,,the ,,science ,,of ,,the ,,stars ,,by ,e4 ,,walter ,,maunder1 ,f4,r4,a4,s4 ,,of ,,the ,,royal ,,observatory1 ,,greenwich ,,author ,,of ,,astronomy ,,without ,a ,,telescope ,,the ,,astronomy ,,of ,,the ,,bible1 ,,etc4 ,,london3 ,t4 ,c4 ,e4 ,c4 ,,jack #fg ,,long ,,acre1 ,w4,c41 ,,and ,,edinburgh ,,new ,,york3 ,,dodge ,,publishing ,,co4 vii ,,contents ,,chap4 ,i4 ,,astronomy ,,before ,,history ,,ii4 ,,astronomy ,,before ,,the ,,telescope ,,iii4 ,,the ,,law ,,of ,,gravitation ,,iv4 ,,astronomical ,,measurements ,v4 ,,the ,,members ,,of ,,the ,,solar ,,system ,,vi4 ,,the ,,system ,,of ,,the ,,stars ,,index #i ,,the ,,science ,,of ,,the ,,stars ,,chapter ,i ,,astronomy ,,before ,,history ,the plan of the present series requires each volume to be complete in about eighty small pages4 ,but no adequate account of the achievements of astronomy can possibly be given within limits so narrow1 for so small a space would not suffice for a mere catalogue of the results which have been obtained2 and in most cases the result alone would be almost meaningless unless some explanation were offered of the way in which it had been reached4 ,all1 therefore1 that can be done in a work of the present size is to take the student to the starting-point of astronomy1 show him the various roads of research which have opened out from it1 and give a brief indication of the character and general direction of each4 ,that which distinguishes astronomy from all the other sciences is this3 it deals with objects that we cannot touch4 ,the heavenly bodies are beyond our reach2 we cannot tamper with them1 or subject them to any form of experiment2 we cannot bring them into our laboratories to analyse or dissect them4 ,we can only watch them and wait for such indications as their #aj own movements may supply4 ,but we are confined to this earth of ours1 and they are so remote2 we are so short-lived1 and they are so long-enduring2 that the difficulty of finding out much about them might well seem insuperable4 ,yet these difficulties have been so far overcome that astronomy is the most advanced of all the sciences1 the one in which our knowledge is the most definite and certain4 ,all science rests on sight and thought1 on ordered observation and reasoned deduction2 but both sight and thought were earlier trained to the service of astronomy than of the other physical sciences4 ,it is here that the highest value of astronomy lies2 in the discipline that it has afforded to man's powers of observation and reflection2 and the real triumphs which it has achieved are not the bringing to light of the beauties or the sensational dimensions and distances of the heavenly bodies1 but the vanquishing of difficulties which might well have seemed superhuman4 ,the true spirit of the science can be far better exemplified by the presentation of some of these difficulties1 and of the methods by which they have been overcome1 than by many volumes of picturesque description or of eloquent rhapsody4 ,there was a time when men knew nothing of astronomy2 like every other science it began from zero4 ,but it is not possible to suppose that such a state of things lasted long1 we know that there was a time when men had noticed that there were two great lights in the sky--a greater light that shone by day1 a lesser light that shone by night--and there were the stars also4 ,and this1 the earliest observation of primitive astronomy1 is preserved for us1 expressed in the simplest possible language1 in the first chapter of the first book #aa of the sacred writings handed down to us by the ,hebrews4 ,this observation1 that there are bodies above us giving light1 and that they are not all equally bright1 is so simple1 so inevitable1 that men must have made it as soon as they possessed any mental power at all4 ,but1 once made1 a number of questions must have intruded themselves3 ,what are these lights8 ,where are they8 ,how far are they off8 ,many different answers were early given to these questions4 ,some were foolish2 some1 though intelligent1 were mistaken2 some1 though wrong1 led eventually to the discovery of the truth4 ,many myths1 many legends1 some full of beauty and interest1 were invented4 ,but in so small a book as this it is only possible to glance at those lines of thought which eventually led to the true solution4 ,as the greater light1 the lesser light1 and the stars were carefully watched1 it was seen not only that they shone1 but that they appeared to move2 slowly1 steadily1 and without ceasing4 ,the stars all moved together like a column of soldiers on the march1 not altering their positions relative to each other4 ,the lesser light1 the ,moon1 moved with the stars1 and yet at the same time among them4 ,the greater light1 the ,sun1 was not seen with the stars2 the brightness of his presence made the day1 his absence brought the night1 and it was only during his absence that the stars were seen2 they faded out of the sky before he came up in the morning1 and did not reappear again until after he passed out of sight in the evening4 ,but there came a time when it was realised that there were stars shining in the sky all day long as well as at night1 and this discovery was one of the greatest and most important ever made1 #ab because it was the earliest discovery of something quite unseen4 ,men laid hold of this fact1 not from the direct and immediate evidence of their senses1 but from reflection and reasoning4 ,we do not know who made this discovery1 nor how long ago it was made1 but from that time onward the eyes with which men looked upon nature were not only the eyes of the body1 but also the eyes of the mind4 ,it followed from this that the ,sun1 like the ,moon1 not only moved with the general host of the stars1 but also among them4 ,if an observer looks out from any fixed station and watches the rising of some bright star1 night after night1 he will notice that it always appears to rise in the same place2 so too with its setting4 ,from any given observing station the direction in which any particular star is observed to rise or set is invariable4 ,not so with the ,sun4 ,we are accustomed to say that the ,sun rises in the east and sets in the west4 ,but the direction in which the ,sun rises in midwinter lies far to the south of the east point2 the direction in which he rises in midsummer lies as far to the north4 ,the ,sun is therefore not only moving with the stars1 but among them4 ,this gradual change in the position of the ,sun in the sky was noticed in many ancient nations at an early time4 ,it is referred to in ,job xxxviii4 #ab3 ,hast thou commanded the morning since thy days2 and caused the dayspring to know his place8 ,and the apparent path of the ,sun on one day is always parallel to its path on the days preceding and following4 ,when1 therefore1 the ,sun rises far to the south of east1 he sets correspondingly far to the south of west1 and at noon he is low down in the south4 ,his course during the day is a short one1 and the daylight #ac is much shorter than the night1 and the ,sun at noon1 being low down in the sky1 has not his full power4 ,the cold and darkness of winter1 therefore1 follows directly upon this position of the ,sun4 ,these conditions are reversed when the ,sun rises in the north-east4 ,the night is short1 the daylight prolonged1 and the ,sun1 being high in the heavens at noon1 his heat is felt to the full4 ,thus the movements of the ,sun are directly connected with the changes of season upon the ,earth4 ,but the stars also are connected with those seasons2 for if we look out immediately after it has become dark after sunset1 we shall notice that the stars seen in the night of winter are only in part those seen in the nights of summer4 ,in the northern part of the sky there are a number of stars which are always visible whenever we look out1 no matter at what time of the night nor what part of the year4 ,if we watch throughout the whole night1 we see that the whole heavens appear to be slowly turning--turning1 as if all were in a single piece--and the pivot about which it is turning is high up in the northern sky4 ,the stars1 therefore1 are divided into two classes4 ,those near this invisible pivot--the ,pole of the ,heavens1 as we term it--move round it in complete circles2 they never pass out of sight1 but even when lowest they clear the horizon4 ,the other stars move round the same pivot in curved paths1 which are evidently parts of circles1 but circles of which we do not see the whole4 ,these stars rise on the eastern side of the heavens and set on the western1 and for a greater or less space of time are lost to sight below the horizon4 ,and some of these stars are visible at one time of the year1 others at another2 some being seen during the #ad whole of the long nights of winter1 others throughout the short nights of summer4 ,this distinction again1 and its connection with the change of the seasons on the earth1 was observed many ages ago4 ,it is alluded to in ,job xxxviii4 #cb3 ,canst thou lead forth the ,signs of the ,zodiac in their season1 or canst thou guide the ,bear with her train8 7,r4,v41 ,margin74 ,the ,signs of the ,zodiac are taken as representing the stars which rise and set1 and therefore have each their season for being led forth1 while the northern stars1 which are always visible1 appearing to be guided in their continual movement round the ,pole of the sky in perfect circles1 are represented by the ,bear with her train4 ,the changes in position of the ,sun1 the greater light1 must have attracted attention in the very earliest ages1 because these changes are so closely connected with the changes of the seasons upon the ,earth1 which affect men directly4 ,the ,moon1 the lesser light1 goes through changes of position like the ,sun1 but these are not of the same direct consequence to men1 and probably much less notice was taken of them4 ,but there were changes of the ,moon which men could not help noticing--her changes of shape and brightness4 ,one evening she may be seen soon after the ,sun has set1 as a thin arch of light1 low down in the sunset sky4 ,on the following evenings she is seen higher and higher in the sky1 and the bow of light increases1 until by the fourteenth day it is a perfect round4 ,then the ,moon begins to diminish and to disappear1 until1 on the twenty-ninth or thirtieth day after the first observation1 she is again seen in the west after sunset as a narrow crescent4 ,this succession of changes gave men an important measure of time1 and1 in an age when artificial means of light were difficult to procure1 moonlight was of the greatest #ae value1 and the return of the moonlit portion of the month was eagerly looked for4 ,these early astronomical observations were simple and obvious1 and of great practical value4 ,the day1 month1 and year were convenient measures of time1 and the power of determining1 from the observation of the ,sun and of the stars1 how far the year had progressed was most important to farmers1 as an indication when they should plough and sow their land4 ,such observations had probably been made independently by many men and in many nations1 but in one place a greater advance had been made4 ,the ,sun and ,moon are both unmistakable1 but one star is very like another1 and1 for the most part1 individual stars can only be recognised by their positions relative to others4 ,the stars were therefore grouped together into ,constellations and associated with certain fancied designs1 and twelve of these designs were arranged in a belt round the sky to mark the apparent path of the ,sun in the course of the year1 these twelve being known as the ,signs of the ,zodiac--the ,ram1 ,bull1 ,twins1 ,crab1 ,lion1 ,virgin1 ,balance1 ,scorpion1 ,archer1 ,goat1 ,water-pourer1 and ,fishes4 ,in the rest of the sky some thirty to thirty-six other groups1 or constellations1 were formed1 the ,bear being the largest and brightest of the constellations of the northern heavens4 ,but these ancient constellations do not cover the entire heavens2 a large area in the south is untouched by them4 ,and this fact affords an indication both of the time when and the place where the old stellar groups were designed1 for the region left untouched was the region below the horizon of #dj ,north latitude1 about #dfjj years ago4 ,it is probable1 therefore1 that the ancient astronomers who carried out this great work #af lived about #bgjj ,b4,c41 and in ,north latitude #cg or #ch4 ,the indication is only rough1 but the amount of uncertainty is not very large2 the constellations must be at least #djjj years old1 they cannot be more than #ejjj4 ,all this was done by prehistoric astronomers2 though no record of the actual carrying out of the work and no names of the men who did it have come down to us4 ,but it is clear from the fact that the ,signs of the ,zodiac are arranged so as to mark out the annual path of the ,sun1 and that they are twelve in number--there being twelve months in the year--that those who designed the constellations already knew that there are stars shining near the ,sun in full daylight1 and that they had worked out some means for determining what stars the ,sun is near at any given time4 ,another great discovery of which the date and the maker are equally unknown is referred to in only one of the ancient records available to us4 ,it was seen that all along the eastern horizon1 from north to south1 stars rise1 and all along the western horizon1 from north to south1 stars set4 ,that is what was seen2 it was the fact observed4 ,there is no hindrance anywhere to the movement of the stars--they have a free passage under the ,earth2 the ,earth is unsupported in space4 ,that is what was thought2 it was the inference drawn4 ,or1 as it is written in ,job xxvi4 #g1 ,he 7,god7 stretcheth out the north over empty space1 and hangeth the earth upon nothing4 ,the ,earth therefore floats unsupported in the centre of an immense star-spangled sphere4 ,and what is the shape of the ,earth8 ,the natural and correct inference is that it is spherical1 and we find in some of the early ,greek writers the arguments which establish this inference as clearly set forth as they would be to-day4 #ag ,the same inference followed1 moreover1 from the observation of a simple fact1 namely1 that the stars as observed from any particular place all make the same angle with the horizon as they rise in the east1 and all set at the same angle with it in the west2 but if we go northward1 we find that angle steadily decreasing2 if we go southward1 we find it increasing4 ,but if the ,earth is round like a globe1 then it must have a definite size1 and that size can be measured4 ,the discoveries noted above were made by men whose names have been lost1 but the name of the first person whom we know to have measured the size of the ,earth was ,,eratosthenes4 ,he found that the ,sun was directly overhead at noon at midsummer at ,syene 7the modern ,assouan71 in ,egypt1 but was #g south of the zenith--the point overhead--at ,alexandria1 and from this he computed the ,earth to be #bej1jjj stadia 7a stadium #fjf feet7 in circumference4 ,another consequence of the careful watch upon the stars was the discovery that five of them were planets2 wandering stars2 they did not move all in one piece with the rest of the celestial host4 ,in this they resemble the ,sun and ,moon1 and they further resemble the ,moon in that1 though too small for any change of shape to be detected1 they change in brightness from time to time4 ,but their movements are more complicated than those of the other heavenly bodies4 ,the ,sun moves a little slower than the stars1 and so seems to travel amongst them from west to east2 the ,moon moves much slower than the stars1 so her motion from west to east is more pronounced than that of the ,sun4 ,but the five planets sometimes move slower than the stars1 sometimes quicker1 and sometimes at the same rate4 ,two of the five1 which we now know as ,mercury #ah and ,venus1 never move far from the ,sun1 sometimes being seen in the east before he rises in the morning1 and sometimes in the west after he has set in the evening4 ,mercury is the closer to the ,sun1 and moves more quickly2 ,venus goes through much the greater changes of brightness4 ,jupiter and ,saturn move nearly at the same average rate as the stars1 ,saturn taking about thirteen days more than a year to come again to the point of the sky opposite to the ,sun1 and ,jupiter about thirty-four days4 ,mars1 the fifth planet1 takes two years and fifty days to accomplish the same journey4 ,these planetary movements were not1 like those of the ,sun and ,moon and stars1 of great and obvious consequence to men4 ,it was important to men to know when they would have moonlight nights1 to know when the successive seasons of the year would return4 ,but it was no help to men to know when ,venus was at her brightest more than when she was invisible4 ,she gave them no useful light1 and she and her companion planets returned at no definite seasons4 ,nevertheless1 men began to make ordered observations of the planets--observations that required much more patience and perseverance than those of the other celestial lights4 ,and they set themselves with the greatest ingenuity to unravel the secret of their complicated and seemingly capricious movements4 ,this was a yet higher development than anything that had gone before1 for men were devoting time1 trouble1 and patient thought1 for long series of years1 to an inquiry which did not promise to bring them any profit or advantage4 ,yet the profit which it actually did bring was of the highest order4 ,it developed men's mental powers2 it led to the devising of #ai instruments of precision for the observations2 it led to the foundation of mathematics1 and thus lay at the root of all our modern mechanical progress4 ,it brought out1 in a higher degree1 ordered observation and ordered thought4 #bj ,,chapter ,,ii ,,astronomy ,,before ,,the ,,telescope ,there was thus a real science of astronomy before we have any history of it4 ,some important discoveries had been made1 and the first step had been taken towards cataloguing the fixed stars4 ,it was certainly known to some of the students of the heavens1 though perhaps only to a few1 that the ,earth was a sphere1 freely suspended in space1 and surrounded on all sides by the starry heavens1 amongst which moved the ,sun1 ,moon1 and the five planets4 ,the general character of the ,sun's movement was also known2 namely1 that he not only moved day by day from east to west1 as the stars do1 but also had a second motion inclined at an angle to the first1 and in the opposite direction1 which he accomplished in the course of a year4 ,to this sum of knowledge1 no doubt1 several nations had contributed4 ,we do not know to what race we owe the constellations1 but there are evidences of an elementary acquaintance with astronomy on the part of the ,chinese1 the ,babylonians1 the ,egyptians1 and the ,jews4 ,but in the second stage of the development of the science the entire credit for the progress made belongs to the ,greeks4 ,the ,greeks1 as a race1 appear to have been very little apt at originating ideas1 but they possessed1 beyond all other races1 the power of developing and perfecting crude ideas which they had obtained from other sources1 #ba and when once their attention was drawn to the movements of the heavenly bodies1 they devoted themselves with striking ingenuity and success to devising theories to account for the appearances presented1 to working out methods of computation1 and1 last1 to devising instruments for observing the places of the luminaries in which they were interested4 ,in the brief space available it is only possible to refer to two or three of the men whose commanding intellects did so much to help on the development of the science4 ,,eudoxus of ,knidus1 in ,asia ,minor 7#djh-cee ,b4,c471 was1 so far as we know1 the first to attempt to represent the movements of the heavenly bodies by a simple mathematical process4 ,his root idea was something like this4 ,the ,earth was in the centre of the universe1 and it was surrounded1 at a great distance from us1 by a number of invisible transparent shells1 or spheres4 ,each of these spheres rotated with perfect uniformity1 though the speed of rotation differed for different spheres4 ,one sphere carried the stars1 and rotated from east to west in about #bc h4 #ef m4 ,the ,sun was carried by another sphere1 which rotated from west to east in a year1 but the pivots1 or poles1 of this sphere were carried by a second1 rotating exactly like the sphere of the stars4 ,this explained how it is that the ecliptic--that is to say1 the apparent path of the ,sun amongst the stars--is inclined #bc- to the equator of the sky1 so that the ,sun is #bc- north of the equator at midsummer and #bc- south of the equator at midwinter1 for the poles of the sphere peculiar to the ,sun were supposed to be #bc- from the poles of the sphere peculiar to the stars4 ,then the ,moon had three spheres2 that which actually carried the ,moon having its poles #e from the poles of the sphere peculiar to the #bb ,sun4 ,these poles were carried by a sphere placed like the sphere of the ,sun1 but rotating in #bg days2 and this1 again1 had its poles in the sphere of the stars4 ,the sphere carrying the ,moon afforded the explanation of the wavy motion of the ,moon to and fro across the ecliptic in the course of a month1 for at one time in the month the ,moon is #e north of the ecliptic1 at another time #e south4 ,the motions of the planets were more difficult to represent1 because they not only have a general daily motion from east to west1 like the stars1 and a general motion from west to east along the ecliptic1 like the ,sun and ,moon1 but from time to time they turn back on their course in the ecliptic1 and retrograde4 ,but the introduction of a third and fourth sphere enabled the motions of most of the planets to be fairly represented4 ,there were thus twenty-seven spheres in all--four for each of the five planets1 three for the ,moon1 three for the ,sun 7including one not mentioned in the foregoing summary71 and one for the stars4 ,these spheres were not1 however1 supposed to be solid structures really existing2 the theory was simply a means for representing the observed motions of the heavenly bodies by computations based upon a series of uniform movements in concentric circles4 ,but this assumption that each heavenly body moves in its path at a uniform rate was soon seen to be contrary to fact4 ,a reference to the almanac will show at once that the ,sun's movement is not uniform4 ,thus for the year #aiaj-aa the solstices and equinoxes fell as given on the next page3 #bc ,epoch ,time ,interval ,winter ,solstice #aiaj ,dec4 #bb d4 #e h4 #ab m4 ,p4,m4 #hi d4 #j h4 #db m4 ,spring ,equinox #aiaa ,mar4 #ba #e #ed ,p4,m4 #ib #ai #da ,summer ,solstice #aial ,june #bb #a #ce ,p4,m4 #ic #ad #dc ,autumn ,equinox #aiaa ,sept4 #bd #d #ah ,a4,m4 #hi #ah #cf ,winter ,solstice #aiaa ,dec4 #bb #aj #ed ,p4,m4 so that the winter half of the year is shorter than the summer half2 the ,sun moves more quickly over the half of its orbit which is south of the equator than over the half which is north of it4 ,the motion of the ,moon is more irregular still1 as we can see by taking out from the almanac the times of new and full moon3 ,new ,moon ,interval to ,full ,moon ,dec4 #aiaj #a d4 #i h4 #aj4g m4 ,p4,m4 #ad d4 #ac h4 #ed4d m4 #ca #d #ba4b ,p4,m4 #ad #f #d4h ,jan4 #aiaa #cj #i #dd4g ,a4,m4 #ad #j #eb4h ,march #a #j #ca4a ,a4,m4 #ac #bc #bg4d #cj #j #cg4h ,p4,m4 #ad #a #eh4h ,april #bh #aj #be4j ,p4,m4 #ad #g #dd4g ,may #bh #f #bd4d ,a4,m4 #ad #ae #bf4c ,june #bf #a #ai4g ,p4,m4 #ad #bc #cc4g ,july #be #h #ab4j ,p4,m4 #ae #f #db4g ,aug4 #bd #d #ad4c ,a4,m4 #ae #aa #db4d ,sept4 #bb #b #cg4d ,p4,m4 #ae #ac #cc4g ,oct4 #bb #d #i4c ,a4,m4 #ae #aa #ch4h ,nov4 #bj #h #di4d ,p4,m4 #ae #f #b4e ,dec4 #bj #c #dj4c ,p4,m4 #ad #ba #di4d #bd ,full ,moon ,interval to ,new ,moon ,dec4 #aiaj #af d #aa h4 #e4a m4 ,a4,m4 #ae d4 #e h4 #af4a m4 ,jan4 #aiaa #ad #aj #bf4j ,p4,m4 #ae #aa #ah4g ,feb4 #ac #aj #cg4e ,a4,m4 #ae #ac #ec4f ,march #ad #aa #eh4e ,p4,m4 #ae #ab #ci4c ,april #ac #b #cf4f ,p4,m4 #ae #g #dh4d ,may #ac #f #i4g ,a4,m4 #ae #j #ad4g ,june #aa #i #ej4g ,p4,m4 #ad #ae #bi4j ,july #aa #j #ec4d ,p4,m4 #ad #g #ah4f ,aug4 #aj #b #ed4g ,a4,m4 #ad #a #ai4f ,sept4 #h #c #ef4g ,p4,m4 #ac #bb #dj4g ,oct4 #h #d #aa4a ,a4,m4 #ac #bc #eh4b ,nov4 #f #c #dh4a ,p4,m4 #ad #e #a4c ,dec4 #f #b #ea4i ,a4,m4 #ad #ab #dh4d ,jan4 #aiab #d #a #ii4g ,p4,m4 #ad #ba #dj4c ,the astronomer who dealt with this difficulty was ,,hipparchus 7about #aij-abj ,b4,c471 who was born at ,nica1 in ,bithynia1 but made most of his astronomical observations in ,rhodes4 ,he attempted to explain these irregularities in the motions of the ,sun and ,moon by supposing that though they really moved uniformly in their orbits1 yet the centre of their orbits was not the centre of the ,earth1 but was situated a little distance from it4 ,this point was called the excentric1 and the line from the excentric to the ,earth was called the line of apsides4 ,but when he tried to deal with the movements of the planets1 he found that there were not enough good observations available for him to build up any satisfactory theory4 ,he therefore devoted himself to the work of making systematic determinations of the places of the planets that he might put his successors in a better position to deal with the problem than he was4 ,his great successor was ,,claudius ,,ptolemy of #be ,alexandria1 who carried the work of astronomical observation from about ,a4,d4 #abg to #aej4 ,he was1 however1 much greater as a mathematician than as an observer1 and he worked out a very elaborate scheme1 by which he was able to represent the motions of the planets with considerable accuracy4 ,the system was an extremely complex one1 but its principle may be represented as follows3 ,if we suppose that a planet is moving round the ,earth in a circle at a uniform rate1 and we tried to compute the place of the planet on this assumption for regular intervals of time1 we should find that the planet gradually got further and further away from the predicted place4 ,then after a certain time the error would reach a maximum1 and begin to diminish1 until the error vanished and the planet was in the predicted place at the proper time4 ,the error would then begin to fall in the opposite direction1 and would increase as before to a maximum1 subsequently diminishing again to zero4 ,this state of things might be met by supposing that the planet was not itself carried by the circle round the earth1 but by an epicycle--i4e4 a circle travelling upon the first circle--and by judiciously choosing the size of the epicycle and the time of revolution the bulk of the errors in the planet's place might be represented4 ,but still there would be smaller errors going through their own period1 and these1 again1 would have to be met by imagining that the first epicycle carried a second1 and it might be that the second carried a third1 and so on4 ,the ,ptolemaic system was more complicated than this brief summary would suggest1 but it is not possible here to do more than indicate the general principles upon which it was founded1 and the numerous other systems or modifications of them produced in the #bf five centuries from ,eudoxus to ,ptolemy must be left unnoticed4 ,the point to be borne in mind is that one fundamental assumption underlay them all1 an assumption fundamental to all science--the assumption that like causes must always produce like effects4 ,it was apparent to the ancient astronomers that the stars--that is to say1 the great majority of the heavenly bodies--do move round the ,earth in circles1 and with a perfect uniformity of motion1 and it seemed inevitable that1 if one body moved round another1 it should thus move4 ,for if the revolving body came nearer to the centre at one time and receded at another1 if it moved faster at one time and slower at another1 then1 the cause remaining the same1 the effect seemed to be different4 ,any complexity introduced by superposing one epicycle upon another seemed preferable to abandoning this great fundamental principle of the perfect uniformity of the actings of ,nature4 ,for more than #acjj years the ,ptolemaic system remained without serious challenge1 and the next great name that it is necessary to notice is that of ,,copernicus 7#adgc-aedc74 ,copernicus was a canon of ,frauenburg1 and led the quiet1 retired life of a student4 ,the great work which made him immortal1 ,de ,revolutionibus1 was the result of many years' meditation and work1 and was not printed until he was on his deathbed4 ,in this work ,copernicus showed that he was one of those great thinkers who are able to look beyond the mere appearance of things and to grasp the reality of the unseen4 ,copernicus realised that the appearance would be just the same whether the whole starry vault rotated every twenty-four hours round an immovable ,earth from east to west or the ,earth rotated from west to east in the midst of the starry sphere2 and1 as the #bg stars are at an immeasurable distance1 the latter conception was much the simpler4 ,extending the idea of the ,earth's motion further1 the supposition that1 instead of the ,sun revolving round a fixed ,earth in a year1 the ,earth revolved round a fixed ,sun1 made at once an immense simplification in the planetary motions4 ,the reason became obvious why ,mercury and ,venus were seen first on one side of the ,sun and then on the other1 and why neither of them could move very far from the ,sun2 their orbits were within the orbit of the ,earth4 ,the stationary points and retrogressions of the planets were also explained2 for1 as the ,earth was a planet1 and as the planets moved in orbits of different sizes1 the outer planets taking a longer time to complete a revolution than the inner1 it followed1 of necessity1 that the ,earth in her motion would from time to time be passed by the two inner planets1 and would overtake the three outer4 ,the chief of the ,ptolemaic epicycles were done away with1 and all the planets moved continuously in the same direction round the ,sun4 ,but no planet's motion could be represented by uniform motion in a single circle1 and ,copernicus had still to make use of systems of epicycles to account for the deviations from regularity in the planetary motions round the ,sun4 ,the ,earth having been abandoned as the centre of the universe1 a further sacrifice had to be made3 the principle of uniform motion in a circle1 which had seemed so necessary and inevitable1 had also to be given up4 ,for the time came when the instruments for measuring the positions of the stars and planets had been much improved1 largely due to ,,tycho ,,brahe 7#aedf-afja71 a ,dane of noble birth1 who was the keenest and most careful observer that astronomy had yet produced4 #bh ,his observations enabled his friend and pupil1 ,,johann ,,kepler1 7#aega-afcj71 to subject the planetary movements to a far more searching examination than had yet been attempted1 and he discovered that the ,sun is in the plane of the orbit of each of the planets1 and also in its line of apsides--that is to say1 the line joining the two points of the orbit which are respectively nearest and furthest from the ,sun4 ,copernicus had not been aware of either of these two relations1 but their discovery greatly strengthened the ,copernican theory4 ,then for many years ,kepler tried one expedient after another in order to find a combination of circular motions which would satisfy the problem before him1 until at length he was led to discard the circle and try a different curve--the oval or ellipse4 ,now the property of a circle is that every point of it is situated at the same distance from the centre1 but in an ellipse there are two points within it1 the foci1 and the sum of the distances of any point on the circumference from these two foci is constant4 ,if the two foci are at a great distance from each other1 then the ellipse is very long and narrow2 if the foci are close together1 the ellipse differs very little from a circle2 and if we imagine that the two foci actually coincide1 the ellipse becomes a circle4 ,when ,kepler tried motion in an ellipse instead of motion in a circle1 he found that it represented correctly the motions of all the planets without any need for epicycles1 and that in each case the ,sun occupied one of the foci4 ,and though the planet did not move at a uniform speed in the ellipse1 yet its motion was governed by a uniform law1 for the straight line joining the planet to the ,sun1 the radius vector1 passed over equal areas of space in equal periods of time4 #bi ,these two discoveries are known as ,kepler's ,first and ,second ,laws4 ,his ,third ,law connects all the planets together4 ,it was known that the outer planets not only take longer to revolve round the ,sun than the inner1 but that their actual motion in space is slower1 and ,kepler found that this actual speed of motion is inversely as the square root of its distance from the ,sun2 or1 if the square of the speed of a planet be multiplied by its distance from the ,sun1 we get the same result in each case4 ,this is usually expressed by saying that the cube of the distance is proportional to the square of the time of revolution4 ,thus the varying rate of motion of each planet in its orbit is not only subject to a single law1 but the very different speeds of the different planets are also all subject to a law that is the same for all4 ,thus the whole of the complicated machinery of ,ptolemy had been reduced to three simple laws1 which at the same time represented the facts of observation much better than any possible development of the ,ptolemaic mechanism4 ,on his discovery of his third law ,kepler had written3 ,the book is written to be read either now or by posterity--,i care not which2 it may well wait a century for a reader1 as ,god has waited #fjjj years for an observer4 ,twelve years after his death1 on ,christmas ,day #afdb 7old style71 near ,grantham1 in ,lincolnshire1 the predestined reader was born4 ,the inner meaning of ,kepler's three laws was brought to light by ,,isaac ,,newton4 #cj ,,chapter ,,iii ,,the ,,law ,,of ,,gravitation ,the fundamental thought which1 recognised or not1 had lain at the root of the ,ptolemaic system1 as indeed it lies at the root of all science1 was that like causes must always produce like effects4 ,upon this principle there seemed to the ancient astronomers no escape from the inference that each planet must move at a uniform speed in a circle round its centre of motion4 ,for1 if there be any force tending to alter the distance of the planet from that centre1 it seemed inevitable that sooner or later it should either reach that centre or be indefinitely removed from it4 ,if there be no such force1 then the planet's distance from that centre must remain invariable1 and if it move at all1 it must move in a circle2 move uniformly1 because there is no force either to hasten or retard it4 ,uniform motion in a circle seemed a necessity of nature4 ,but all this system1 logical and inevitable as it had once seemed1 had gone down before the assault of observed facts4 ,the great example of uniform circular motion had been the daily revolution of the star sphere2 but this was now seen to be only apparent1 the result of the rotation of the ,earth4 ,the planets revolved round the ,sun1 but the ,sun was not in the centre of their motion2 they moved1 not in circles1 but in ellipses2 not at a uniform speed1 but at a speed which diminished with the increase of their distance from #ca the ,sun4 ,there was need1 therefore1 for an entire revision of the principles upon which motion was supposed to take place4 ,the mistake of the ancients had been that they supposed that continued motion demanded fresh applications of force4 ,they noticed that a ball1 set rolling1 sooner or later came to a stop2 that a pendulum1 set swinging1 might swing for a good time1 but eventually came to rest2 and1 as the forces that were checking the motion--that is to say1 the friction exercised by the ground1 the atmosphere1 and the like--did not obtrude themselves1 they were overlooked4 ,newton brought out into clear statement the true conditions of motion4 ,a body once moving1 if acted upon by no force whatsoever1 must continue to move forward in a straight line at exactly the same speed1 and that for ever4 ,it does not require any maintaining force to keep it going4 ,if any change in its speed or in its direction takes place1 that change must be due to the introduction of some further force4 ,this principle1 that1 if no force acts on a body in motion1 it will continue to move uniformly in a straight line1 is ,newton's ,first ,law of ,motion4 ,his ,second lays it down that1 if force acts on a body1 it produces a change of motion proportionate to the force applied1 and in the same direction4 ,and the ,third ,law states that when one body exerts force upon another1 that second body reacts with equal force upon the first4 ,the problem of the motions of the planets was1 therefore1 not what kept them moving1 but what made them deviate from motion in a straight line1 and deviate by different amounts4 ,it was quite clear1 from the work of ,kepler1 that the force deflecting the planets from uniform motion in a #cb straight line lay in the ,sun4 ,the facts that the ,sun lay in the plane of the orbits of all the planets1 that the ,sun was in one of the foci of each of the planetary ellipses1 that the straight line joining the ,sun and planet moved for each planet over equal areas in equal periods of time1 established this fact clearly4 ,but the amount of deflection was very different for different planets4 ,thus the orbit of ,mercury is much smaller than that of the ,earth1 and is travelled over in a much shorter time1 so that the distance by which ,mercury is deflected in a course of an hour from movement in a straight line is much greater than that by which the ,earth is deflected in the same time1 ,mercury falling towards the ,sun by about #aei miles1 whilst the fall of the ,earth is only about #bc4i miles4 ,the force drawing ,mercury towards the ,sun is therefore #f4ff times that drawing the ,earth1 but #f4ff is the square of #b4eh1 and the ,earth is #b4eh times as far from the ,sun as ,mercury4 ,similarly1 the fall in an hour of ,jupiter towards the ,sun is about #j4hh miles1 so that the force drawing the ,earth is #bg times that drawing ,jupiter towards the ,sun4 ,but #bg is the square of #e4b1 and ,jupiter is #e4b times as far from the ,sun as the ,earth4 ,similarly with the other planets4 ,the force1 therefore1 which deflects the planets from motion in a straight line1 and compels them to move round the ,sun1 is one which varies inversely as the square of the distance4 ,but the ,sun is not the only attracting body of which we know4 ,the old ,ptolemaic system was correct to a small extent2 the ,earth is the centre of motion for the ,moon1 which revolves round it at a mean distance of #bch1hjj miles1 and in a period of #bg d4 #g h4 #dc m4 ,hence the circumference of her orbit is #a1ejj1dej miles1 and the length of the straight line which she would travel #cc in one second of time1 if not deflected by the ,earth1 is #bhbh feet4 ,in this distance the deviation of a circle from a straight line is one inch divided by #ah4ff4 ,but we know from experiment that a stone let fall from a height of #aic inches above the ,earth's surface will reach the ground in exactly one second of time4 ,the force drawing the stone to the ,earth1 therefore1 is #aic x #ah4ff2 i4e4 #cfja times as great as that drawing the ,moon4 ,but the stone is only #a_/ccj of a mile from the ,earth's surface1 while the ,moon is #bch1hjj miles away--more than #gh million times as far4 ,the force1 therefore1 would seem not to be diminished in the proportion that the distance is increased--much less in the proportion of its square4 ,but ,newton proved that a sphere of uniform density1 or made up of any number of concentric shells of uniform density1 attracted a body outside itself1 just as if its entire mass was concentrated at its centre4 ,the distance of the stone from the ,earth must therefore be measured1 not from the ,earth's surface1 but from its centre2 in other words1 we must consider the stone as being distant from the ,earth1 not some #af feet1 but #cifc miles4 ,this is very nearly one-sixtieth of the ,moon's distance1 and the square of #fj is #cfjj4 ,the ,earth's pull upon the ,moon1 therefore1 is almost exactly in the inverse square of the distance as compared with its pull on the stone4 ,kepler's book had found its reader4 ,his three laws were but three particular aspects of ,newton's great discovery that the planets moved under the influence of a force1 lodged in the ,sun1 which varied inversely as the square of their distances from it4 ,but ,newton's work went far beyond this1 for he showed that the same law governed the motion of the ,moon round the #cd ,earth and the motions of the satellites revolving round the different planets1 and also governed the fall of bodies upon the ,earth itself4 ,it was universal throughout the solar system4 ,the law1 therefore1 is stated as of universal application4 ,every particle of matter in the universe attracts every other particle with a force varying inversely as the square of the distance between them1 and directly as the product of the masses of the two particles4 ,and ,newton further proved that if a body1 projected in free space and moving with any velocity1 became subject to a central force acting1 like gravitation1 inversely as the square of the distance1 it must revolve in an ellipse1 or in a closely allied curve4 ,these curves are what are known as the conic sections--that is1 they are the curves found when a cone is cut across in different directions4 ,their relation to each other may be illustrated thus4 ,if we have a very powerful light emerging from a minute hole1 then1 if we place a screen in the path of the beam of light1 and exactly at right angles to its axis1 the light falling on the screen will fill an exact circle4 ,if we turn the screen so as to be inclined to the axis of the beam1 the circle will lengthen out in one direction1 and will become an ellipse4 ,if we turn the screen still further1 the ellipse will lengthen and lengthen1 until at last1 when the screen has become parallel to one of the edges of the beam of light1 the ellipse will only have one end2 the other will be lost4 ,for it is clear that that edge of the beam of light which is parallel to the screen can never meet it4 ,the curve now shown on the screen is called a parabola1 and if the screen is turned further yet1 the boundaries of the light falling upon it become divergent1 and we have a fourth curve1 the hyperbola4 ,bodies moving under the influence of #ce gravitation can move in any of these curves1 but only the circle and ellipse are closed orbits4 ,a particle moving in a parabola or hyperbola can only make one approach to its attracting body2 after such approach it continually recedes from it4 ,as the circle and parabola are only the two extreme forms of an ellipse1 the two foci being at the same point for the circle and at an infinite distance apart for the parabola1 we may regard all orbits under gravitation as being ellipses of one form or another4 ,from his great demonstration of the law of gravitation1 ,newton went on to apply it in many directions4 ,he showed that the ,earth could not be truly spherical in shape1 but that there must be a flattening of its poles4 ,he showed also that the ,moon1 which is exposed to the attractions both of the ,earth and of the ,sun1 and1 to a sensible extent1 of some of the other planets1 must show irregularities in her motion1 which at that time had not been noticed4 ,the ,moon's orbit is inclined to that of the ,earth1 cutting its plane in two opposite points1 called the nodes4 ,it had long been observed that the position of the nodes travelled round the ecliptic once in about nineteen years4 ,newton was able to show that this was a consequence of the ,sun's attraction upon the ,moon4 ,and he further made a particular application of the principle thus brought out1 for1 the ,earth not being a true sphere1 but flattened at the poles and bulging at the equator1 the equatorial belt might be regarded as a compact ring of satellites revolving round the ,earth's equator4 ,this1 therefore1 would tend to retrograde precisely as the nodes of a single satellite would1 so that the axis of the equatorial belt of the ,earth--in other words1 the axis of the ,earth--must revolve round the pole of the ecliptic4 #cf ,consequently the pole of the heavens appears to move amongst the stars1 and the point where the celestial equator crosses the equator necessarily moves with it4 ,this is what we know as the ,precession of the ,equinoxes1 and it is from our knowledge of the fact and the amount of precession that we are able to determine roughly the date when the first great work of astronomical observation was accomplished1 namely1 the grouping of the stars into constellations by the astronomers of the prehistoric age4 ,the publication of ,newton's great work1 the ,principia 7,the ,mathematical ,principles of ,natural ,philosophy71 in which he developed the ,laws of ,motion1 the significance of ,kepler's ,three ,planetary ,laws1 and the ,law of ,universal ,gravitation1 took place in #afhg1 and was due to his friend ,,edmund ,,halley1 to whom he had confided many of his results4 ,that he was the means of securing the publication of the ,principia is ,halley's highest claim to the gratitude of posterity1 but his own work in the field which ,newton had opened was of great importance4 ,newton had treated comets as moving in parabolic orbits1 and ,halley1 collecting all the observations of comets that were available to him1 worked out the particulars of their orbits on this assumption1 and found that the elements of three were very closely similar1 and that the interval between their appearances was nearly the same1 the comets having been seen in #aeca1 #afjg1 and #afhb4 ,on further consulting old records he found that comets had been observed in #adef1 #acgh1 and #acja4 ,he concluded that these were different appearances of the same object1 and predicted that it would be seen again in #ageh1 or1 according to a later and more careful computation1 in #agei4 ,as the time for its return drew near1 ,,clairaut #cg computed with the utmost care the retardation which would be caused to the comet by the attractions of ,jupiter and ,saturn4 ,the comet made its predicted nearest approach to the ,sun on ,march #ac1 #agei1 just one month earlier than ,clairaut had computed4 ,but in its next return1 in #ahce1 the computations effected by ,,pont,,coulant were only two days in error1 so carefully had the comet been followed during its unseen journey to the confines of the solar system and back again1 during a period of seventy-five years4 ,pontcoulant's exploit was outdone at the next return by ,drs4 ,,cowell and ,,crommelin1 of ,greenwich ,observatory1 who not only computed the time of its perihelion passage--that is to say1 its nearest approach to the ,sun--for ,april #af1 #aiaj1 but followed the comet back in its wanderings during all its returns to the year #bdj ,b4,c4 ,halley's ,comet1 therefore1 was the first comet that was known to travel in a closed orbit and to return to the neighbourhood of the ,sun4 ,not a few small or telescopic comets are now known to be periodic1 but ,halley's is the only one which has made a figure to the naked eye4 ,notices of it occur not a few times in history2 it was the comet like a flaming sword which ,josephus described as having been seen over ,jerusalem not very long before the destruction by ,titus4 ,it was also the comet seen in the spring of the year when ,william the ,conqueror invaded ,england1 and was skilfully used by that leader as an omen of his coming victory4 ,the law of gravitation had therefore enabled men to recognise in ,halley's ,comet an addition to the number of the primary bodies in the solar system--the first addition that had been made since prehistoric times4 ,on ,march #ac1 #agha1 ,sir ,,william ,,herschel #ch detected a new object1 which he at first supposed to be a comet1 but afterwards recognised as a planet far beyond the orbit of ,saturn4 ,this planet1 to which the name of ,uranus was finally given1 had a mean distance from the ,sun nineteen times that of the ,earth1 and a diameter four times as great4 ,this was a second addition to the solar system1 but it was a discovery by sight1 not by deduction4 ,the first day of the nineteenth century1 ,january #a1 #ahja1 was signalised by the discovery of a small planet by ,,piazzi4 ,the new object was lost for a time1 but it was redetected on ,december #ca of the same year4 ,this planet lay between the orbits of ,mars and ,jupiter--a region in which many hundreds of other small bodies have since been found4 ,the first of these minor planets was called ,ceres2 the next three to be discovered are known as ,pallas1 ,juno1 and ,vesta4 ,beside these four1 two others are of special interest3 one1 ,eros1 which comes nearer the ,sun than the orbit of ,mars--indeed at some oppositions it approaches the ,earth within #ac1jjj1jjj miles1 and is therefore1 next to the ,moon1 our nearest neighbour in space2 the other1 ,achilles1 moves at a distance from the ,sun equal to that of ,jupiter4 ,ceres is much the largest of all the minor planets2 indeed is larger than all the others put together4 ,yet the ,earth exceeds ,ceres #djjj times in volume1 and #gjjj times in mass1 and the entire swarm of minor planets1 all put together1 would not equal in total volume one-fiftieth part of the ,moon4 ,the search for these small bodies rendered it necessary that much fuller and more accurate maps of the stars should be made than had hitherto been attempted1 and this had an important bearing on the next great event in the development of gravitational astronomy4 #ci ,the movements of ,uranus soon gave rise to difficulties4 ,it was found impossible1 satisfactorily1 to reconcile the earlier and later observations1 and in the tables of ,uranus1 published by ,,bouvard in #ahba1 the earlier observations were rejected4 ,but the discrepancies between the observed and calculated places for the planet soon began to reappear and quickly increase1 and the suggestion was made that these discrepancies were due to an attraction exercised by some planet as yet unknown4 ,thus ,mrs4 ,somerville in a little book on the connection of the physical sciences1 published in #ahcf1 wrote1 ,possibly it 7that is1 ,uranus7 may be subject to disturbances from some unseen planet revolving about the ,sun beyond the present boundaries of our system4 ,if1 after the lapse of years1 the tables formed from a combination of numerous observations should still be inadequate to represent the motions of ,uranus1 the discrepancies may reveal the existence1 nay1 even the mass and orbit of a body placed for ever beyond the sphere of vision4 ,in #ahdc ,,john ,c4 ,,adams1 who had just graduated as ,senior ,wrangler at ,cambridge1 proceeded to attack the problem of determining the position1 orbit1 and mass of the unknown body by which on this assumption ,uranus was disturbed1 from the irregularities evident in the motion of that planet4 ,the problem was one of extraordinary intricacy1 but by ,september #ahde ,adams had obtained a first solution1 which1 he submitted to ,,airy1 the ,astronomer ,royal4 ,as1 however1 he neglected to reply to some inquiries made by ,airy1 no search for the new planet was instituted in ,england until the results of a new and independent worker had been published4 ,the same problem had been attacked by a well-known and very gifted ,french mathematician1 ,u4 ,j4 ,j4 ,,leverrier1 and #dj in ,june #ahdf he published his position for the unseen planet1 which proved to be in close accord with that which ,adams had furnished to ,airy nine months before4 ,on this ,airy stirred up ,challis1 the ,director of the ,cambridge ,observatory1 which then possessed the most powerful telescope in ,england1 to search for the planet1 and ,challis commenced to make charts1 which included more than #cjjj stars1 in order to make sure that the stranger should not escape his net4 ,leverrier1 on the other hand1 communicated his result to the ,berlin ,observatory1 where they had just received some of the star charts prepared by ,dr4 ,bremiker in connection with the search for minor planets4 ,the ,berlin observer1 ,dr4 ,galle1 had therefore nothing to do but to compare the stars in the field1 upon which he turned his telescope1 with those shown on the chart2 a star not in the chart would probably be the desired stranger4 ,he found it1 therefore1 on the very first evening1 ,september #bc1 #ahdf1 within less than four diameters of the ,moon of the predicted place4 ,the same object had been observed by ,challis at ,cambridge on ,august #d and #ab1 but he was deferring the reduction of his observations until he had completed his scrutiny of the zone1 and hence had not recognised it as different from an ordinary star4 ,this discovery of the planet now known as ,neptune1 which had been disturbing the movement of ,uranus1 has rightly been regarded as the most brilliant triumph of gravitational astronomy4 ,it was the legitimate crown of that long intellectual struggle which had commenced more than #bjjj years earlier1 when the first ,greek astronomers set themselves to unravel the apparently aimless wanderings of the planets in the assured faith that they would find them obedient unto law4 #da ,but of what use was all this effort8 ,what is the good of astronomy8 ,the question is often asked1 but it is the question of ignorance4 ,the use of astronomy is the development which it has given to the intellectual powers of man4 ,directly the problem of the planetary motions was first attempted1 it became necessary to initiate mathematical processes in order to deal with it1 and the necessity for the continued development of mathematics has been felt in the same connection right down to the present day4 ,when the ,greek astronomers first began their inquiries into the planetary movements they hoped for no material gain1 and they received none4 ,they laboured2 we have entered into their labours4 ,but the whole of our vast advances in mechanical and engineering science--advances which more than anything else differentiate this our present age from all those which have preceded it--are built upon our command of mathematics and our knowledge of the laws of motion--a command and a knowledge which we owe directly to their persevering attempts to advance the science of astronomy1 and to follow after knowledge1 not for any material rewards which she had to offer1 but for her own sake4 #db ,,chapter ,,iv ,,astronomical ,,measurements ,the old proverb has it that ,science is measurement1 and of none of the sciences is this so true as of the science of astronomy4 ,indeed the measurement of time by observation of the movements of the heavenly bodies was the beginning of astronomy4 ,the movement of the ,sun gave the day1 which was reckoned to begin either at sunrise or at sunset4 ,the changes of the ,moon gave the month1 and in many languages the root meaning of the word for ,moon is measurer4 ,the apparent movement of the ,sun amongst the stars gave a yet longer division of time1 the year1 which could be determined in a number of different ways1 either from the ,sun alone1 or from the ,sun together with the stars4 ,a very simple and ancient form of instrument for measuring this movement of the ,sun was the obelisk1 a pillar with a pointed top set up on a level pavement4 ,such obelisks were common in ,egypt1 and one of the most celebrated1 known as ,cleopatra's ,needle1 now stands on the ,thames ,embankment4 ,as the ,sun moved in the sky1 the shadow of the pillar moved on the pavement1 and midday1 or noon1 was marked when the shadow was shortest4 ,the length of the shadow at noon varied from day to day2 it was shortest at mid-summer1 and longest at midwinter1 i4e4 at the summer and winter solstices4 ,twice in the year the shadow of the pillar pointed due west at sunrise1 and due east at #dc sunset--that is to say1 the shadow at the beginning of the day was in the same straight line as at its end4 ,these two days marked the two equinoxes of spring and autumn4 ,the obelisk was a simple means of measuring the height and position of the ,sun1 but it had its drawbacks4 ,the length of the shadow and its direction did not vary by equal amounts in equal times1 and if the pavement upon which the shadow fell was divided by marks corresponding to equal intervals of time for one day of the year1 the marks did not serve for all other days4 ,but if for the pillar a triangular wall was substituted--a wall rising from the pavement at the south and sloping up towards the north at such an angle that it seemed to point to the invisible pivot of the heavens1 round which all the stars appeared to revolve--then the shadow of the wall moved on the pavement in the same manner every day1 and the pavement if marked to show the hours for one day would show them for any day4 ,the sundials still often found in the gardens of country houses or in churchyards are miniatures of such an instrument4 ,but the ,greek astronomers devised other and better methods for determining the positions of the heavenly bodies4 ,obelisks or dials were of use only with the ,sun and ,moon which cast shadows4 ,to determine the position of a star1 sights like those of a rifle were employed1 and these were fixed to circles which were carefully divided1 generally into #cfj degrees4 ,as there are #cfe days in a year1 and as the ,sun makes a complete circuit of the ,zodiac in this time1 it moves very nearly a degree in a day4 ,the twelve ,signs of the ,zodiac are therefore each #cj in length1 and each #dd takes on the average a double-hour to rise or set4 ,while the ,sun and ,moon are each about half a degree in diameter1 i4e4 about one-sixtieth of the length of a ,sign1 and therefore take a double-minute to rise or set4 ,each degree of a circle is therefore divided into #fj minutes1 and each minute may be divided into #fj seconds4 ,as the ,sun or ,moon are each about half a degree1 or1 more exactly1 #cb minutes in diameter1 it is clear that1 so long as astronomical observations were made by the unaided sight1 a minute of arc 7written #a'7 was the smallest division of the circle that could be used4 ,a cord or wire can indeed be detected when seen projected against a moderately bright background if its thickness is a second of arc 7written #a7--;a sixtieth of a minute--but the wire is merely perceived1 not properly defined4 ,tycho ,brahe had achieved the utmost that could be done by the naked eye1 and it was the certainty that he could not have made a mistake in an observation in the place of the planet ,mars amounting to as much as #h minutes of arc--that is to say1 of a quarter the apparent diameter of the ,moon--that made ,kepler finally give up all attempts to explain the planetary movements on the doctrine of circular orbits and to try movements in an ellipse4 ,but a contemporary of ,kepler1 as gifted as he was himself1 but in a different direction1 was the means of increasing the observing power of the astronomer4 ,,galileo ,,galilei 7#aefd-afdb71 of a noble ,florentine family1 was appointed ,lecturer in ,mathematics at the ,university of ,pisa4 ,here he soon distinguished himself by his originality of thought1 and the ingenuity and decisiveness of his experiments4 ,up to that time it had been taught that of #de two bodies the heavier would fall to the ground more quickly than the lighter4 ,galileo let fall a #ajj-lb4 weight and a #a-lb4 weight from the top of the ,leaning ,tower1 and both weights reached the pavement together4 ,by this and other ingenious experiments he laid a firm foundation for the science of mechanics1 and he discovered the laws of motion which ,newton afterwards formulated4 ,he heard that an instrument had been invented in ,holland which seemed to bring distant objects nearer1 and1 having himself a considerable knowledge of optics1 it was not long before he made himself a little telescope4 ,he fixed two spectacle glasses1 one for long and one for short sight1 in a little old organ-pipe1 and thus made for himself a telescope which magnified three times4 ,before long he had made another which magnified thirty times1 and1 turning it towards the heavenly bodies1 he discovered dark moving spots upon the ,sun1 mountains and valleys on the ,moon1 and four small satellites revolving round ,jupiter4 ,he also perceived that ,venus showed phases--that is to say1 she changed her apparent shape just as the ,moon does--and he found the ,milky ,way to be composed of an immense number of small stars4 ,these discoveries were made in the years #afji-aa4 ,a telescope consists in principle of two parts--an object-glass1 to form an image of the distant object1 and an eye-piece1 to magnify it4 ,the rays of light from the heavenly body fall on the object-glass1 and are so bent out of their course by it as to be brought together in a point called the focus4 ,the light-gathering power of the telescope1 therefore1 depends upon the size of the object-glass1 and is proportional to its area4 ,but the size of the image depends upon the focal length of the telescope1 i4e4 upon the distance that the focus #df is from the object-glass4 ,thus a small disc1 an inch in diameter--such as a halfpenny--will exactly cover the full ,moon if held up nine feet away from the eye2 and necessarily the image of the full ,moon made by an object-glass of nine-feet focus will be an inch in diameter4 ,the eye-piece is a magnifying-glass or small microscope applied to this image1 and by it the image can be magnified to any desired amount which the quality of the object-glass and the steadiness of the atmosphere may permit4 ,this little image of the ,moon1 planet1 or group of stars lent itself to measurement4 ,a young ,english gentleman1 ,,gascoigne1 who afterwards fell at the ,battle of ,marston ,moor1 devised the micrometer for this purpose4 ,the micrometer usually has two frames1 each carrying one or more very thin threads--usually spider's threads--and the frames can be moved by very fine screws1 the number of turns or parts of a turn of each screw being read off on suitable scales4 ,by placing one thread on the image of one star1 and the other on the image of another1 the apparent separation of the two can be readily and precisely measured4 ,within the last thirty years photography has immensely increased the ease with which astronomical measurements can be made4 ,the sensitive photographic plate is placed in the focus of the telescope1 and the light of ,sun1 ,moon1 or stars1 according to the object to which the telescope is directed1 makes a permanent impression on the plate4 ,thus a picture is obtained1 which can be examined and measured in detail at any convenient time afterwards2 a portion of the heavens is1 as it were1 brought actually down to the astronomer's study4 ,it was long before this great advance was effected4 #dg ,the first telescopes were very imperfect1 for the rays of different colour proceeding from any planet or star came to different foci1 so that the image was coloured1 diffused1 and ill-defined4 ,the first method by which this difficulty was dealt with was by making telescopes of enormously long focal length2 #hj1 #ajj1 or #aej feet were not uncommon1 but these were at once cumbersome and unsteady4 ,sir ,isaac ,newton therefore discarded the use of object-glasses1 and used curved mirrors in order to form the image in the focus1 and succeeded in making two telescopes on this principle of reflection4 ,others followed in the same direction1 and a century later ,sir ,,william ,,herschel was most skilful and successful in making reflectors1 his largest being #dj feet in focal length1 and thus giving an image of the ,moon in its focus of nearly #d- inches diameter4 ,but in #agbi ,,chester ,,moor ,,hall found that by combining two suitable lenses together in the object-glass he could get over most of the colour difficulty1 and in #ageh the optician ,,dollond began to make object-glasses that were almost free from the colour defect4 ,from that time onward the manufacture of refractors1 as object-glass telescopes are called1 has improved2 the glass has been made more transparent and more perfect in quality1 and larger in size1 and the figure of the lens improved4 ,the largest refractor now in use is that of the ,yerkes ,observatory1 ,wisconsin1 ,u4,s4,a41 and is #dj inches in aperture1 with a focal length of #fe feet1 so that the image of the ,moon in its focus has a diameter of more than #g inches4 ,at present this seems to mark the limit of size for refractors1 and the difficulty of getting good enough glass for so large a lens is very great indeed4 ,reflectors have therefore come again into favour1 as mirrors can be made larger #dh than any object-glass4 ,thus ,lord ,rosse's great telescope was #f feet in diameter2 and the most powerful telescope now in action is the great #e-;foot mirror of the ,mt4 ,wilson ,observatory1 ,california1 with a focal length1 as sometimes used1 of #aej feet4 ,thus its light-gathering power is about #fj1jjj times that of the unaided eye1 and the full ,moon in its focus is #ag inches in diameter2 such is the enormous increase to man's power of sight1 and consequently to his power of learning about the heavenly bodies1 which the development of the telescope has afforded to him4 ,the measurement of time was the first purpose for which men watched the heavenly bodies2 a second purpose was the measurement of the size of the ,earth4 ,if at one place a star was observed to pass exactly overhead1 and if at another1 due south of it1 the same star was observed to pass the meridian one degree north of the zenith1 then by measuring the distance between the two places the circumference of the whole ,earth would be known1 for it would be #cfj times that amount4 ,in this way the size of the ,earth was roughly ascertained #bjjj years before the invention of the telescope4 ,but with the telescope measures of much greater precision could be made1 and hence far more difficult problems could be attacked4 ,one great practical problem was that of finding out the position of a ship when out of sight of land4 ,the ancient ,phoenician and ,greek navigators had mostly confined themselves to coasting voyages along the shores of the ,mediterranean ,sea1 and therefore the quick recognition of landmarks was the first requisite for a good sailor4 ,but when1 in #adib1 ,columbus had brought a new continent to light1 and long voyages were freely taken across the great oceans1 it became an urgent #di necessity for the navigator to find out his position when he had been out of sight of any landmark for weeks4 ,this necessity was especially felt by the nations of ,western ,europe1 the countries facing the ,atlantic with the ,new ,world on its far-distant other shore4 ,spain1 ,france1 ,england1 and ,holland1 all were eager competitors for a grasp on the new lands1 and therefore were earnest in seeking a solution of the problem of navigation4 ,the latitude of the ship could be found out by observing the height of the ,sun at noon1 or of the ,pole ,star at night1 or in several other ways4 ,but the longitude was more difficult4 ,as the ,earth turns on its axis1 different portions of its surface are brought in succession under the ,sun1 and if we take the moment when the ,sun is on the meridian of any place as its noon1 as twelve o'clock for that place1 then the difference of longitude between any two places is essentially the difference in their local times4 ,it was possible for the sailor to find out when it was local noon for him1 but how could he possibly find out what time it was at that moment at the port from which he had sailed1 perhaps several weeks before8 ,the ,moon and stars supplied eventually the means for giving this information4 ,for the ,moon moves amongst the stars1 as the hand of a clock moves amongst the figures of a dial1 and it became possible at length to predict for long in advance exactly where amongst the stars the ,moon would be1 for any given time1 of any selected place4 ,when this method was first suggested1 however1 neither the motion of the ,moon nor the places of the principal stars were known with sufficient accuracy1 and it was to remedy this defect1 and put navigation upon #ej a sound basis1 that ,,charles ,,ii4 founded ,greenwich ,observatory in the year #afge1 and appointed ,,flamsteed the first ,astronomer ,royal4 ,in the year #agfg ,,maskelyne1 the fifth ,astronomer ,royal1 brought out the first volume of the ,nautical ,almanac1 in which the positions of the ,moon relative to certain stars were given for regular intervals of ,greenwich time4 ,much about the same period the problem was solved in another way by the invention of the chronometer1 by ,,john ,,harrison1 a ,yorkshire carpenter4 ,the chronometer was a large watch1 so constructed that its rate was not greatly altered by heat or cold1 so that the navigator had ,greenwich time with him wherever he went4 ,the new method in the hands of ,,captain ,,cook and other great navigators led to a rapid development of navigation and the discovery of ,australia and ,new ,zealand1 and a number of islands in the ,pacific4 ,the building up of the vast oceanic commerce of ,great ,britain and of her great colonial empire1 both in ,north ,america and in the ,southern ,oceans1 has arisen out of the work of the ,royal ,observatory1 ,greenwich1 and has had a real and intimate connection with it4 ,to observe the motions of the ,moon1 ,sun1 and planets1 and to determine with the greatest possible precision the places of the stars have been the programme of ,greenwich ,observatory from its foundation to the present time4 ,other great national observatories have been ,copenhagen1 founded in #afcg2 ,paris1 in #affg2 ,berlin1 in #agjj2 ,st4 ,petersburg1 in #agbe1 superseded by that of ,pulkowa1 in #ahci2 and ,washington1 in #ahdb2 while not a few of the great universities have also efficient observatories connected with them4 ,of the directly practical results of astronomy1 the #ea promotion of navigation stands in the first rank4 ,but the science has never been limited to merely utilitarian inquiries1 and the problem of measuring celestial distances has followed on inevitably from the measurement of the ,earth4 ,the first distance to be attacked was that of the nearest companion to the ,earth1 i4e4 the ,moon4 ,it often happens on our own planet that it is required to find the distance of an object beyond our reach4 ,thus a general on the march may come to a river and need to know exactly how broad it is1 that he may prepare the means for bridging it4 ,such problems are usually solved on the following principle4 ,let ,a be the distant object4 ,then if the direction of ,a be observed from each of two stations1 ,b and ,c1 and the distance of ,b from ,c be measured1 it is possible to calculate the distances of ,a from ,b and from ,c4 ,the application of this principle to the measurement of the ,moon's distance was made by the establishment of an observatory at the ,cape of ,good ,hope1 to co-operate with that of ,greenwich4 ,it is1 of course1 not possible to see ,greenwich ,observatory from the ,cape1 or vice versa1 but the stars1 being at an almost infinite distance1 lie in the same direction from both observatories4 ,what is required then is to measure the apparent distance of the ,moon from the same stars as seen from ,greenwich and as seen from the ,cape1 and1 the distance apart of the two observatories being known1 the distance of the ,moon can be calculated4 ,this was a comparatively easy problem4 ,the next step in celestial measurement was far harder2 it was to find the distance of the ,sun4 ,the ,sun is #djj times as far off as the ,moon1 and therefore it seems to be practically in the same direction as seen from each of #eb the two observatories1 and1 being so bright1 stars cannot be seen near it in the telescope4 ,but by carefully watching the apparent movements of the planets their relative distances from the ,sun can be ascertained1 and were known long before it was thought possible that we should ever know their real distances4 ,thus ,venus never appears to travel more than #dg #ae' from the ,sun4 ,this means that her distance from the ,sun is a little more than seven-tenths of that of the ,earth4 ,if1 therefore1 the distance of one planet from the ,sun can be measured1 or the distance of one planet from the ,earth1 the actual distances of all the planets will follow4 ,we know the proportions of the parts of the solar system1 and1 if we can fix the scale of one of the parts1 we fix the scale of all4 ,it has been found possible to determine the distance of ,mars1 of several of the minor planets1 and especially of ,eros1 a very small minor planet that sometimes comes within #ac1jjj1jjj miles of the ,earth1 or seven times nearer to us than is the ,sun4 ,from the measures of ,eros1 we have learned that the ,sun is separated from us by very nearly #ic1jjj1jjj miles--an unimaginable distance4 ,perhaps the nearest way of getting some conception of this vast interval is by remembering that there are only #ca1eef1ibf seconds of time in a year4 ,if1 therefore1 an express train1 travelling #fj miles an hour--a mile a minute--set out for the ,sun1 and travelled day and night without cease1 it would take more than #ahj years to accomplish the journey4 ,but this astronomical measure has led on to one more daring still4 ,the earth is on one side of the ,sun in ,january1 on the other in ,july4 ,at these two dates1 therefore1 we are occupying stations #ahf1jjj1jjj miles #ec apart1 and can ascertain the apparent difference in direction of the stars as viewed from the two points ,but the astonishing result is that this enormous change in the position of the ,earth makes not the slightest observable difference in the position of most of the stars4 ,a few1 a very few1 do show a very slight difference4 ,the nearest star to us is about #bhj1jjj times as far from us as the ,sun2 this is ,alpha ,centauri1 the brightest star in the constellation of the ,centaur and the third brightest star in the sky4 ,sirius1 the brightest star1 is twice this distance4 ,some forty or fifty stars have had their distances roughly determined2 but the stars in general far transcend all our attempts to plumb their distances4 ,but1 from certain indirect hints1 it is generally supposed that the mass of stars in the ,milky ,way are something like #cjj1jjj1jjj times as far from us as we are from our ,sun4 ,thus far1 then1 astronomy has led us in the direction or measurement4 ,it has enabled us to measure the size of the ,earth upon which we live1 and to find out the position of a ship in the midst of the trackless ocean4 ,it has also enabled us to cast a sounding-line into space1 to show how remote and solitary the earth moves through the void1 and to what unimaginable lengths the great stellar universe1 of which it forms a secluded atom1 stretches out towards infinity4 #ed ,,chapter ,v ,,the ,,members ,,of ,,the ,,solar ,,system ,astronomical measurement has not only given us the distances of the various planets from the ,sun2 it has also furnished us1 as in the annexed table1 with their real diameters1 and1 as a consequence of the law of gravitation1 with their densities and weights1 and the force of gravity at their surfaces4 ,and these numerical details are of the first importance in directing us as to the inferences that we ought to draw as to their present physical conditions4 ,the theory of ,copernicus deprived the ,earth of its special position as the immovable centre of the universe1 but raised it to the rank of a planet4 ,it is therefore a heavenly body1 yet needs no telescope to bring it within our ken2 bad weather does not hide it from us1 but rather shows it to us under new conditions4 ,we find it to be a globe of land and water1 covered by an atmosphere in which float changing clouds2 we have mapped it1 and we find that the land and water are always there1 but their relations are not quite fixed2 there is give and take between them4 ,we have found of what elements the land and water consist1 and how these elements combine with each other or dissociate4 ,in a word1 the ,earth is the heavenly body that we know the best1 and with it we must compare and contrast all the others4 ,before the invention of the telescope there were but #ee two other heavenly bodies--the ,sun and the ,moon--that appeared as orbs showing visible discs1 and even in their cases nothing could be satisfactorily made out as to their conditions4 ,now each of the five planets known to the ancients reveals to us in the telescope a measurable disc1 and we can detect significant details on their surfaces4 ,,the ,,moon is the one object in the heavens which does not disappoint a novice when he first sees it in the telescope4 ,every detail is hard1 clear-cut1 and sharp2 it is manifest that we are looking at a globe1 a very rough globe1 with hills and mountains1 plains and valleys1 the whole in such distinct relief that it seems as if it might be touched4 ,no clouds ever conceal its details1 no mist ever softens its outlines2 there are no half-lights1 its shadows are dead black1 its high lights are molten silver4 ,certain changes of illumination go on with the advancing age of the ,moon1 as the crescent broadens out to the half1 the half to the full1 and the full1 in its turn1 wanes away2 but the lunar day is nearly thirty times as long as that of the ,earth1 and these changes proceed but slowly4 ,the full ,moon1 as seen by the naked eye1 shows several vague dark spots1 which most people agree to fancy as like the eyes1 nose1 and mouth of a broad1 sorrowful face4 ,the ordinary astronomical telescope inverts the image1 so the eyes of the ,moon are seen in the lower part of the field of the telescope as a series of dusky plains stretching right across the disc4 ,but in the upper part1 near the left-hand corner of the underlip1 there is a bright1 round spot1 from which a number of bright streaks radiate--suggesting a peeled orange with its stalk1 and the lines marking the sections radiating from it4 ,this bright spot has been called after the great #ef ,mean distance from ,sun4 ,period ,velocity ,class4 ,name4 ,earth's ,in millions of revolution4 in orbit4 ,eccentricity4 distance of miles4 ,in years4 ,miles per #a4 sec4 ,terrestrial ,mercury #j4chg #cf4j #j4bd #bi4g #j4bjef ,planets ,venus #j4gbc #fg4b #j4fb #ba4i #j4jjfh ,earth #a4jjj #ib4i #a4jj #ah4e #j4jafh ,mars #a4ebd #ada4e #a4hh #ae4j #j4jicc ,minor ,eros #a4deh #ace4e #a4gf #ae4e #j4bbbh ,planets ,ceres #b4gfg #beg4a #d4fj #aa4a #j4jgfc ,achilles #e4bec #dhh4j #ab4jd #h4a #j4jeji ,major ,jupiter #e4bjc #dhc4c #aa4hf #h4a #j4jdhc ,planets ,saturn #i4eci #hhf4f #bi4df #f4j #j4jefa ,uranus #ai4ahc #agha4i #hd4jb #d4b #j4jdfc ,neptune #cj4jee #bgia4f #afd4gh #c4d #j4jjij #eg ,mean diameter4 ,surface4 ,volume4 ,mass4 ,name4 ,symbol4 ,in miles4 ,earth#a4 ,earth#a4 ,earth#a4 ,earth#a4 ,sun ,sun #hffdjj #aji4dbb #aaigc4 #acajacj4 #ccbjjj4 ,moon ,moon #bafc #j4bgc #j4jge #j4jb #j4jab ,mercury ,mercury #cjcj #j4chc #j4adg #j4jf #j4jdh ,venus ,venus #ggjj #j4igb #j4ide #j4ib #j4hbj ,earth ,earth #giah #a4jjj #a4jjj #a4jj #a4jjj ,mars ,mars #dbcj #j4ecd #j4bhe #j4ae #j4ajg ,jupiter ,jupiter #hfejj #aj4ibd #aai4c #acjd4 #cag4g ,saturn ,saturn #gcjjj #i4bai #he4j #ghc4 #id4h ,uranus ,uranus #caijj #d4jbi #af4b #fe4 #ad4f ,neptune ,neptune #cdhjj #d4cie #ai4c #he4 #ag4j #eh ,light ,gravity4 and heat ,albedo2 ,density4 ,fall in received i4e4 re- ,earth ,water ,earth feet per from ,sun4 ,time of rotation flecting ,name4 #a4 #a4 #a4 sec4 ,earth#a4 on axis4 power4 d4 h4 m4 ,sun #j4be #a4ci #bg4fe #ddd4fj 444 #be #d #dh 444 ,moon #j4fa #c4ci #j4ag #b4gc #a4jj #bg #g #dc #j4ag d4 h4 m4 s4 ,mercury #j4he #d4gb #j4dc #f4ia #f4fg #hh 787 #j4ad ,venus #j4hi #d4id #j4hb #ac4ai #a4ia #bc #ba #bc 787 #j4gf ,earth #a4jj #e4ee #a4jj #af4jh #a4jj #bc #ef #d #j4ej 787 ,mars #j4ga #c4ib #j4ch #f4aa #j4dc #bd #cg #bc #j4bb h4 m4 ,jupiter #j4bd #a4cb #b4fe #db4fa #j4jcg #i #ee #j4fb ,saturn #j4ac #j4gb #a4ah #ah4ig #j4jaa #aj #ad #j4gb ,uranus #j4bb #a4bb #j4ij #ad4dg #j4jjc #i #cj 787 #j4fj ,neptune #j4bj #a4aa #j4hi #ad4ca #j4jja 787 #j4eb #ei ,danish astronomer1 ,tycho1 and is one of the most conspicuous objects of the full ,moon4 ,the contrasts of the ,moon are much more pronounced when she is only partly lit up4 ,then the mountains and valleys stand out in the strongest relief1 and it becomes clear that the general type of formation on the ,moon is that of rings--rings of every conceivable size1 from the smallest point that the telescope can detect up to some of the great dusky plains themselves1 hundreds of miles in diameter4 ,these rings are so numerous that ,galileo described the ,moon as looking as full of eyes as a peacock's tail4 ,the right eye of the moonface1 as we see it in the sky1 is formed by a vast dusky plain1 nearly as large as ,france and ,germany put together1 to which has been given the name of the ,sea of ,rains 7,mare ,imbrium71 and just below this 7as seen in the telescope7 is one of the most perfect and beautiful of all the lunar rings--a great ring-plain1 #ef miles in diameter1 called after the thinker who revolutionised men's ideas of the solar system1 ,copernicus4 ,copernicus1 like ,tycho1 is the centre of a set of bright streaks2 and a neighbouring but smaller ring1 bearing the great name of ,kepler1 stands in a like relation to another set4 ,the most elevated region of the ,moon is immediately in the neighbourhood of the great stalk of the orange1 ,tycho4 ,here the rings are crowded together as closely as they can be packed2 more closely in many places1 for they intrude upon and overlap each other in the most intricate manner4 ,a long chain of fine rings stretches from this disturbed region nearly to the centre of the disc1 where the great ,alexandrian astronomer is commemorated by a vast walled plain1 #fj considerably larger than the whole of ,wales1 and known as ,ptolemus4 ,the distinctness of the lunar features shows at once that the ,moon is in an altogether different condition from that of the ,earth4 ,here the sky is continually being hidden by cloud1 and hence the details of the surface of the ,earth as viewed from any other planet must often be invisible1 and even when actual cloud is absent there is a more permanent veil of dust1 which must greatly soften and confuse terrestrial outlines4 ,the clearness1 therefore1 with which we perceive the lunar formations proves that there is little or no atmosphere there4 ,nor is there any sign upon it of water1 either as seas or lakes or running streams4 ,yet the ,moon shows clearly that in the past it has gone through great and violent changes4 ,the gradation is so complete from the little craterlets1 which resemble closely1 in form and size1 volcanic craters on the ,earth1 up to the great ring-plains1 like ,copernicus or ,tycho1 or formations larger still1 that it seems natural to infer not only that the smaller craters were formed by volcanic eruption1 like the similar objects with which we are acquainted on our own ,earth1 but that the others1 despite their greater sizes1 had a like origin4 ,in consequence of the feebler force of gravity on the ,moon1 the same explosive force there would carry the material of an eruption much further than on the ,earth4 ,the darker1 low-lying districts of the ,moon give token of changes of a different order4 ,it is manifest that the material of which the floors of these plains is composed has invaded1 broken down1 and almost submerged many of the ring-formations4 ,sometimes half #fa of a ring has been washed away2 sometimes just the outline of a ring can still be traced upon the floor of the sea2 sometimes only a slight breach has been made in the wall4 ,so it is clear that the ,moon was once richer in the great crater-like formations than it is to-day1 but a lava-flood has overflowed at least one-third of its area4 ,more recent still are the bright streaks1 or rays1 which radiate in all directions from ,tycho1 and from some of the other ring-plains4 ,it is evident from these different types of structure on the ,moon1 and from the relations which they bear to each other1 that the lunar surface has passed through several successive stages1 and that its changes tended1 on the whole1 to diminish in violence as time went on2 the minute crater pits with which the surface is stippled having been probably the last to form4 ,but the #cjj years during which the ,moon has been watched with the telescope have afforded no trace of any continuance of these changes4 ,she has had a stormy and fiery past2 but nothing like the events of those bygone ages disturbs her serenity to-day4 ,and yet we must believe that change does take place on the ,moon even now1 because during the #ced hours of its long day the ,sun beats down with full force on the unprotected surface1 and during the equally long night that surface is exposed to the cold of outer space4 ,every part of the surface must be exposed in turn to an extreme range of temperature1 and must be cracked1 torn1 and riven by alternate expansion and contraction4 ,apart from this slow1 wearing process1 and a very few rather doubtful cases in which a minute alteration of some surface detail has been suspected1 our sister planet1 the ,moon1 shows herself as changeless and inert1 without any appreciable trace of air or water or any sign #fb of life--a dead world1 with all its changes and activities in the past4 ,,mars1 after the ,moon1 is the planet whose surface we can study to best advantage4 ,its orbit lies outside that of the ,earth1 so that when it is nearest to us it turns the same side to both the ,sun and ,earth1 and we see it fully illuminated4 ,mercury and ,venus1 on the contrary1 when nearest us are between us and the ,sun1 and turn their dark sides to us4 ,when fully illuminated they are at their greatest distance1 and appear very small1 and1 being near the ,sun1 are observed with difficulty4 ,these three are intermediate in size between the ,moon and the ,earth4 ,in early telescopic days it was seen that ,mars was an orange-coloured globe with certain dusky markings upon it1 and that these markings slowly changed their place--that1 in short1 it was a world rotating upon its axis1 and in a period not very different from that of the ,earth4 ,the rotation period of ,mars has indeed been fixed to the one-hundredth part of a second of time2 it is #bd h4 #cg m4 #bb4fg s4 ,and this has been possible because some of the dusky spots observed in the seventeenth century can be identified now in the twentieth4 ,some of the markings on ,mars1 like our own continents and seas1 and like the craters on the ,moon1 are permanent features2 and many charts of the planet have been constructed4 ,other markings are variable4 ,since the planet rotates on its axis1 the positions of its poles and equator are known1 its equator being inclined to its orbit at an angle of #bd #ej'1 while the angle in the case of the ,earth is #bc #bg'4 ,the times when its seasons begin and end are therefore known2 and it is found that the spring of its northern hemisphere lasts #aii of our #fc days1 the summer #ahc1 the autumn #adg1 and the winter #aeh4 ,round the pole in winter a broad white cap forms1 which begins to shrink as spring comes on1 and may entirely disappear in summer4 ,no corresponding changes have been observed on the ,moon1 but it is easy to find an analogy to them on the ,earth4 ,round both our poles a great cap of ice and snow is spread--a cap which increases in size as winter comes on1 and diminishes with the advance of summer--and it seems a reasonable inference to suppose that the white polar caps of ,mars are1 like our own1 composed of ice and snow4 ,from time to time indications have been observed of the presence on ,mars of a certain amount of cloud4 ,familiar dark markings have1 for a short time1 been interrupted1 or been entirely hidden1 by white bands1 and have recovered their ordinary appearance later4 ,with rotation on its axis and succession of seasons1 with atmosphere and cloud1 with land and water1 with ice and snow1 ,mars would seem to be a world very similar to our own4 ,this was the general opinion up to the year #ahgg1 when ,,schiaparelli announced that he had discovered a number of very narrow1 straight1 dark lines on the planet--lines to which he gave the name of canali--that is1 channels4 ,this word was unfortunately rendered into ,english by the word canals1 and1 as a canal means a waterway artificially made1 this mistranslation gave the idea that ,mars was inhabited by intelligent beings1 who had dug out the surface of the planet into a network of canals of stupendous length and breadth4 ,the chief advocate of this theory is ,,lowell1 an ,american observer1 who has given very great attention #fd to the study of the planet during the last seventeen years4 ,his argument is that the straight lines1 the canals1 which he sees on the planet1 and the round dots1 the oases1 which he finds at their intersections1 form a system so obviously unnatural1 that it must be the work of design--of intelligent beings4 ,the canals are to him absolutely regular and straight1 like lines drawn with ruler and pen-and-ink1 and the oases are exactly round4 ,but1 on the one hand1 the best observers1 armed with the most powerful telescopes1 have often been able to perceive that markings were really full of irregular detail1 which ,lowell has represented as mere hard straight lines and circular dots1 and1 on the other hand1 the straight line and the round dot are the two geometric forms which all very minute objects must approach in appearance4 ,that we cannot see irregularities in very small and distant objects is no proof at all that irregularities do not exist in them1 and it has often happened that a marking which appeared a typical canal when ,mars was at a great distance lost that appearance when the planet was nearer4 ,astronomers1 therefore1 are almost unanimous that there is no reason for supposing that any of the details that we see on the surface of ,mars are artificial in their origin4 ,and indeed the numerical facts that we know about the planet render it almost impossible that there should be any life upon it4 ,if we turn to the table1 we see that in size1 volume1 density1 and force of gravity at its surface1 ,mars lies between the ,moon and the ,earth1 but is nearer the ,moon4 ,this has an important bearing as to the question of the planet's atmosphere4 ,on the ,earth we pass through half the atmosphere by ascending a mountain #fe that is three and a third miles in height2 on ,mars we should have to ascend nearly nine miles4 ,if the atmospheric pressure at the surface of ,mars were as great as it is at the surface of the ,earth1 his atmosphere would be far deeper than ours and would veil the planet more effectively4 ,but we see the surface of ,mars with remarkable distinctness1 almost as clearly1 when its greater distance is allowed for1 as we see the ,moon4 ,it is therefore accepted that the atmospheric pressure at the surface of ,mars must be very slight1 probably much less than at the top of our very highest mountains1 where there is eternal snow1 and life is completely absent4 ,but ,mars compares badly with the ,earth in another respect4 ,it receives less light and heat from the ,sun in the proportion of three to seven4 ,this we may express by saying that ,mars1 on the whole1 is almost as much worse off than the ,earth as a point on the ,arctic ,circle is worse off than a point on the ,equator4 ,the mean temperature of the ,earth is taken as about #fj of the ,fahrenheit thermometer 7say1 #ae ,cent472 the mean temperature of ,mars must certainly be considerably below freezing-point1 probably near #j ,f4 ,here on our ,earth the boiling-point of water is #bab1 and1 since the mean temperature is #fj and water freezes at #cb1 it is normally in the liquid state4 ,on ,mars it must normally be in the solid state--ice1 snow1 or frost1 or the like4 ,but with so rare an atmosphere water will boil at a low temperature1 and it is not impossible that under the direct rays of the ,sun--that is to say1 at midday of the torrid zone of ,mars--ice may not only melt1 but water boil by day1 condensing and freezing again during the night4 ,,newcomb1 the foremost astronomer of his day1 concluded that during #ff the night of ,mars1 even in the equatorial regions1 the surface of the planet probably falls to a lower temperature than any we ever experienced on our globe4 ,if any water exists1 it must not only be frozen1 but the temperature of the ice must be far below the freezing point4444 ,the most careful calculation shows that if there are any considerable bodies of water on our neighbouring planet1 they exist in the form of ice1 and can never be liquid to a depth of more than one or two inches1 and that only within the torrid zone and during a few hours each day4 ,with regard to the snow caps of ,mars1 ,newcomb thought it not possible that any considerable fall of snow could ever take place4 ,he regarded the white caps as simply due to a thin deposit of hoar frost1 and it cannot be deemed wonderful that such should gradually disappear1 when it is remembered that each of the two poles of ,mars is in turn presented to the ,sun for more than #cjj consecutive days4 ,newcomb's conclusion was3 ,thus we have a kind of ,martian meteorological changes1 very slight indeed1 and seemingly very different from those of our ,earth1 but yet following similar lines on their small scale4 ,for snowfall substitute frostfall2 instead of 7the barometer reading7 feet or inches say fractions of a millimetre1 and instead of storms or wind substitute little motions of an air thinner than that on the top of the ,himalayas1 and we shall have a general description of ,martian meteorology4 ,we conclude1 then1 that ,mars is not so inert a world as the ,moon1 but1 though some slight changes of climate or weather take place upon it1 it is quite unfitted for the nourishment and development of the different forms of organic life4 ,of ,,mercury we know very little4 ,it is smaller than ,mars but larger than the ,moon1 but it differs from them #fg both in that it is much nearer the ,sun1 and receives1 therefore1 many times the light and heat1 surface for surface4 ,we should expect1 therefore1 that water on ,mercury would exist in the gaseous state instead of in the solid state as on ,mars4 ,the little planet reflects the sunlight only feebly1 and shows no evidence of cloud4 ,a few markings have been made out on its surface1 and the best observers agree that it appears to turn the same face always to the ,sun4 ,this would imply that the one hemisphere is in perpetual darkness and cold1 the other1 exposed to an unimaginable fiery heat4 ,,venus is nearly of the same size as the ,earth1 and the conditions as to the arrangement of its atmosphere1 the force of gravity at its surface1 must be nearly the same as on our own world4 ,but we know almost nothing of the details of its surface2 the planet is very bright1 reflecting fully seven-tenths of the sunlight that falls upon it4 ,it would seem that1 in general1 we see nothing of the actual details of the planet1 but only the upper surface of a very cloudy atmosphere4 ,owing to the fact that ,venus shows no fixed definite marking that we can watch1 it is still a matter of controversy as to the time in which it rotates upon its axis4 ,schiaparelli and some other observers consider that it rotates in the same time as it revolves round the ,sun4 ,others believe that it rotates in a little less than twenty-four hours4 ,if this be so1 and there is any body in the solar system other than the ,earth1 which is adapted to be the home of life1 then the planet ,venus is that one4 ,,the ,,sun1 like the ,moon1 presents a visible surface to the naked eye1 but one that shows no details4 ,in the telescope the contrast between it and the ,moon is very great1 and still greater is the contrast which is brought #fh out by the measurements of its size1 volume1 and weight4 ,but the really significant difference is that the ,sun is a body giving out light and heat1 not merely reflecting them4 ,without doubt this last difference is connected most closely with the difference in size4 ,the ,moon is cold1 dead1 unchanging1 because it is a small world2 the ,sun is bright1 fervent1 and undergoes the most violent change1 because it is an exceedingly large world4 ,the two bodies--the ,sun and ,moon--appear to the eye as being about the same size1 but since the ,sun is #djj times as far off as the ,moon it must be #djj times the diameter4 ,that means that it has #djj times #djj1 or #afj1jjj times the surface and #djj times #djj times #djj1 or #fd1jjj1jjj times the volume4 ,the ,sun and ,moon1 therefore1 stand at the very extremes of the scale4 ,the heat of the ,sun is so great that there is some difficulty in observing it in the telescope4 ,it is not sufficient to use a dark glass in order to protect the eye1 unless the telescope be quite a small one4 ,some means have to be employed to get rid of the greater part of the heat and light4 ,the simplest method of observing is to fix a screen behind the eyepiece of a telescope and let the image of the ,sun be projected upon the screen1 or the sensitive plate may be substituted for the screen1 and a photograph obtained1 which can be examined at leisure afterwards4 ,as generally seen1 the surface of the ,sun appears to be mottled all over by a fine irregular stippling4 ,this stippling1 though everywhere present1 is not very strongly marked1 and a first hasty glance might overlook it4 ,from time to time1 however1 dark spots are seen1 of ever-changing form and size4 ,by watching these1 ,galileo proved that the ,sun rotated on its axis in a little more than twenty-five days1 and in the #fi nineteenth century ,,schwabe proved that the sunspots were not equally large and numerous at all times1 but that there was a kind of cycle of a little more than eleven years in average length4 ,at one time the ,sun would be free from spots2 then a few small ones would appear2 these would gradually become larger and more numerous2 then a decline would follow1 and another spotless period would succeed about eleven years after the first4 ,as a rule1 the increase in the spots takes place more quickly than the decline4 ,most of the spot-groups last only a very few days1 but about one group in four lasts long enough to be brought round by the rotation of the ,sun a second time2 in other words1 it continues for about a month4 ,in a very few cases spots have endured for half a year4 ,an ordinary form for a group of spots is a long stream drawn out parallel to the ,sun's equator1 the leading spot being the largest and best defined4 ,it is followed by a number of very small irregular and ill-developed spots1 and the train is brought up by a large spot1 sometimes even larger than the leader1 but by no means so regular in form or so well defined4 ,the leading spot for a short time moves forward much faster than its followers1 at a speed of about #hjjj miles per day4 ,the small middle spots then gradually die out1 or rather seem to be overflowed by the bright material of the solar surface1 the photosphere1 as it is called2 the spot in the rear breaks up a little later1 and the leader1 which is now almost circular1 is left alone1 and may last in this condition for some weeks4 ,finally1 it slowly contracts or breaks up1 and the disturbance comes to an end4 ,this is the course of development of many long-lived spot-groups1 but all do not conform to the same type4 #gj ,the very largest spots are indeed usually quite different in their appearance and history4 ,in size1 sunspots vary from the smallest dot that can be discovered in the telescope up to huge rents with areas that are to be counted by thousands of millions of square miles2 the great group of ,february #aije had an area of #d1jjj1jjj1jjj square miles1 a thousand times the area of ,europe4 ,closely associated with the macul1 as the spots were called by the first observers1 are the facul--long1 branching lines of bright white light1 bright as seen even against the dazzling background of the ,sun itself1 and looking like the long lines of foam of an incoming tide4 ,these are often associated with the spots2 the spots are formed between their ridges1 and after a spot-group has disappeared the broken waves of facul will sometimes persist in the same region for quite a long time4 ,the facul clearly rise above the ordinary solar surface2 the spots as clearly are depressed a little below it2 because from time to time we see the bright material of the surface pour over spots1 across them1 and sometimes into them4 ,but there is no reason to believe that the spots are deep1 in proportion either to the ,sun itself or even to their own extent4 ,sunspots are not seen in all regions of the ,sun4 ,it is very seldom that they are noted in a higher solar latitude than #dj1 the great majority of spots lying in the two zones between #e and #be latitude on either side of the equator4 ,facul1 on the other hand1 though most frequent in the spot zones1 are observed much nearer the two poles4 ,it is very hard to find analogies on our ,earth for sunspots and for their peculiarities of behaviour4 ,some #ga of the earlier astronomers thought they were like terrestrial volcanoes1 or rather like the eruptions from them4 ,but if there were a solid nucleus to the ,sun1 and the spots were eruptions from definite areas of the nucleus1 they would all give the same period of rotation4 ,but sunspots move about freely on the solar surface1 and the different zones of that surface rotate in different times1 the region of the equator rotating the most quickly4 ,this alone is enough to show that the ,sun is essentially not a solid body4 ,yet far down below the photosphere something approaching to a definite structure must already be forming4 ,for there is a well-marked progression in the zones of sunspots during the eleven-year cycle4 ,at a time when spots are few and small1 known as the sunspot minimum1 they begin to be seen in fairly high latitudes4 ,as they get more numerous1 and many of them larger1 they frequent the medium zones4 ,when the ,sun is at its greatest activity1 known as the sunspot maximum1 they are found from the highest zone right down to the equator4 ,then the decline sets in1 but it sets in first in the highest zones1 and when the time of minimum has come again the spots are close to the equator4 ,before these have all died away1 a few small spots1 the heralds of a new cycle of activity1 begin to appear in high latitudes4 ,this law1 called after ,,sp,,rer1 its discoverer1 indicates that the origin and source of sunspot activity lie within the ,sun4 ,at one time it was thought that sunspots were due to some action of ,jupiter--for ,jupiter moves round the ,sun in #aa4h years1 a period not very different from the sunspot cycle--or to some meteoric stream4 ,but ,sprer's ,law could not be imposed by some influence from without4 ,still sunspots1 once formed1 may be influenced by the ,earth1 and perhaps by other #gb planets also1 for ,,mrs4 ,,walter ,,maunder has shown that the numbers and areas of spots tend to be smaller on the western half of the disc1 as seen from the ,earth1 than on the eastern1 while considerably more groups come into view at the east edge of the ,sun than pass out of view at the west edge1 so that it would appear as if the ,earth had a damping effect upon the spots exposed to it4 ,but the ,sun is far greater than it ordinarily appears to us4 ,twice every year1 and sometimes oftener1 the ,moon1 when new1 comes between the ,earth and the ,sun1 and we have an ,eclipse of the ,sun1 the dark body of the ,moon hiding part1 or all1 of the greater light4 ,the ,sun and ,moon are so nearly of the same apparent size that an eclipse of the ,sun is total only for a very narrow belt of the ,earth's surface1 and1 as the ,moon moves more quickly than the ,sun1 the eclipse only remains total for a very short time--seven minutes at the outside1 more usually only two or three4 ,north or south of that belt the ,moon is projected1 so as to leave uncovered a part of the ,sun north or south of the ,moon4 ,a total eclipse1 therefore1 is rare at any particular place1 and if a man were able to put himself in the best possible position on each occasion1 it would cost him thirty years to secure an hour's accumulated duration4 ,eclipses of the ,moon are visible over half the world at one time1 for there is a real loss to the ,moon of her light4 ,her eclipses are brought about when1 in her orbit1 she passes behind the ,earth1 and the ,earth1 being between the ,sun and the ,moon1 cuts off from the latter most of the light falling upon her2 not quite all2 a small portion reaches her after passing through the thickest part of the ,earth's atmosphere1 so that the #gc ,moon in an eclipse looks a deep copper colour1 much as she does when rising on a foggy evening4 ,total eclipses of the ,sun have well repaid all the efforts made to observe them4 ,it is a wonderful sight to watch the blackness of darkness slowly creeping over the very fountain of light until it is wholly and entirely hidden2 to watch the colours fade away from the landscape and a deathlike1 leaden hue pervade all nature1 and then to see a silvery1 star-like halo1 flecked with bright little rose-coloured flames1 flash out round the black disc that has taken the place of the ,sun4 ,these rose-coloured flames are the solar prominences1 and the halo is the corona1 and it is to watch these that astronomers have made so many expeditions hither and thither during the last seventy years4 ,the prominences1 or red flames1 can be observed1 without an eclipse1 by means of the spectroscope1 but1 as the work of the spectroscope is to form the subject of another volume of this series1 it is sufficient to add here that the prominences are composed of various glowing gases1 chiefly of hydrogen1 calcium1 and helium4 ,these and other gases form a shell round the ,sun1 about #cjjj miles in depth1 to which the name chromosphere has been given4 ,it is out of the chromosphere that the prominences arise as vast irregular jets and clouds4 ,ordinarily they do not exceed #dj or #ej thousand miles in height1 but occasionally they extend for #bjj or even #cjj thousand miles from the ,sun4 ,their changes are as remarkable as their dimensions2 huge jets of #ej or #ajj thousand miles have been seen to form1 rise1 and disappear within an hour or less1 and movements have been chronicled of #bjj or #cjj miles in a single second of time4 ,prominences are largest and most frequent when #gd sunspots and facul are most frequent1 and fewest when those are fewest4 ,the corona1 too1 varies with the sunspots4 ,at the time of maximum the corona sends forth rays and streamers in all directions1 and looks like the conventional figure of a star on a gigantic scale4 ,at minimum the corona is simpler in form1 and shows two great wings1 east and west1 in the direction of the ,sun's equator1 and round both of his poles a number of small1 beautiful jets like a crest of feathers4 ,some of the streamers or wings of the corona have been traced to an enormous distance from the ,sun4 ,mrs4 ,walter ,maunder photographed one ray of the corona of #ahih to a distance of #f millions of miles4 ,,langley1 in the clear air of ,pike's ,peak1 traced the wings of the corona of #ahgh with the naked eye to nearly double this distance4 ,but the rapid changes of sunspots and the violence of some of the prominence eruptions are but feeble indications of the most wonderful fact concerning the ,sun1 i4e4 the enormous amount of light and heat which it is continually giving off4 ,here we can only put together figures which by their vastness escape our understanding4 ,sunlight is to moonlight as #fjj1jjj is to #a1 so that if the entire sky were filled up with full moons1 they would not give us a quarter as much light as we derive from the ,sun4 ,the intensity of sunlight exceeds by far any artificial light2 it is #aej times as bright as the calcium light1 and three or four times as bright as the brightest part of the electric arc light4 ,the amount of heat radiated by the ,sun has been expressed in a variety of different ways2 ,c4 ,a4 ,,young very graphically by saying that if the ,sun were encased in a shell of ice #fd feet deep1 its heat would melt the shell in one minute1 and that if a bridge of ice could be #ge formed from the ,earth to the ,sun1 #b- miles square in section and #ic millions of miles long1 and the entire solar radiation concentrated upon it1 in one second the ice would be melted1 in seven more dissipated into vapour4 ,the ,earth derives from the ,sun not merely light and heat1 but1 by transformation of these1 almost every form of energy manifest upon it2 the energy of the growth of plants1 the vital energy of animals1 are only the energy received from the ,sun1 changed in its expression4 ,the question naturally arises1 ,if the ,sun1 to which the ,earth is indebted for nearly everything1 passes through a change in its activity every eleven years or so1 how is the ,earth affected by it8 ,it would seem at first sight that the effect should be great and manifest4 ,a sunspot1 like that of ,february #aije1 one thousand times as large as ,europe1 into which worlds as large as our ,earth might be poured1 like peas into a saucer1 must mean1 one might think1 an immense falling off of the solar heat4 ,yet it is not so4 ,for even this great sunspot was but small as compared with the ,sun as a whole4 ,had it been dead black1 it would have stopped out much less than #a per cent4 of the ,sun's heat2 and even the darkest sunspot is really very bright4 ,and the more spots there are1 the more numerous and brighter are the facul2 so that we do not know certainly which of the two phases1 maximum or minimum1 means the greater radiation4 ,if the weather on the ,earth answers to the sunspot cycle1 the connection is not a simple one2 as yet no connection has been proved4 ,thus two of the worst and coldest summers experienced in ,england fell the one in #ahfj1 the other in #ahgi1 i4e4 at #gf maximum and minimum respectively4 ,so1 too1 the hot summers of #ahic and #aiaa were also1 the one at maximum and the other at minimum2 and ordinary average years have fallen at both the phases just the same4 ,yet there is an answer on the part of the ,earth to these solar changes4 ,the ,earth itself is a kind of magnet1 possessing a magnetism of which the intensity and direction is always changing4 ,to watch these changes1 very sensitive magnets are set up1 and a slight daily to-and-fro swing is noticed in them2 this swing is more marked in summer than in winter1 but it is also more marked at times of the sunspot maximum than at minimum1 showing a dependence upon the solar activity4 ,yet more1 from time to time the magnetic needle undergoes more or less violent disturbance2 in extreme cases the electric telegraph communication has been disturbed all over the world1 as on ,september #be1 #aiji1 when the submarine cables ceased to carry messages for several hours4 ,in most cases when such a magnetic storm occurs1 there is an unusually large or active spot on the ,sun4 ,the writer was able in #aijd to further prove that such storms have a marked tendency to recur when the same longitude of the ,sun is presented again towards the ,earth4 ,thus in ,february #ahib1 when a very large spot was on the ,sun1 a violent magnetic storm broke out4 ,the spot passed out of sight and the storm ceased1 but in the following month1 when the spot reached exactly the same apparent place on the ,sun's disc1 the storm broke out again4 ,such magnetic disturbances are therefore due to streams of particles driven off from limited areas of the ,sun1 probably in the same way that the long1 #gg straight rays of the corona are driven off4 ,such streams of particles1 shot out into space1 do not spread out equally in all directions1 like the rays of light and heat1 but are limited in direction1 and from time to time they overtake the ,earth in its orbit1 and1 striking it1 cause a magnetic storm1 which is felt all over the ,earth at practically the same moment4 ,,jupiter is1 after the ,sun1 much the largest member of the solar system1 and it is a peculiarly beautiful object in the telescope4 ,even a small instrument shows the little disc striped with many delicately coloured bands or belts1 broken by white clouds and dark streaks1 like a windy sky at sunset4 ,and it changes while being watched1 for1 though #djj1jjj1jjj miles away from us1 it rotates so fast upon its axis that its central markings can actually be seen to move4 ,this rapid rotation1 in less than ten hours1 is the most significant fact about ,jupiter4 ,for different spots have different rotation periods1 even in the same latitude1 proving that we are looking down not upon any solid surface of ,jupiter1 but upon its cloud envelope--an envelope swept by its rapid rotation and by its winds into a vast system of parallel currents4 ,one object on ,jupiter1 the great ,red ,spot1 has been under observation since #ahgh1 and possibly for #bjj years before that4 ,it is a large1 oval object fitted in a frame of the same shape4 ,the spot itself has often faded and been lost since #ahgh1 but the frame has remained4 ,the spot is in size and position relative to ,jupiter much as ,australia is to the ,earth1 but while ,australia moves solidly with the rest of the ,earth in the daily rotation1 neither gaining on ,south ,america nor losing on ,africa1 the ,red ,spot on ,jupiter sees many other spots and clouds pass it by1 and does not even #gh retain the same rate of motion itself from one year to another4 ,no other marking on ,jupiter is so permanent as this4 ,from time to time great round white clouds form in a long series as if shot up from some eruption below1 and then drawn into the equatorial current4 ,from time to time the belts themselves change in breadth1 in colour1 and complexity4 ,jupiter is emphatically the planet of change4 ,and such change means energy1 especially energy in the form of heat4 ,if ,jupiter possessed no heat but that it derived from the ,sun1 it would be colder than ,mars1 and therefore an absolutely frozen globe4 ,but these rushing winds and hurrying clouds are evidences of heat and activity--a native heat much above that of our ,earth4 ,while ,mars is probably nearer to the ,moon than to the ,earth in its condition1 ,jupiter has probably more analogies with the ,sun4 ,the one unrivalled distinction of ,,saturn is its ,ring4 ,nothing like this exists elsewhere in the solar system4 ,everywhere else we see spherical globes2 this is a flat disc1 but without its central portion4 ,it surrounds the planet1 lying in the plane of its equator1 but touches it nowhere1 a gap of #gjjj miles intervening4 ,it appears to be circular1 and is #db1jjj miles in breadth4 ,yet it is not1 as it appears to be1 a flat continuous surface4 ,it is in reality made up of an infinite number of tiny satellites1 mere dust or pebbles for the most part1 but so numerous as to look from our distance like a continuous ring1 or rather like three or four concentric rings1 for certain divisions have been noticed in it--an inner broad division called after its discoverer1 ,,cassini1 and an outer1 fainter1 narrower one discovered by ,,encke4 ,the innermost part of the ring is dusky1 fainter #gi than the planet or the rest of the ring1 and is known as the crape-ring4 ,of ,saturn itself we know little2 it is further off and fainter than ,jupiter1 and its details are not so pronounced1 but in general they resemble those of ,jupiter4 ,the planet rotates quickly--in #aj h4 #ad m4--its markings run into parallel belts1 and are diversified by spots of the same character as on ,jupiter4 ,saturn is probably possessed of no small amount of native heat4 ,,uranus and ,,neptune are much smaller bodies than ,jupiter and ,saturn1 though far larger than the ,earth4 ,but their distance from the ,earth and ,sun makes their discs small and faint1 and they show little in the telescope beyond a hint of belts like those of ,jupiter2 so that1 as with that planet1 the surfaces that they show are almost certainly the upper surfaces of a shell of cloud4 ,in general1 therefore1 the rule appears to hold good throughout the solar system that a very large body is intensely hot and in a condition of violent activity and rapid change2 that smaller bodies are less hot and less active1 until we come down to the smallest1 which are cold1 inert1 and dead4 ,our own ,earth1 midway in the series1 is itself cold1 but is placed at such a distance from the ,sun as to receive from it a sufficient but not excessive supply of light and heat1 and the changes of the ,earth are such as not to prohibit but to nourish and support the growth and development of the various forms of life4 ,the smallest members of the solar system are known as ,,meteors4 ,these are often no more than pebbles or particles of dust1 moving together in associated orbits round the ,sun4 ,they are too small and too scattered to be seen in open space1 and become visible to us only #hj when their orbits intersect that of the earth1 and the earth actually encounters them4 ,they then rush into our atmosphere at a great speed1 and become highly heated and luminous as they compress the air before them2 so highly heated that most are vapourised and dissipated1 but a few reach the ground4 ,as they are actually moving in parallel paths at the time of one of these encounters1 they appear from the effect of perspective to diverge from a point1 hence called the radiant4 ,some showers occur on the same date of every year2 thus a radiant in the constellation ,lyra is active about ,april #ba1 giving us meteors1 known as the ,lyrids2 and another in ,perseus in ,august1 gives us the ,perseids4 ,other radiants are active at intervals of several years2 the most famous of all meteoric showers1 that of the ,leonids1 from a radiant in ,leo1 was active for many centuries every thirty-third year2 and another falling in the same month1 ,november1 came from a radiant in ,andromeda every thirteen years4 ,in these four cases and in some others the meteors have been found to be travelling along the same path as a comet4 ,it is therefore considered that meteoric swarms are due to the gradual break up of comets2 indeed the comet of the ,andromeda shower1 known from one of its observers as ,biela's1 was actually seen to divide into two in ,december #ahde1 and has not been observed as a comet since #aheb1 though the showers connected with it1 giving us the meteors known as the ,andromedes1 have continued to be frequent and rich4 ,meteors1 therefore1 are the smallest1 most insignificant1 of all the celestial bodies2 and the shining out of a meteor is the last stage of its history--its death2 after death it simply goes to add an infinitesimal trifle to the dust of the earth4 #ha ,,chapter ,,vi ,,the ,,system ,,of ,,the ,,stars ,the first step towards our knowledge of the starry heavens was made when the unknown and forgotten astronomers of #bgjj ,b4,c4 arranged the stars into constellations1 for it was the first step towards distinguishing one star from another4 ,when one star began to be known as the star in the eye of the ,bull1 and another as the star in the shoulder of the ,giant1 the heavens ceased to display an indiscriminate crowd of twinkling lights2 each star began to possess individuality4 ,the next step was taken when ,hipparchus made his catalogue of stars 7#abi ,b4,c471 not only giving its name to each star1 but measuring and fixing its place--a catalogue represented to us by that of ,claudius ,ptolemy 7,a4,d4 #acg74 ,the third step was taken when ,,bradley1 the third ,astronomer ,royal1 made1 at ,greenwich1 a catalogue of more than #cjjj star-places determined with the telescope4 ,a century later ,,argelander made the great ,bonn ,zone catalogue of #ccj1jjj stars1 and now a great photographic catalogue and chart of the entire heavens have been arranged between eighteen observatories of different countries4 ,this great chart when complete will probably present #cj millions of stars in position and brightness4 #hb ,the question naturally arises1 ,why so many stars8 ,what conceivable use can be served by catalogues of #cj millions or even of #cjjj stars8 ,and so far as strictly practical purposes are concerned1 the answer must be that there is none4 ,thus ,,maskelyne1 the fifth ,astronomer ,royal1 restricted his observations to some thirty-six stars1 which were all that he needed for his ,nautical ,almanac1 and these1 with perhaps a few additions1 would be sufficient for all purely practical ends4 ,but there is in man a restless1 resistless passion for knowledge--for knowledge for its own sake--that is always compelling him to answer the challenge of the unknown4 ,the secret hid behind the hills1 or across the seas1 has drawn the explorer in all ages2 and the secret hid behind the stars has been a magnet not less powerful4 ,so catalogues of stars have been made1 and made again1 and enlarged and repeated2 instruments of ever-increasing delicacy have been built in order to determine the positions of stars1 and observations have been made with ever-increasing care and refinement4 ,it is knowledge for its own sake that is longed for1 knowledge that can only be won by infinite patience and care4 ,the chief instrument used in making a star catalogue is called a transit circle2 two great stone pillars are set up1 each carrying one end of an axis1 and the axis carries a telescope4 ,the telescope can turn round like a wheel1 in one direction only2 it points due north or due south4 ,a circle carefully divided into degrees and fractions of a degree is attached to the telescope4 ,in the course of the twenty-four hours every star above the horizon of the observatory must come at least once within the range of this telescope1 and at that moment the observer points the telescope to the #hc star1 and notes the time by his clock when the star crossed the spider's threads1 which are fitted in the focus of his eye-piece4 ,he also notes the angle at which the telescope was inclined to the horizon by reading the divisions of his circle4 ,for by these two--the time when the star passed before the telescope and the angle at which the telescope was inclined--he is able to fix the position of the star4 ,but why should catalogues be repeated8 ,when once the position of a star has been observed1 why trouble to observe it again8 ,will not the record serve in perpetuity8 ,the answers to these questions have been given by star catalogues themselves1 or have come out in the process of making them4 ,the ,earth rotates on its axis and revolves round the ,sun4 ,but that axis also has a rolling motion of its own1 and gives rise to an apparent motion of the stars called ,precession4 ,hipparchus discovered this effect while at work on his catalogue1 and our knowledge of the amount of ,precession enables us to fix the date when the constellations were designed4 ,similarly1 ,bradley discovered two further apparent motions of the stars--,aberration and ,nutation4 ,of these1 the first arises from the fact that the light coming from the stars moves with an inconceivable speed1 but does not cross from star to ,earth instantly2 it takes an appreciable1 even a long1 time to make the journey4 ,but the ,earth is travelling round the ,sun1 and therefore continually changing its direction of motion1 and in consequence there is an apparent change in the direction in which the star is seen4 ,the change is very small1 for though the ,earth moves #ah- miles in a second1 light travels #aj1jjj times as fast4 ,stars therefore are deflected from their true positions by ,aberration1 by #hd an extreme amount of #bj4dg of arc1 that being the angle shown by an object that is slightly more distant than #aj1jjj times its diameter4 ,the axis of the ,earth not only rolls on itself1 but it does so with a slight staggering1 nodding motion1 due to the attractions of the ,sun and ,moon1 known as ,nutation4 ,and the axis does not remain fixed in the solid substance of the ,earth1 but moves about irregularly in an area of about #fj feet in diameter4 ,the positions of the north and south poles are therefore not precisely fixed1 but move1 producing what is known as the ,variation of ,latitude4 ,then star-places have to be corrected for the effect of our own atmosphere1 i4e4 refraction1 and for errors of the instruments by which their places are determined4 ,and when all these have been allowed for1 the result stands out that different stars have real movement of their own--their ,proper ,motions4 ,no stars are really fixed2 the name fixed stars is a tradition of a time when observation was too rough to detect that any of the heavenly bodies other than the planets were in motion4 ,but nothing is fixed4 ,the ,earth on which we stand has many different motions2 the stars are all in headlong flight4 ,and from this motion of the stars it has been learned that the ,sun too moves4 ,when ,copernicus overthrew the ,ptolemaic theory and showed that the ,earth moves round the ,sun1 it was natural that men should be satisfied to take this as the centre of all things1 fixed and immutable4 ,it is not so4 ,just as a traveller driving through a wood sees the trees in front apparently open out and drift rapidly past him on either hand1 and then slowly close together behind him1 so ,sir ,,william ,,herschel showed that the stars in one #he part of the heavens appear to be opening out1 or slowly moving apart1 while in the opposite part there seems to be a slight tendency for them to come together1 and in a belt midway between the two the tendency is for a somewhat quicker motion toward the second point4 ,and the explanation is the same in the one case as in the other--the real movement is with the observer4 ,the ,sun with all its planets and smaller attendants is rushing onward1 onward1 towards a point near the borders of the constellations ,lyra and ,hercules1 at the rate of about twelve miles per second4 ,part of the ,proper ,motions of the stars are thus only apparent1 being due to the actual motion of the ,sun--the ,sun's ,way1 as it is called--but part of the ,proper ,motions belong to the stars themselves2 they are really in motion1 and this not in a haphazard1 random manner4 ,for recently ,,kapteyn and other workers in the same field have brought to light the fact of ,star-,drift1 i4e4 that many of the stars are travelling in associated companies4 ,this may be illustrated by the seven bright stars that make up the well-known group of the ,plough1 or ,charles's ,wain1 as country people call it4 ,for the two stars of the seven that are furthest apart in the sky are moving together in one direction1 and the other five in another4 ,another result of the close study of the heavens involved in the making of star catalogues has been the detection of ,,double ,,stars--stars that not only appear to be near together but are really so4 ,quite a distinct and important department of astronomy has arisen dealing with the continual observation and measurement of these objects4 ,for many double stars are in motion round each other in obedience to the law of gravitation1 and their orbits have been computed4 #hf ,some of these systems contain three or even four members4 ,but in every case the smaller body shines by its own light2 we have no instance in these double stars of a sun attended by a planet2 in each case it is a sun with a companion sun4 ,the first double star to be observed as such was one of the seven stars of the ,plough4 ,it is the middle star in the ,plough handle1 and has a faint star near it that is visible to any ordinarily good sight4 ,star catalogues and the work of preparing them have brought out another class--,,variable ,,stars4 ,as the places of stars are not fixed1 so neither are their brightnesses1 and some change their brightness quickly1 even as seen by the naked eye4 ,one of these is called ,algol1 i4e4 the ,demon ,star1 and is in the constellation ,perseus4 ,the ancient ,greeks divided all stars visible to the naked eye into six classes1 or magnitudes1 according to their brightness1 the brightest stars being said to be of the first magnitude1 those not quite so bright of the second1 and so on4 ,algol is then usually classed as a star of the second magnitude1 and for two days and a half it retains its brightness unchanged4 ,then it begins to fade1 and for four and a half hours its brightness declines1 until two-thirds of it has gone4 ,no further change takes place for about twenty minutes1 after which the light begins to increase again1 and in another four and a half hours it is as bright as ever1 to go through the same changes again after another interval of two days and a half4 ,algol is a double star1 but1 unlike those stars that we know under that name1 the companion is dark1 but is nearly as large as its sun1 and is very close to it1 moving round it in a little less than three days4 ,at one point of its orbit it comes between ,algol and the ,earth1 #hg and ,algol suffers1 from our point of view1 a partial eclipse4 ,there are many other cases of variable stars of this kind in which the variation is caused by a dark companion moving round the bright star1 and eclipsing it once in each revolution2 and the diameters and distances of some of these have been computed1 showing that in some cases the two stars are almost in contact4 ,in some instances the companion is a dull but not a dark star2 it gives a certain amount of light4 ,when this is the case there is a fall of light twice in the period--once when the fainter star partly eclipses the brighter1 once when the brighter star partly eclipses the fainter4 ,but not all variable stars are of this kind4 ,there is a star in the constellation ,cetus which is sometimes of the second magnitude1 at which brightness it may remain for about a fortnight4 ,then it will gradually diminish in brightness for nine or ten weeks1 until it is lost to the unassisted sight1 and after six months of invisibility it reappears and increases during another nine or ten weeks to another maximum4 ,mira1 i4e4 wonderful star1 as this variable is called1 is about #ajjj times as bright at maximum as at minimum1 but some maxima are fainter than others2 neither is the period of variation always the same4 ,it is clear that variation of this kind cannot be caused by an eclipse1 and though many theories have been suggested1 the long-period variables1 of which ,mira is the type1 as yet remain without a complete explanation4 ,more remarkable still are the ,,new ,,stars--stars that suddenly burst out into view1 and then quickly fade away1 as if a beacon out in the stellar depths had suddenly been fired4 ,one of these suggested to ,hipparchus the need for a catalogue of the #hh stars2 another1 the so-called ,pilgrim ,star1 in the year #aegb was the means of fixing the attention of ,tycho ,brahe upon astronomy2 a third in #afjd was observed and fully described by ,kepler4 ,the real meaning of these new1 or temporary1 stars was not understood until the spectroscope was applied to astronomy4 ,they will therefore be treated in the volume of this series to be devoted to that subject4 ,it need only be mentioned here that their appearance is evidently due to some kind of collision between celestial bodies1 producing an enormous and instantaneous development of light and heat4 ,these ,new ,stars do not occur in all parts of the heavens4 ,even a hasty glance at the sky will show that the stars are not equally scattered1 but that a broad belt apparently made up of an immense number of very small stars divides them into two parts4 ,,the ,,milky ,,way1 or ,,galaxy1 as this belt is called1 bridges the heavens at midnight1 early in ,october1 like an enormous arch1 resting one foot on the horizon in the east1 and the other in the west1 and passing through the ,zenith1 i4e4 the point overhead4 ,it is on this belt of small stars--on the ,milky ,way--that ,new ,stars are most apt to break out4 ,the region of the ,milky ,way is richer in stars than are the heavens in general4 ,but it varies itself also in richness in a remarkable degree4 ,in some places the stars1 as seen on some of the wonderful photographs taken by ,e4 ,e4 ,,barnard1 seem almost to form a continuous wall2 in other places1 close at hand1 barren spots appear that look inky black by contrast4 ,and the ,star ,clusters1 stars evidently crowded together1 are frequent in the ,milky ,way4 ,and yet again beside the stars the telescope reveals #hi to us the ,,nebul4 ,some of these are the ,irregular ,nebul--wide-stretching1 cloudy1 diffused masses of filmy light1 like the ,great ,nebula in ,orion4 ,others are faint but more defined objects1 some of them with small circular discs1 and looking like a very dim ,uranus1 or even like ,saturn--that is to say1 like a planet with a ring round its equator4 ,this class are therefore known as ,planetary ,nebul1 and1 when bright enough to show traces of colour1 appear green or greenish blue4 ,these are1 however1 comparatively rare4 ,other of these faint1 filmy objects are known as the ,white ,nebul1 and are now counted by thousands4 ,they affect the spiral form4 ,sometimes the spiral is seen fully presented2 sometimes it is seen edgewise2 sometimes more or less foreshortened1 but in general the spiral character can be detected4 ,and these ,white ,nebul appear to shun the ,galaxy as much as the ,planetary ,nebula2 and ,star ,clusters prefer it2 indeed the part of the northern heavens most remote from the ,milky ,way is simply crowded with them4 ,it can be by no accident or chance that in the vast edifice of the heavens objects of certain classes should crowd into the belt of the ,milky ,way1 and other classes avoid it2 it points to the whole forming a single growth1 an essential unity4 ,for there is but one belt in the heavens1 like the ,milky ,way1 a belt in which small stars1 ,new ,stars1 and ,planetary ,nebul find their favourite home2 and that belt encircles the entire heavens2 and similarly that belt is the only region from which the ,white ,nebul appear to be repelled4 ,the ,milky ,way forms the foundation1 the strong and buttressed wall of the celestial building2 the ,white ,nebul close in the roof of its dome4 #ij ,and how vast may that structure be--how far is it from wall to wall8 ,that1 as yet1 we can only guess4 ,but the stars whose distances we can measure1 the stars whose drifting we can watch1 almost infinitely distant as they are1 carry us but a small part of the way4 ,still1 from little hints gathered here and there1 we are able to guess that1 though the nearest star to us is nearly #cjj1jjj times as far as the ,sun1 yet we must overpass the distance of that star #ajjj times before we shall have reached the further confines of the ,galaxy4 ,nor is the end in sight even there4 ,this is1 in briefest outline1 the ,story of ,astronomy4 ,it has led us from a time when men were acquainted with only a few square miles of the ,earth1 and knew nothing of its size and shape1 or of its relation to the moving lights which shone down from above1 on to our present conception of our place in a universe of suns of which the vastness1 glory1 and complexity surpass our utmost powers of expression4 ,the science began in the desire to use ,sun1 ,moon1 and stars as timekeepers1 but as the exercise of ordered sight and ordered thought brought knowledge1 knowledge began to be desired1 not for any advantage it might bring1 but for its own sake4 ,and the pursuit itself has brought its own reward in that it has increased men's powers1 and made them keener in observation1 clearer in reasoning1 surer in inference4 ,the pursuit indeed knows no ending2 the questions to be answered that lie before us are now more numerous than ever they have been1 and the call of the heavens grows more insistent3 ,,lift ,,up ,,your ,,eyes ,,on ,,high4 #ia ,,books ,,to ,,read ,,popular ,,general ,,descriptions3-- ,sir ,r4 ,s4 ,ball4--,star-,land4 7,cassell47 ,agnes ,giberne4---,sun1 ,moon and ,stars4 7,seeley47 ,w4 ,t4 ,lynn4--,celestial ,motions4 7,stanford47 ,a4 ,w4 ,maunder4---,the ,heavens and their ,story4 7,culley47 ,simon ,newcomb4--,astronomy for ,everybody4 7,isbister47 ,,for ,,beginners ,,in ,,observation3-- ,w4 ,f4 ,denning4--,telescopic ,work for ,starlight ,evenings4 7,taylor ,francis47 ,e4 ,w4 ,maunder4--,astronomy without a ,telescope4 7,thacker47 ,arthur ,p4 ,norton4--,a ,star ,atlas and ,telescopic ,handbook4 7,gall ,inglis47 ,garrett ,p4 ,serviss4--,astronomy with an ,opera-,glass4 7,appleton47 ,,star-,,atlases3-- ,rev4 ,j4 ,gall--,an ,easy ,guide to the ,constellations4 7,gall and ,inglis47 ,e4 ,m',clure and ,h4 ,j4 ,klein4--,star-,atlas4 7,society for ,promoting ,christian ,knowledge47 ,r4 ,a4 ,proctor4--,new ,star ,atlas4 7,longmans47 ,,astronomical ,,instruments ,,and ,,methods3-- ,sir ,g4 ,b4 ,airy4--,popular ,astronomy2 ,lectures delivered at ,ipswich4 7,macmillan47 ,e4 ,w4 ,maunder4--,royal ,observatory1 ,greenwich2 its ,history and ,work4 7,religious ,tract ,society47 #ib ,,general ,,text-,,books3-- ,clerke1 ,fowler ,gore4--,concise ,astronomy4 7,hutchinson47 ,simon ,newcomb4--,popular ,astronomy4 7,macmillan47 ,c4 ,a4 ,young4--,manual of ,astronomy4 7,ginn47 ,,special ,,subjects3-- ,rev4 ,e4 ,ledger4--,the ,sun2 its ,planets and ,satellites4 7,stanford47 ,c4 ,a4 ,young4--,the ,sun4 7,kegan ,paul47 ,mrs4 ,todd4--,total ,eclipses4 7,sampson ,low47 ,nasmyth and ,carpenter4--,the ,moon4 7,john ,murray47 ,percival ,lowell4--,mars4 7,longmans47 ,ellen ,m4 ,clerke4--,jupiter4 7,stanford47 ,e4 ,a4 ,proctor4--,saturn and its ,system4 7,longmans47 ,w4 ,t4 ,lynn4--,remarkable ,comets4 7,stanford47 ,e4 ,w4 ,maunder4--,the ,astronomy of the ,bible4 7,hodder and ,stoughton47 ,,historical3-- ,w4 ,w4 ,bryant4--,history of ,astronomy4 7,methuen47 ,agnes ,m4 ,clerke4--,history of ,astronomy in the ,nineteenth ,century4 7,a4 ,c4 ,black47 ,george ,forbes4--,history of ,astronomy4 7,watts47 ,,biographical3-- ,sir ,e4 ,s4 ,ball4--,great ,astronomers4 7,isbister47 ,agnes ,m4 ,clerke4--,the ,herschels and ,modern ,astronomy4 7,cassell47 ,sir ,o4 ,lodge4--,pioneers of ,science4 7,macmillan47 #ic ,,index ,,aberration1 #hc ,achilles 7,minor planet71 #ch ,adams1 ,john ,c41 #ci ,airy1 #ci ,algol1 #hf ,andromedes 7,meteors71 #hj ,apsides1 #bd1 #bh ,argelander1 #ha ,,barnard1 ,e4 ,e41 #hh ,bear1 ,the1 #ad ,biela's ,comet1 #hj ,bouvard1 #ci ,bradley1 #ha1 #hc ,bremiker1 #dj ,,catalogues 7star71 #ha-hc ,centauri1 ,alpha1 #ec ,ceres 7,minor planet71 #ch ,challis1 #dj ,charles ,,ii41 #ej ,chromosphere1 #gc ,chronometer1 #ej ,clairaut1 #cf ,columbus1 #dh ,comets1 #cf ,comet1 ,halley's1 #cg ---- ,biela's1 #hj ,conic ,sections1 #cd ,constellations1 the1 #ae ---- date of1 #af ,cook1 ,capt41 #ej ,copernicus1 #bf1 #ed1 #hd ,copernicus 7,lunar crater71 #ei1 #fj ,corona1 #gc ,cowell1 #cg ,crommelin1 #cg ,,degrees1 #dc ,dollond1 #dg ,double stars1 #he ,,earth1 form of1 #af ---- size of1 #ag1 #cc ,eclipses1 #gb ,ecliptic1 #ba ,ellipse1 #bh ,epicycle1 #be ,eratosthenes1 #ag ,eros 7,minor planet71 #ch1 #eb ,eudoxus1 #ba ,excentric1 #bd ,eye-piece1 #de ,,facul1 #gj ,flamsteed1 #ej ,,galileo1 #dd ,galle1 #dj ,gascoigne1 #df ,gravitation1 ,law of1 #cd ,,hall1 ,,chester ,,moor1 #dg ,halley1 #cf ,halley's ,comet1 #cg ,harrison1 ,john1 #ej ,herschel1 ,sir ,w41 #cg1 #dg1 #hd ,hipparchus1 #bd1 #ha1 #hc1 #hg ,hyperbola1 #cd ,,job1 ,book of1 #ab1 #ad ,juno 7,minor planet71 #ch ,jupiter1 #ah1 #cb1 #gg-gh ,,kapteyn1 #he ,kepler1 #bh1 #dd1 #hh ,kepler's ,laws1 #bi ,kepler 7,lunar crater71 #ei ,,langley1 #gd ,latitude1 ,variation of1 #hd ,leonids 7,meteors71 #hj ,leverrier1 #ci ,lowell1 #fc1 #fd ,lyrids 7,meteors71 #hj ,,magnetic ,,storm1 #gf ,magnetism1 ,earth's1 #gf ,magnitudes of stars1 #hf ,mare ,imbrium1 #ei ,mars1 #ah1 #eb1 #fb-ff ---- ,canals of1 #fc ,maskelyne1 #ej1 #hb ,maunder1 ,mrs4 ,walter1 #gb1 #gd ,mercury1 #ag1 #ah1 #bg1 #cb1 #ff-fg ,meteors1 #gi1 #hj ,micrometer1 #df ,milky ,way1 #ec1 #hh ,minor ,planets1 #ch1 #eb ,minutes of arc1 #dd ,mira1 #hg ,moon1 #aa1 #ad1 #ba1 #cb1 #cc1 #di1 #ee-fb ---- distance of1 #ea ,nautical ,almanac1 #ej1 #hb ,navigation1 #di ,nebul1 #hi ,neptune1 #dj1 #gi ,newcomb1 #fe ,new stars1 #hg ,newton1 #bi1 #ca1 #dg ,newton's ,laws of motion1 #ca ,nodes1 #ce ,nutation1 #hc1 #hd ,,oases of ,mars1 #fd ,obelisks1 #db ,object glass1 #de ,observatories1 ,berlin1 #ej ---- ,copenhagen1 #ej ---- ,greenwich1 #ej ---- ,mt4 ,wilson1 #dh ---- ,paris1 #ej ---- ,pulkowa1 #ej ---- ,st4 ,petersburg1 #ej ---- ,washington1 #ej ---- ,yerkes1 #dg ,,pallas 7,minor planet71 #ch ,parabola1 #cd ,perseids 7,meteors71 #hj ,photography1 #df ,photosphere1 #fi ,pilgrim star1 #hh ,piazzi1 #ch ,planets1 #ag ,pole of the ,heavens1 #ac ,pontcoulant1 #cg ,precession of the ,equinoxes1 #cf1 #hc ,principia1 #cf ,prominences1 #gc ,ptolemus 7,lunar crater71 #fj ,ptolemy1 #bd1 #ha ,,radiant ,,points1 #hj ,radius ,vector1 #bh ,reflectors1 #dg ,refractors1 #dg ,,saturn1 #ah1 #gh-gi ,schiaparelli1 #fc ,schwabe1 #fi ,seconds of arc1 #dd ,sirius1 #ec ,solar ,system1 ,tables of1 #ef-eh ,somerville1 ,mrs41 #hi ,spheres1 ,planetary1 #ba ,sprer1 #ga ,sprer's ,law1 #ga ,star catalogues1 #ha-hc ---- clusters1 #hh ---- drift1 #he ,stars1 fixed1 #hd ---- proper motions of1 #hd ,sun1 #aa1 #ab1 #ad1 #ba1 #cb1 #fg-gg ---- distance of1 #ea ---- dials1 #dc ,sun spots1 #fi ---- spot maximum1 #ga ---- ---- minimum1 #ga ,sun's ,way1 #he ,,telescope1 ,invention of1 #de ,transit ,circle1 #hb ,tycho ,brahe1 #bg1 #dd1 #hh ,tycho 7,lunar crater71 #ei1 #fj1 #fa ,,uranus1 #ch1 #gi ,,variable stars1 #hf ---- ----1 ,long period1 #hg ,venus1 #ah1 #bg1 #fg ,vesta 7,minor planet71 #ch ,,young1 ,c4 ,a41 #gd ,,zenith1 #ag1 #hh ,zodiac1 ,signs of1 #ad1 #ae1 #af1 #dc ,printed by ,,ballantyne1 ,,hanson ,co4 ,edinburgh ,london 99 99 99 99 99 ,we have nothing but the highest praise for these little books1 and no one who examines them will have anything else4--,westminster ,gazette1 #bbnd ,june #aiab4 ,,the ,,people',s ,,books ,,the ,,first ,,ninety ,,volumes ,the volumes issued are marked with an asterisk ,,science #a4 ,the ,foundations of ,science 4 4 4 ,by ,w4 ,c4 ,d4 ,whetham1 ,m4,a41 ,f4,r4,s4 #b4 ,embryology--,the ,beginnings of ,life 4 4 4 ,by ,prof4 ,gerald ,leighton1 ,m4,d4 #c4 ,biology 4 4 4 ,by ,prof4 ,w4 ,d4 ,henderson1 ,m4,a4 #d4 ,zoology3 ,the ,study of ,animal ,life 4 4 4 ,by ,prof4 ,e4 ,w4 ,mac,bride1 ,m4,a41 ,f4,r4,s4 #e4 ,botany2 ,the ,modern ,study of ,plants 4 4 4 ,by ,m4 ,c4 ,stopes1 ,d4,sc41 ,ph4,d41 ,f4,l4,s4 #f4 ,bacteriology 4 4 4 ,by ,w4 ,e4 ,carnegie ,dickson1 ,m4,d4 #g4 ,the ,structure of the ,earth 4 4 4 ,by ,prof4 ,t4 ,g4 ,bonney1 ,f4,r4,s4 #h4 ,evolution 4 4 4 ,by ,e4 ,s4 ,goodrich1 ,m4,a41 ,f4,r4,s4 #i4 ,darwin 4 4 4 ,by ,prof4 ,w4 ,garstang1 ,m4,a41 ,d4,sc4 #aj4 ,heredity 4 4 4 ,by ,j4 ,a4 ,s4 ,watson1 ,b4,sc4 #aa4 ,inorganic ,chemistry 4 4 4 ,by ,prof4 ,e4 ,c4 ,c4 ,baly1 ,f4,r4,s4 #ab4 ,organic ,chemistry 4 4 4 ,by ,prof4 ,j4 ,b4 ,cohen1 ,b4,sc41 ,f4,r4,s4 #ac4 ,the ,principles of ,electricity 4 4 4 ,by ,norman ,k4 ,campbell1 ,m4,a4 #ad4 ,radiation 4 4 4 ,by ,p4 ,phillips1 ,d4,sc4 #ae4 ,the ,science of the ,stars 4 4 4 ,by ,e4 ,w4 ,maunder1 ,f4,r4,a4,s4 #af4 ,the ,science of ,light 4 4 4 ,by ,p4 ,phillips1 ,d4,sc4 #ag4 ,weather ,science 4 4 4 ,by ,r4 ,g4 ,k4 ,lempfert1 ,m4,a4 #ah4 ,hypnotism and ,self-,education 4 4 4 ,by ,a4 ,m4 ,hutchison1 ,m4,d4 #ai4 ,the ,baby3 ,a ,mother's ,book 4 4 4 ,by a ,university ,woman4 #bj4 ,youth and ,sex--,dangers and ,safeguards for ,boys and ,girls 4 4 4 ,by ,mary ,scharlieb1 ,m4,d41 ,m4,s41 and ,f4 ,arthur ,sibly1 ,m4,a41 ,,ll4,d4 #ba4 ,marriage and ,motherhood 4 4 4 ,by ,h4 ,s4 ,davidson1 ,m4,b41 ,f4,r4,c4,s4,e4 #bb4 ,lord ,kelvin 4 4 4 ,by ,a4 ,russell1 ,m4,a41 ,d4,sc41 ,m4,i4,e4,e4 #bc4 ,huxley 4 4 4 ,by ,professor ,g4 ,leighton1 ,m4,d4 #bd4 ,sir ,william ,huggins and ,spectroscopic ,astronomy 4 4 4 ,by ,e4 ,w4 ,maunder1 ,f4,r4,a4,s41 of the ,royal ,observatory1 ,greenwich4 #fb4 ,practical ,astronomy 4 4 4 ,by ,h4 ,macpherson1 ,jr41 ,f4,r4,a4,s4 #fc4 ,aviation 4 4 4 ,by ,sydney ,f4 ,walker1 ,r4,n4 #fd4 ,navigation 4 4 4 ,by ,william ,hall1 ,r4,n41 ,b4,a4 #fe4 ,pond ,life 4 4 4 ,by ,e4 ,c4 ,ash1 ,m4,r4,a4,c4 #ff4 ,dietetics 4 4 4 ,by ,alex4 ,bryce1 ,m4,d41 ,d4,p4,h4 ,,philosophy ,,and ,,religion #be4 ,the ,meaning of ,philosophy 4 4 4 ,by ,prof4 ,a4 ,e4 ,taylor1 ,m4,a41 ,f4,b4,a4 #bf4 ,henri ,bergson 4 4 4 ,by ,h4 ,wildon ,carr1 ,litt4,d4 #bg4 ,psychology 4 4 4 ,by ,h4 ,j4 ,watt1 ,m4,a41 ,ph4,d41 ,d4,phil4 #bh4 ,ethics 4 4 4 ,by ,canon ,rashdall1 ,d4,litt41 ,f4,b4,a4 #bi4 ,kant's ,philosophy 4 4 4 ,by ,a4 ,d4 ,lindsay1 ,m4,a4 #cj4 ,the ,teaching of ,plato 4 4 4 ,by ,a4 ,d4 ,lindsay1 ,m4,a4 #fg4 ,aristotle 4 4 4 ,by ,prof4 ,a4 ,e4 ,taylor1 ,m4,a41 ,f4,b4,a4 #fh4 ,friedrich ,nietzsche 4 4 4 ,by ,m4 ,a4 ,mgge4 #fi4 ,eucken3 ,a ,philosophy of ,life 4 4 4 ,by ,a4 ,j4 ,jones1 ,m4,a41 ,b4,sc41 ,ph4,d4 #gj4 ,the ,experimental ,psychology of ,beauty 4 4 4 ,by ,c4 ,w4 ,valentine1 ,b4,a41 ,d4,phil4 #ga4 ,the ,problem of ,truth 4 4 4 ,by ,h4 ,wildon ,carr1 ,litt4,d4 #ca4 ,buddhism 4 4 4 ,by ,prof4 ,t4 ,w4 ,rhys ,davids1 ,m4,a41 ,f4,b4,a4 #cb4 ,roman ,catholicism 4 4 4 ,by ,h4 ,b4 ,coxon4 ,preface1 ,mgr4 ,r4 ,h4 ,benson4 #cc4 ,the ,oxford ,movement 4 4 4 ,by ,wilfrid ,ward4 #cd4 ,the ,bible and ,criticism 4 4 4 ,by ,w4 ,h4 ,bennett1 ,d4,d41 ,litt4,p41 and ,w4 ,f4 ,adeney1 ,d4,d4 #ce4 ,cardinal ,newman 4 4 4 ,by ,wilfrid ,meynell4 #gb4 ,the ,church of ,england 4 4 4 ,by ,rev4 ,canon ,masterman4 #gc4 ,anglo-,catholicism 4 4 4 ,by ,a4 ,e4 ,manning ,foster4 #gd4 ,the ,free ,churches 4 4 4 ,by ,rev4 ,edward ,shillito1 ,m4,a4 #ge4 ,judaism 4 4 4 ,by ,ephraim ,levine1 ,m4,a4 #gf4 ,theosophy 4 4 4 ,by ,annie ,besant4 ,,history #cf4 ,the ,growth of ,freedom 4 4 4 ,by ,h4 ,w4 ,nevinson4 #cg4 ,bismarck and the ,origin of the ,german ,empire 4 4 4 ,by ,professor ,f4 ,m4 ,powicke4 #ch4 ,oliver ,cromwell 4 4 4 ,by ,hilda ,johnstone1 ,m4,a4 #ci4 ,mary ,queen of ,scots 4 4 4 ,by ,e4 ,o',neill1 ,m4,a4 #dj4 ,cecil ,john ,rhodes1 #ahec-aijb 4 4 4 ,by ,ian ,d4 ,colvin4 #da4 ,julius ,csar 4 4 4 ,by ,hilary ,hardinge4 #db4 ,england in the ,making 4 4 4 ,by ,prof4 ,f4 ,j4 ,c4 ,hearnshaw1 ,m4,a41 ,,ll4,d4 #dc4 ,england in the ,middle ,ages 4 4 4 ,by ,e4 ,o',neill1 ,m4,a4 #dd4 ,the ,monarchy and the ,people 4 4 4 ,by ,w4 ,t4 ,waugh1 ,m4,a4 #de4 ,the ,industrial ,revolution 4 4 4 ,by ,arthur ,jones1 ,m4,a4 #df4 ,empire and ,democracy 4 4 4 ,by ,g4 ,s4 ,veitch1 ,m4,a41 ,litt4,d4 #fa4 ,home ,rule 4 4 4 ,by ,l4 ,g4 ,redmond ,howard4 ,preface by ,robert ,harcourt1 ,m4,p4 #gg4 ,nelson 4 4 4 ,by ,h4 ,w4 ,wilson4 #gh4 ,wellington and ,waterloo 4 4 4 ,by ,major ,g4 ,w4 ,redway4 ,,social ,,and ,,economic #dg4 ,women's ,suffrage 4 4 4 ,by ,m4 ,g4 ,fawcett1 ,,ll4,d4 #dh4 ,the ,working of the ,british ,system of ,government to-day 4 4 4 ,by ,prof4 ,ramsay ,muir1 ,m4,a4 #di4 ,an ,introduction to ,economic ,science 4 4 4 ,by ,prof ,h4 ,o4 ,meredith4 ,m4,a4 #ej4 ,socialism 4 4 4 ,by ,b4 ,b4 ,kirkman1 ,b4,a4 #gi4 ,medival ,socialism 4 4 4 ,by ,bede ,jarrett1 ,o4,p41 ,m4,a4 #hj4 ,syndicalism 4 4 4 ,by ,j4 ,h4 ,harley1 ,m4,a4 #ha4 ,labour and ,wages 4 4 4 ,by ,h4 ,m4 ,hallsworth1 ,m4,a41 ,b4,sc4 #hb4 ,co-operation 4 4 4 ,by ,joseph ,clayton4 #hc4 ,insurance as a ,means of ,investment 4 4 4 ,by ,w4 ,a4 ,robertson1 ,f4,f4,a4 #ib4 ,the ,training of the ,child 4 4 4 ,by ,g4 ,spiller ,,letters #ea4 ,shakespeare 4 4 4 ,by ,prof4 ,c4 ,h4 ,herford1 ,litt4,d4 #eb4 ,wordsworth 4 4 4 ,by ,rosaline ,masson4 #ec4 ,pure ,gold--,a ,choice of ,lyrics and ,sonnets 4 4 4 by ,h4 ,c4 ,o',neill #ed4 ,francis ,bacon 4 4 4 ,by ,prof4 ,a4 ,r4 ,skemp1 ,m4,a4 #ee4 ,the ,bronts 4 4 4 ,by ,flora ,masson4 #ef4 ,carlyle 4 4 4 ,by ,l4 ,mac,lean ,watt4 #eg4 ,dante 4 4 4 ,by ,a4 ,g4 ,ferrers ,howell4 #eh4 ,ruskin 4 4 4 ,by ,a4 ,blyth ,webster1 ,m4,a4 #ei4 ,common ,faults in ,writing ,english 4 4 4 ,by ,prof4 ,a4 ,r4 ,skemp1 ,m4,a4 #fj4 ,a ,dictionary of ,synonyms 4 4 4 ,by ,austin ,k4 ,gray1 ,b4,a4 #hd4 ,classical ,dictionary 4 4 4 ,by ,miss ,a4 ,e4 ,stirling #he4 ,a ,history of ,english ,literature 4 4 4 ,by ,a4 ,compton-,rickett1 ,,ll4,d4 #hf4 ,browning 4 4 4 ,by ,prof4 ,a4 ,r4 ,skemp1 ,m4,a4 #hg4 ,charles ,lamb 4 4 4 ,by ,flora ,masson4 #hh4 ,goethe 4 4 4 ,by ,prof4 ,c4 ,h4 ,herford1 ,litt4,d4 #hi4 ,balzac 4 4 4 ,by ,frank ,harris #ij4 ,rousseau 4 4 4 ,by ,f4 ,b4 ,kirkman1 ,b4,a4 #ia4 ,ibsen 4 4 4 ,by ,hilary ,hardinge4 #ic4 ,tennyson 4 4 4 ,by ,aaron ,watson ,,london ,,and ,,edinburgh3 ,t4 ,c4 ,e4 ,c4 ,,jack ,,new ,,york3 ,,dodge ,,publishing ,,co4 ,transcriber's ,note3 ,italicized text is indicated with underscores4 ,bold text is indicated with plus signs4 ,numbers inside curly braces1 e4g4 #ii are page numbers4