanus1 ! ,riv] ,po4 ,its />s are n bright b !y appe> 9 significant r[s & /reams4 ,ab\t ! fir/ ( ,febru>y ! con/ellation ,auriga is on ! m]idian n f> f ov]head1 ,capella ly+ t[>d ! we/4 ,directly "u ,auriga1 two r conspicu\s />s m>k ! tips ( ! horns ( ,taurus1 imag9$ z a gigantic bull1 & s\? ( ^!1 ) its c5tre on ! equator1 sc9tillates ! magnific5t con/ellation ,orion1 ! mo/ spl5did 9 all ! sky1 ) two great fir/-magnitude />s1 "o1 9 ! %\ld] ( ! imag9>y giant1 ( an orange hue1 call$ ,betelguese1 & ! o!r 9 ! foot1 ( a blue-:ite radiance1 call$ ,rigel4 ,betwe5 ^! is /ret*$ ! /raight l9e ( ! 8belt10 consi/+ ( ?ree beautiful second-magnitude />s1 ab\t a degree & a half ap>t4 ,_! names1 beg9n+ ) ! we/]n "o1 are ,m9taka1 ,alnilam1 & ,alnitah4 ,directly "u ! belt1 9 ! mid/ ( a %ort r[ ( fa9t />s call$ ! 8sword10 is ! great ,orion nebula4 ,x w be obs]v$ t ! ?ree />s ( ! belt po9t1 ?\gh n exactly1 t[>d ! brighte/ ( all />s1 ,sirius1 9 ! con/ellation ,canis ,major1 ! ,great ,dog1 :i* is se5 advanc+ f ! ea/4 ,"u ,orion is a little con/ellation nam$ ,lepus1 ! ,h>e4 ,! fir/ ( ,m>* ! region ov]head is occupi$ by ! v fa9t con/ellation ,lynx4 ,s\? ( x1 & a/ride ! ecliptic1 appe> ! con/ellations ,gem9i1 ! ,tw9s1 & ,canc]1 ! ,crab4 ,^!1 l ,taurus1 belong to ! zodiac4 ,! ,tw9s are we/w>d f ,canc]1 & are m>k$ by two ne>ly equal />s1 ab\t five degrees ap>t4 ,! m we/]ly & nor!rly "o is ,ca/or & ! o!r is ,pollux4 ,canc] is m>k$ by a small clu/] ( fa9t />s call$ ,prsepe1 ! ,mang] 7also sometimes ! ,beehive74 ,directly s\? ( ! ,tw9s1 is ! bright lone /> ,procyon1 9 ! con/ellation ,canis ,m9or1 ! ,little ,dog4 ,sirius & ! o!r />s ( ,canis ,major1 :i* make a /rik+ figure1 are se5 s\?-we/ ( ,procyon4 ,! fir/ ( ,april ! zodiac con/ellation ,leo is ne> ! m]idian1 recognisable by a sickle-%ap$ figure m>k+ ! head & brea/ ( ! imag9>y ,lion4 ,! bright /> at ! end ( ! h&le ( ! sickle is ,regulus4 ,above ,leo1 betwe5 x & ! ,great ,dipp]1 appe>s a gr\p ( />s belong+ to ! small con/ellation ,leo ,m9or1 ! ,little ,lion4 ,f>!r s\? is a w9d+ ribbon ( />s indicat+ ! con/ellation ,hydra1 ! ,wat] ,s]p5t4 ,its *ief />1 ,alph>d1 ( a slightly r$di% t9t1 is se5 we/ ( ! m]idian & a few degrees s\? ( ! equator4 ,at ! beg9n+ ( ,may1 :5 ! ,great ,dipp] is ne>ly ov]head1 ! small con/ellation ,canes ,v5atici1 ! ,hunt+ ,dogs1 is se5 directly "u ! h&le ( ! ,dipp]1 & s\? ( t a cobwebby spot1 consi/+ ( m9ute />s1 indicates ! position ( ! con/ellation ,coma ,b]5ices1 ,b]5ice's ,hair4 ,/ill f>!r s\?1 :]e ! ecliptic & ! equator cross1 at ! autumnal equ9ox1 is ! l>ge con/ellation ,virgo1 ! ,virg91 also "o ( ! zodiacal b&4 ,its *ief /> ,spica1 a pure :ite gem1 is se5 "s #bj ea/ ( ! m]idian4 ,bel[ & we/w>d f ,virgo1 & s\? ( ! equator1 are ! con/ellations ,crat]1 ! ,cup1 & ,corvus1 ! ,cr[4 ,! />s ( ,hydra cont9ue to run ea/w>d bel[ ^! con/ellations4 ,! we/]nmo/1 ,crat]1 consi/s ( small />s =m+ a rude semicircle op5 t[>d ! ea/1 :ile ,corvus1 :i* possesses bright] />s1 has ! =m ( a quadrilat]al4 ,! fir/ ( ,june ! great gold5 /> ,arcturus1 ^: position may be f\nd by runn+ ! eye along ! curve ( ! h&le ( ! ,great ,dipp]1 & cont9u+ onw>d a di/ance equal to ! :ole l5g? ( ! ,dipp]1 is se5 approa*+ ! m]idian f ! ea/ & high ov]head4 ,?is sup]b /> is ! lead] ( ! con/ellation ,botes1 ! ,be>-,driv]4 ,spica 9 ,virgo is n[ a little we/ ( ! m]idian4 ,! fir/ ( ,july1 :5 ! c5tre ( ,draco is on ! m]idian nor? ( ! z5i?1 ! exquisite circlet ( />s call$ ,corona ,borealis1 ! ,nor!rn ,cr[n1 is ne>ly ov]head4 ,a %ort di/ance nor?-ea/ ( x appe>s a d\ble-quadrilat]al figure1 m>k+ out ! con/ellation ,h]cules1 :ile directly s\? ( ! ,cr[n a crook$ l9e ( />s tr5d+ ea/w>d indicates ! con/ellation ,s]p5s1 ! ,s]p5t4 ,s\?-we/ ( ,s]p5s1 two widely sep>at$ b ne>ly equal />s ( ! second magnitude di/+ui% ! zodiacal con/ellation ,libra1 ! ,balance2 :ile l[] d[n t[>d ! s\?-ea/ appe>s ! brilliant r$ /> ,ant>es1 9 ! con/ellation ,scorpio1 likewise belong+ to ! zodiac4 ,on ! fir/ ( ,augu/ ! head ( ,draco is on ! m]idian ne> ! z5i?1 & s\? ( x is se5 ,h]cules1 t[>d ! we/1 & ! exce$+ly brilliant /> ,vega1 9 ! con/ellation ,lyra1 ! ,lyre1 t[>d ! ea/4 ,vega1 or ,alpha ,lyr1 has few rivals = beauty4 ,its light has a decid$ blui%-:ite tone1 :i* is greatly acc5tuat$ :5 x is view$ ) a telescope4 ,s\? ( ,h]cules two or ?ree r[s ( r l>ge1 widely sep>at$ />s m>k ! con/ellation ,ophiu*us1 ! ,s]p5t-,be>]4 ,?is ext5ds across ! equator4 ,bel[ x1 9 a ri* "p ( ! ,milky ,way1 is ,scorpio1 ^: w9d+ l9e1 beg9n+ ) ,ant>es we/ ( ! m]idian1 t]m9ates a consid]able di/ance ea/ ( ! m]idian 9 a pair ( />s repres5t+ ! uplift$ /+ ( ! imag9>y mon/]4 ,! fir/ ( ,septemb] ! ,milky ,way runs directly ov]head1 & 9 ! mid/ ( x %9es ! l>ge & /rik+ figure call$ ! ,nor!rn ,cross1 9 ! con/ellation ,cygnus1 ! ,swan4 ,! bright /> at ! head ( ! ,cross is nam$ ,d5ib4 ,bel[ ! ,cross & 9 ! ea/]n edge ( ! ,milky ,way is ! con/ellation ,aquila1 ! ,eagle1 m>k$ by a bright />1 ,altair1 ) a small] "o on ea* side & n f> away4 ,l[ 9 ! s\?1 a little we/ ( ! m]idian & p>tly imm]s$ 9 ! brighte/ portion ( ! ,milky ,way1 is ! zodiacal con/ellation ,sagitt>ius1 ! ,ar*]4 ,x is di/+ui%$ by a gr\p ( />s sev]al ( :i* =m ! figure ( ! upturn$ b[l ( a dipp]1 sometimes call$ ! ,milk ,dipp]4 ,ea/ ( ,cygnus & ,aquila a diamond-%ap$ figure m>ks ! small con/ellation ,delph9us1 ! ,dolph94 ,at ! op5+ ( ,octob]1 :5 ,d5ib is ne> ! m]idian1 ! sky directly 9 ! s\? is n v brilliant4 ,l[ d[n1 s\? ( ! equator1 is se5 ! zodiacal con/ellation ,capricornus1 ! ,goat1 ) a noticeable pair ( />s 9 ! head ( ! imag9>y animal4 ,on ! fir/ ( ,novemb]1 :5 ,cassiopeia is approa*+ ! m]idian ov]head1 ! ,great ,squ>e1 9 ! con/ellation ,pegasus1 is on ! m]idian s\? ( ! z5i?1 :ile s\?-we/ ( ,pegasus ! zodiacal con/ellation ,aqu>ius1 ! ,wat]-,be>]1 appe>s on ! ecliptic4 ,a curi\s scrawl+ ,y-%ap$ figure 9 ! upp] "p ( ,aqu>ius s]ves z a m>k to id5tify ! con/ellation4 ,?irty degrees s\? ( ?is %9es ! bright /> ,fomalhaut1 9 ! con/ellation ,piscis ,au/ralis1 ! ,s\!rn ,fi%4 ,! two />s =m+ ! ea/]n side ( ! ,great ,squ>e ( ,pegasus are int]e/+ because1 l ,caph 9 ,cassiopeia1 !y lie close to ! l9e ( ! equ9octial colure4 ,! nor!rn "o is call$ ,alph]atz & ! s\!rn ,gamma ,pegasi4 ,alph]atz is a /> claim$ by two con/ellations1 s9ce x n only m>ks "o corn] ( ! squ>e ( ,pegasus1 b x also s]ves to indicate ! head ( ! maid5 9 ! celebrat$ con/ellation ( ,&rom$a4 ,! fir/ ( ,decemb]1 ,&rom$a is se5 ne>ly ov]head1 s\? ( ,cassiopeia4 ,! con/ellation is m>k$ by a r[ ( ?ree second-magnitude />s1 beg9n+ on ! ea/ ) ,alph]atz & t]m9at+ ne> ,p]seus ) ,almaack4 ,! c5tral /> is nam$ ,mira*4 ,a few degrees nor?-we/ ( ,mira* glimm]s ! great ,&rom$a nebula4 ,bel[ ,&rom$a1 we/ ( ! m]idian1 appe>s ! zodiacal con/ellation ,aries1 ! ,ram1 indicat$ by a gr\p ( ?ree />s1 =m+ a triangle1 ! brighte/ ( :i* is call$ ,hamal4 ,s\?-we/]ly f ,aries is ! zodiacal con/ellation ,pisces1 ! ,fi%es1 :i* consi/s ma9ly ( fa9t />s arrang$ 9 pairs & runn+ f> t[>d ! we/ along ! c\rse ( ! ecliptic1 :i* crosses ! equator at ! v]nal equ9ox1 ne> ! we/]n end ( ! con/ellation4 ,s\? ( ,pisces & ,aries is ! broad con/ellation ,cetus1 ! ,:ale1 m>k$ by a numb] ( l>ge quadrilat]al & p5tagonal figures1 =m$ by its />s4 ,ne> ! c5tre ( ?is con/ellation1 b n ord9>ily visible to ! nak$ eye1 is ! celebrat$ v>iable ,mira1 also kn[n z ,omicron ,ceti4 ,) a little application any p]son c le>n to recognise ^! con/ellations1 ev5 ) ! slight aid "h (f]$1 & if he does1 he w f9d ! k ?us acquir$ z delightful z x is useful4 ---------------------------------------- -------------------------------- ,,index ,ab]ration1 #hh ,alpha ,c5tauri1 #ade ,alpha ,draconis1 #fb ,altitude1 #ac1 #ad ,&rom$a1 nebula 91 #bcf ,ant>es1 #bd ,aphelion1 #h ,april1 aspect ( sky 91 #bea ,arcturus1 #bbd ,a/]oids1 !1 #ahg ,atmosph]e1 !1 #hb ,augu/1 aspect ( sky 91 #bec ,azimu?1 #ac1 #ad ,beta ,aurig1 #bcj ,b9>y 7& multiple7 />s1 #bbf ,b9>ies1 spectroscopic1 #bbg ,cal5d>1 !1 #aag ,calorie1 measure ( sun's heat1 #abi ,cassiopeia1 #bb ,*eops1 pyramid (1 #fb ,*romosph]e1 ( sun1 #ace ,circles1 v]tical1 #ae2 altitude1 #ad2 division (1 #af ,circumpol> />s1 #bbd ,clock1 ! a/ronomical1 #da1 #ij ,comets1 #bja1 #bjc ,con/ellations1 !1 #bda ,corona1 ( ! sun1 #ace ,coronium1 #aeb ,corona ,borealis1 #bd ,day1 *ange (1 #ii1 #aja ,day & night1 #if ,decemb]1 aspect ( sky 91 #bef ,decl9ation1 #ca1 #ce ,dipp]1 ! ,great1 #ba2 ! ,little1 #bc ,e>?1 description ( !1 #fg2 weight (1 #gc ,eclipses1 #afc ,ecliptic1 #dc ,elem5ts1 *emical1 9 ! sun1 #aea ,equator1 #ch ,equ9oxes1 #de ,eros1 ! a/]oid1 #adb ,febru>y1 aspect ( sky 91 #bdi ,galaxy1 !1 #bcc ,gravitation1 laws (1 #gj 8,gre5wi* ( ! ,sky10 #cb ,heav5s1 app>5t motion (1 #ai ,helium1 #aeb ,horizon1 #aj1 #ab2 dip (1 #hf ,h\r ,circles1 #ca ,janu>y1 aspect ( sky 91 #bdh ,july1 aspect ( sky 91 #beb ,june1 aspect ( sky 91 #beb ,jupit]1 ! planet1 #ahi ,kepl]1 laws (1 #agc ,latitude1 t]re/rial1 #cj2 cele/ial1 #di1 #ea ,light1 pressure (1 #bjf ,longitude1 t]re/rial1 #cj2 cele/ial1 #di1 #ea ,magnitudes1 /ell>1 #bbj ,m>*1 aspect ( sky 91 #bej ,m>s1 ! planet1 #aha ,may1 aspect ( sky 91 #bea ,m]cury1 ! planet1 #age ,meteors1 #baj ,meteorites1 #bad ,milky ,way1 !1 #bcc ,molecules1 escape f planet>y atmosph]es1 #aeh ,moon1 description (1 #aec2 h[ e>? controls1 #ge ,motion1 prop]1 ( sol> sy/em1 #bcb ,nadir1 #aa ,nebul1 #bcf ,nebul> ,hypo!sis1 #bci ,neptune1 ! planet1 #aii ,noon-l9e1 #ab ,nor? ,po9t1 #bj ,nor? ,pole1 #bg1 #cg ,nor? ,/>1 #bj1 #bg1 #fa ,novemb]1 aspect ( sky 91 #bee ,nutation1 #fc ,oblique sph]e1 #ci ,octob]1 aspect ( sky 91 #bed ,oppositions1 ( ,m>s1 #ahc ,orbits ( planets1 #g1 #eb ,orion1 nebula 91 #bcf ,p>allax1 #acf1 #aci ,p>allel sph]e1 #ch ,p]ihelion1 #h ,phases ( ! moon1 #afj ,photography1 9 a/ronomy1 #bdj ,photometry1 ( />s1 #bbj ,photosph]e ( sun1 #acd ,planets1 description ( !1 #agb ,pol> circles1 #aaa ,precession ( ! ,equ9oxes1 #ee ,refraction1 #hc1 #he ,"r ,asc5sion1 #ca1 #ce ,"r sph]e1 #ci ,rotation1 effect ( e>?'s1 #e ,saturn1 description (1 #aie ,seasons1 !1 #ajd1 #ajg2 secul> *ange (1 #aae ,septemb]1 aspect ( sky 91 #bed ,signs ( ,zodiac1 #ed ,spectroscopic analysis1 #ade1 #adg2 %ift+ ( l9es1 #bbg ,sph]e1 cele/ial1 #i ,sol/ices1 !1 #dg ,/>s1 ! fix$1 #bai2 daily revolution (1 #be2 h[ to locate1 #bi2 lum9osity (1 #bbb2 spectra (1 #bba2 v>iable1 #bbe2 tempor>y1 #bce2 d\ble & multiple1 #bbf ,sun1 ! description (1 #abg ,sun-spots1 #aca ,temp]ature1 ( sun1 #acj ,tempor>y />s1 #bce ,tides1 !1 #gf1 #gi ,"t1 #hi2 sid]eal & sol>1 #ic ,transits1 ( ,v5us1 #adj1 #ada ,tropics1 !1 #aaa ,univ]se1 appe>ance (1 #h ,uranus1 description (1 #aii ,ursa ,major1 #ba ,v>iable />s1 #bbe ,vega1 #fc ,v5us1 ! planet1 #agh ,v]nal ,equ9ox1 #cb1 #db1 #df ,z5i?1 #aj ,zodiac1 #ej ---------------------------------------- -------------------------------- ,transcrib]'s ,notes3 ,new orig9al cov] art includ$ ) ?is e,book is grant$ to ! public doma94 ,miss+ or obscur$ punctuation 0 sil5tly correct$4 ,typographical errors 7 sil5tly correct$4 ,inconsi/5t spell+ & hyph5ation 7 made consi/5t only :5 a pr$om9ant =m 0 f\nd 9 ?is book4 ,text t3 0 9 italics is enclos$ by und]scores 7italics72 0 9 bold by is enclos$ by 8equal0 signs 7bold74 ,! use ( a c>et 77 be=e a lett]1 or lett]s1 %[s t ! foll[+ lett] or lett]s 0 int5d$ to be a sup]script1 z 9 ,st ,b>?olomew or #aj? ,c5tury4 ,sup]scripts are us$ to indicate numb]s rais$ to a p[]4 ,9 ?is pla9 text docum5t1 !y are repres5t$ by *>act]s l ?is3 8,p#c0 or 8#ajah01 i4e4 ,p cub$ or #aj to ! #ah? p[]4 ,v>iables 9 =mul sometimes use subscripts1 :i* look l ?is3 8,a#j04 ,?is w\ld be read 8,a sub #j04