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 THE SCIENCE OF THE STARS
 
 THE SCIENCE OF 
 THE STARS 
 
 By E. WALTER MAUNDER, F.R.A.S. 
 
 OP THE ROYAL OBSERVATORY, GREENWICH 
 
 AUTHOR OF "ASTRONOMY WITHOUT A TELESCOPE" 
 "THE ASTRONOMY OP THE BIBLE," ETC. 
 
 LONDON: T. C. & E. C. JACK 
 67 LONG ACRE, W.C., AND EDINBURGH 
 NEW YORK: DODGE PUBLISHING CO.
 
 O^f /22PO, 
 
 CONTENTS 
 
 CHAP. TAGE 
 
 I. ASTRONOMY BEFORE HISTORY .... 9 
 
 II. ASTRONOMY BEFORE THE TELESCOPE . . 20 
 
 III. THE LAW OF GRAVITATION .... 30 
 
 IV. ASTRONOMICAL MEASUREMENTS ... 42 
 V. THE MEMBERS OF THE SOLAR SYSTEM . 54 
 
 VI. THE SYSTEM OF THE STARS . . . .81 
 INDEX 93 
 
 vil
 
 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 pages. But no 
 adequate account of the achievements of astronomy 
 can possibly be given within limits so narrow, for so 
 small a space would not suffice for a mere catalogue of 
 the results which have been obtained ; and in most 
 cases the result alone would be almost meaningless 
 unless some explanation were offered of the way in 
 which it had been reached. All, therefore, that can be 
 done in a work of the present size is to take the student 
 to the starting-point of astronomy, show him the various 
 roads of research which have opened out from it, and 
 give a brief indication of the character and general 
 direction of each. 
 
 That which distinguishes astronomy from all the 
 other sciences is this : it deals with objects that we 
 cannot touch. The heavenly bodies are beyond our 
 reach ; we cannot tamper with them, or subject them 
 to any form of experiment ; we cannot bring them into 
 our laboratories to analyse or dissect them. We can 
 only watch them and wait for such indications as their
 
 10 SCIENCE OF THE STARS 
 
 own movements may supply. But we are confined to 
 this earth of ours, and they are so remote ; we are so 
 short-lived, and they are so long-enduring; that the 
 difficulty of finding out much about them might well 
 seem insuperable. 
 
 Yet these difficulties have been so far overcome that 
 astronomy is the most advanced of all the sciences, the 
 one in winch our knowledge is the most definite and 
 certain. All science rests on sight and thought, on 
 ordered observation and reasoned deduction ; but both 
 sight and thought were earlier trained to the service of 
 astronomy than of the other physical sciences. 
 
 It is here that the highest value of astronomy lies ; 
 in the discipline that it has afforded to man's powers 
 of observation and reflection ; 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 bodies, but the vanquishing of diffi- 
 culties which might well have seemed superhuman. 
 The true spirit of the science can be far better exempli- 
 fied by the presentation of some of these difficulties, 
 and of the methods by which they have been over- 
 come, than by many volumes of picturesque description 
 or of eloquent rhapsody. 
 
 There was a time when men knew nothing of astro- 
 nomy ; like every other science it began from zero. 
 But it is not possible to suppose that such a state of 
 things lasted long , 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 day, a lesser 
 light that shone by night — and there were the stars 
 also. And this, the earliest observation of primitive 
 astronomy, is preserved for us, expressed in the simplest 
 possible language, in the first chapter of the first book
 
 ASTRONOMY BEFORE HISTORY 11 
 
 of the sacred writings handed down to us by the 
 Hebrews. 
 
 This observation, that there are bodies above us 
 giving light, and that they are not all equally bright, 
 is so simple, so inevitable, that men must have made 
 it as soon as they possessed any mental power at all. 
 But, once made, a number of questions must have 
 intruded themselves : " What are these lights ? Where 
 are they ? How far are they off ? " 
 
 Many different answers were early given to these 
 questions. Some were foolish ; some, though intelli- 
 gent, were mistaken ; some, though wrong, led eventu- 
 ally to the discovery of the truth. Many myths, many 
 legends, some full of beauty and interest, were invented. 
 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 solution. 
 
 As the greater light, the lesser light, and the stars 
 were carefully watched, it was seen not only that they 
 shone, but that they appeared to move ; slowly, steadily, 
 and without ceasing. The stars all moved together like 
 a column of soldiers on the march, not altering their 
 positions relative to each other. The lesser light, the 
 Moon, moved with the stars, and yet at the same time 
 among them. The greater light, the Sun, was not seen 
 with the stars ; the brightness of his presence made 
 the day, his absence brought the night, and it was 
 only during his absence that the stars were seen ; they 
 faded out of the sky before he came up in the morning, 
 and did not reappear again until after he passed out 
 of sight in the evening. But there came a time when 
 it was realised that there were stars shining in the sky 
 all day long as well as at night, and this discovery was 
 one of the greatest and most important ever made,
 
 12 SCIENCE OF THE STARS 
 
 because it was the earliest discovery of something quite 
 unseen. Men laid hold of this fact, not from the direct 
 and immediate evidence of their senses, but from re- 
 flection and reasoning. We do not know who made 
 this discovery, nor how long ago it was made, but from 
 that time onward the eyes with which men looked 
 upon nature were not only the eyes of the body, but 
 also the eyes of the mind. 
 
 It followed from this that the Sun, like the Moon, 
 not only moved with the general host of the stars, 
 but also among them. If an observer looks out from 
 any fixed station and watches the rising of some bright 
 star, night after night, he will notice that it always 
 appears to rise in the same place ; so too with its 
 setting. From any given observing station the direc- 
 tion in which any particular star is observed to rise or 
 set is invariable. 
 
 Not so with the Sun. We are accustomed to say 
 that the Sun rises in the east and sets in the west. But 
 the direction in which the Sun rises in midwinter lies 
 far to the south of the east point ; the direction in winch 
 he rises in midsummer lies as far to the north. The 
 Sun is therefore not only moving with the stars, but 
 among them. This gradual change in the position of 
 the Sun in the sky was noticed in many ancient nations 
 at an early time. It is referred to in Job xxxviii. 12 : 
 " Hast thou commanded the morning since thy days ; 
 and caused the dayspring to know his place ? " 
 
 And the apparent path of the Sun on one day is 
 always parallel to its path on the days preceding and 
 following. When, therefore, the Sun rises far to the 
 south of east, he sets correspondingly far to the south 
 of west, and at noon he is low down in the south. His 
 course during the day is a short one, and the daylight
 
 ASTRONOMY BEFORE HISTORY 13 
 
 is much shorter than the night, and the Sun at noon, 
 being low down in the sky, has not his full power. The 
 cold and darkness of winter, therefore, follows directly 
 upon this position of the Sun. These conditions are 
 reversed when the Sun rises in the north-east. The 
 night is short, the daylight prolonged, and the Sun, 
 being high in the heavens at noon, his heat is felt to 
 the full. 
 
 Thus the movements of the Sun are directly con- 
 nected with the changes of season upon the Earth. 
 But the stars also are connected with those seasons ; 
 for if we look out immediately after it has become dark 
 after sunset, we shall notice that the stars seen in the 
 night of winter are only in part those seen in the nights 
 of summer. 
 
 In the northern part of the sky there are a number 
 of stars which are always visible whenever we look out, 
 no matter at what time of the night nor what part of the 
 year. If we watch throughout the whole night, we see 
 that the whole heavens appear to be slowly turning — 
 turning, as if all were in a single piece — and the pivot 
 about which it is turning is high up in the northern 
 sky. The stars, therefore, are divided into two classes. 
 Those near this invisible pivot — the " Pole " of the 
 Heavens, as we term it — move round it in complete 
 circles ; they never pass out of sight, but even when 
 lowest they clear the horizon. The other stars move 
 round the same pivot in curved paths, which are evi- 
 dently parts of circles, but circles of which we do not 
 see the whole. These stars rise on the eastern side of 
 the heavens and set on the western, and for a greater 
 or less space of time are lost to sight below the horizon. 
 And some of these stars are visible at one time of the 
 year, others at another ; some being seen during the
 
 14 SCIENCE OF THE STARS 
 
 whole of the long nights of winter, others throughout 
 the short nights of summer. This distinction again, 
 and its connection with the change of the seasons on 
 the earth, was observed many ages ago. It is alluded 
 to in Job xxxviii. 32 : " Canst thou lead forth the 
 Signs of the Zodiac in their season, or canst thou guide 
 the Bear with her train ? " (R.V., Margin). The Signs 
 of the Zodiac are taken as representing the stars which 
 rise and set, and therefore have each their season for 
 being "led forth," while the northern stars, which are 
 always visible, appearing to be "guided" in their con- 
 tinual movement round the Pole of the sky in perfect 
 circles, are represented by " the Bear with her train." 
 
 The changes in position of the Sun, the greater light, 
 must have attracted attention in the very earliest ages, 
 because these changes are so closely connected with 
 the changes of the seasons upon the Earth, which affect 
 men directly. The Moon, the lesser light, goes through 
 changes of position like the Sun, but these are not of 
 the same direct consequence to men, and probably 
 much less notice was taken of them. But there were 
 changes of the Moon which men could not help noticing — 
 her changes of shape and brightness. One evening she 
 may be seen soon after the Sun has set, as a thin arch 
 of light, low down in the sunset sky. On the following 
 evenings she is seen higher and higher in the sky, 
 and the bow of light increases, until by the fourteenth 
 day it is a perfect round. Then the Moon begins to 
 diminish and to disappear, until, on the twenty-ninth 
 or thirtieth day after the first observation, she is again 
 seen in the west after sunset as a narrow crescent. This 
 succession of changes gave men an important measure 
 of time, and, in an age when artificial means of light 
 were difficult to procure, moonlight was of the greatest
 
 ASTRONOMY BEFORE HISTORY 15 
 
 value, and the return of the moonlit portion of the 
 month was eagerly looked for. 
 
 Theee early astronomical observations were simple 
 and obvious, and of great practical value. The day, 
 month, and year were convenient measures of time, and 
 the power of determining, from the observation of the 
 Sun and of the stars, how far the year had progressed 
 was most important to farmers, as an indication when 
 they should plough and sow their land. Such ob- 
 servations had probably been made independently by 
 many men and in many nations, but in one place a 
 greater advance had been made. The Sun and Moon 
 are . both unmistakable, but one star is very like 
 another, and, for the most part, individual stars can 
 only be recognised by their positions relative to others. 
 The stars were therefore grouped together into Con- 
 stellations and associated with certain fancied designs, 
 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 year, these twelve being known as 
 the "Signs of the Zodiac " — the Ram, Bull, Twins, Crab, 
 Lion, Virgin, Balance, Scorpion, Archer, Goat, Water- 
 pourer, and Fishes. In the rest of the sky some thirty 
 to thirty-six other groups, or constellations, were formed, 
 the Bear being the largest and brightest of the con- 
 stellations of the northern heavens. 
 
 But these ancient constellations do not cover the 
 entire heavens ; a large area in the south is untouched 
 by them. And this fact affords an indication both of 
 the time when and the place where the old stellar groups 
 were designed, for the region left untouched was the 
 region below the horizon of 40° North latitude, about 
 4600 years ago. It is probable, therefore, that the 
 ancient astronomers who carried out this great work
 
 16 SCIENCE OF THE STARS 
 
 lived about 2700 B.C., and in North latitude 37° or 38°. 
 The indication is only rough, but the amount of uncer- 
 tainty is not very large ; the constellations must be at 
 least 4000 years old, they cannot be more than 5000. 
 
 All this was done by prehistoric astronomers ; though 
 no record of the actual carrying out of the work and 
 no names of the men who did it have come down to 
 us. 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 Sun, and that they are twelve in number — there 
 being twelve months in the year — that those who de- 
 signed the constellations already knew that there are 
 stars shining near the Sun in full daylight, and that 
 they had worked out some means for determining what 
 stars the Sun is near at any given time. 
 
 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 us. It was seen that 
 all along the eastern horizon, from north to south, stars 
 rise, and all along the western horizon, from north to 
 south, stars set. That is what was seen ; it was the 
 fact observed. There is no hindrance anywhere to the 
 movement of the stars — they have a free passage under 
 the Earth ; the Earth is unsupported in space, That 
 is what was thought ; it was the inference drawn. Or, 
 as it is written in Job xxvi. 7, " He (God) stretcheth 
 out the north over empty space, and hangeth the earth 
 upon nothing." 
 
 The Earth therefore floats unsupported in the centre 
 of an immense star-spangled sphere. And what is the 
 shape of the Earth ? The natural and correct inference 
 is that it is spherical, and we find in some of the early 
 Greek writers the arguments which establish this in- 
 ference as clearly set forth as they would be to-day.
 
 ASTRONOMY BEFORE HISTORY 17 
 
 The same inference followed, moreover, from the ob- 
 servation of a simple fact, namely, that the stars as 
 observed from any particular place all make the same 
 angle with the horizon as they rise in the east, and all 
 set at the same angle with it in the west ; but if we go 
 northward, we find that angle steadily decreasing ; if 
 we go southward, we find it increasing. But if the Earth 
 is round like a globe, then it must have a definite size, 
 and that size can be measured. The discoveries noted 
 above were made by men whose names have been lost, 
 but the name of the first person whom we know to have 
 measured the size of the Earth was Eratosthenes. 
 He found that the Sun was directly overhead at noon at 
 midsummer at Syene (the modern Assouan), in Egypt, 
 but was 7° south of the " zenith " — the point over- 
 head — a t Alexandria, and from this he computed the 
 Earth to be 250,000 stadia (a stadium = 606 feet) in 
 circumference. 
 
 Another consequence of the careful watch upon the 
 stars was the discovery that five of them were planets ; 
 " wandering " stars ; they did not move all in one 
 piece with the rest of the celestial host. In this they 
 resemble the Sun and Moon, and they further resemble 
 the Moon in that, though too small for any change of 
 shape to be detected, they change in brightness from 
 time to time. But their movements are more compli- 
 cated than those of the other heavenly bodies. The 
 Sun moves a little slower than the stars, and so seems 
 to travel amongst them from west to east ; the Moon 
 moves much slower than the stars, so her motion from 
 west to east is more pronounced than that of the Sun. 
 But the five planets sometimes move slower than the 
 stars, sometimes quicker, and sometimes at the same 
 rate. Two of the five, which we now know as Mercury
 
 18 SCIENCE OF THE STARS 
 
 and Venus, never move far from the Sun, sometimes 
 being seen in the east before he rises in the morning, 
 and sometimes in the west after he has set in the 
 evening. Mercury is the closer to the Sun, and moves 
 more quickly ; Venus goes through much the greater 
 changes of brightness. Jupiter and Saturn move 
 nearly at the same average rate as the stars, Saturn 
 taking about thirteen days more than a year to come 
 again to the point of the sky opposite to the Sun, and 
 Jupiter about thirty-four days. Mars, the fifth planet, 
 takes two years and fifty days to accomplish the same 
 journey. 
 
 These planetary movements were not, like those of 
 the Sun and Moon and stars, of great and obvious con- 
 sequence to men. It was important to men to know 
 when they would have moonlight nights, to know 
 when the successive seasons of the year would return. 
 But it was no help to men to know when Venus was 
 at her brightest more than when she was invisible. 
 She gave them no useful light, and she and her com- 
 panion planets returned at no definite seasons. Never- 
 theless, men began to make ordered observations of the 
 planets — observations that required much more patience 
 and perseverance than those of the other celestial 
 lights. And they set themselves with the greatest in- 
 genuity to unravel the secret of their complicated and 
 seemingly capricious movements. 
 
 This was a yet higher development than anything 
 that had gone before, for men were devoting time, 
 trouble, and patient thought, for long series of years, 
 to an inquiry which did not promise to bring them any 
 profit or advantage. Yet the profit which it actually 
 did bring was of the highest order. It developed 
 men's mental powers ; it led to the devising of
 
 ASTRONOMY BEFORE HISTORY 19 
 
 instruments of precision for the observations ; it led 
 to the foundation of mathematics, and thus lay at the 
 root of all our modern mechanical progress. It brought 
 out, in a higher degree, ordered observation and ordered 
 thought. 
 
 B
 
 CHAPTER II 
 
 ASTRONOMY BEFORE THE TELESCOPE 
 
 There was thus a real science of astronomy before we 
 have any history of it. Some important discoveries 
 had been made, and the first step had been taken to- 
 wards cataloguing the fixed stars. It was certainly 
 known to some of the students of the heavens, though 
 perhaps only to a few, that the Earth was a sphere, 
 freely suspended in space, and surrounded on all sides 
 by the starry heavens, amongst which moved the Sun, 
 Moon, and the five planets. The general character of 
 the Sun's movement was also known ; namely, that he 
 not only moved day by day from east to west, as the 
 stars do, but also had a second motion inclined at an 
 angle to the first, and in the opposite direction, which 
 he accomplished in the course of a year. 
 
 To this sum of knowledge, no doubt, several nations 
 had contributed. We do not know to what race we 
 owe the constellations, but there are evidences of an 
 elementary acquaintance with astronomy on the part 
 of the Chinese, the Babylonians, the Egyptians, and 
 the Jews. But in the second stage of the development 
 of the science the entire credit for the progress made 
 belongs to the Greeks. 
 
 The Greeks, as a race, appear to have been very 
 little apt at originating ideas, but they possessed, beyond 
 all other races, the power of developing and perfecting 
 crude ideas which they had obtained from other sources, 
 
 20
 
 ASTRONOMY BEFORE THE TELESCOPE 21 
 
 and when once their attention was drawn to the move- 
 ments of the heavenly bodies, they devoted them- 
 selves with striking ingenuity and success to devising 
 theories to account for the appearances presented, to 
 working out methods of computation, and, last, to 
 devising instruments for observing the places of the 
 luminaries in which they were interested. 
 
 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 science. Etjdoxus of Knidus, in Asia Minor (408- 
 355 B;C), was, so far as we know, the first to attempt 
 to represent the movements of the heavenly bodies by 
 a simple mathematical process. His root idea was 
 something like this. The Earth was in the centre of the 
 universe, and it was surrounded, at a great distance from 
 us, by a number of invisible transparent shells, or 
 spheres. Each of these spheres rotated with perfect 
 uniformity, though the speed of rotation differed for 
 different spheres. One sphere carried the stars, and 
 rotated from east to west in about 23 h. 56 m. 
 The Sun was carried by another sphere, which rotated 
 from west to east in a year, but the pivots, or poles, of 
 this sphere were carried by a second, rotating exactly 
 like the sphere of the stars. This explained how it is 
 that the ecliptic — that is to say, the apparent path of 
 the Sun amongst the stars — is inclined 234° to the 
 equator of the sky, so that the Sun is 234° north of the 
 equator at midsummer and 234° south of the equator 
 at midwinter, for the poles of the sphere peculiar to 
 the Sun were supposed to be 23 4° from the poles of the 
 sphere peculiar to the stars. Then the Moon had three 
 spheres; that which actually carried the Moon having 
 its poles 5° from the poles of the sphere peculiar to the
 
 22 SCIENCE OF THE STARS 
 
 San. These poles were carried by a sphere placed like 
 the sphere of the Sun, but rotating in 27 days ; and 
 this, again, had its poles in the sphere of the stars. 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 month, for at one time in the 
 month the Moon is 5° north of the ecliptic, at another 
 time 5° south. The motions of the planets were more 
 difficult to represent, because they not only have a 
 general daily motion from east to west, like the stars, 
 and a general motion from west to east along the 
 ecliptic, like the Sun and Moon, but from time to time 
 they turn back on their course in the ecliptic, and 
 " retrograde." But the introduction of a third and 
 fourth sphere enabled the motions of most of the planets 
 to be fairly represented. There were thus twenty-seven 
 spheres in all — four for each of the five planets, three for 
 the Moon, three for the Sun (including one not men- 
 tioned in the foregoing summary), and one for the 
 stars. These spheres were not, however, supposed to 
 be solid structures really existing ; 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 circles. 
 
 But this assumption that each heavenly body moves 
 in its path at a uniform rate was soon seen to be con- 
 trary to fact. A reference to the almanac will show 
 at once that the Sun's movement is not uniform. Thus 
 for the year 1910-11 the solstices and equinoxes fell as 
 given on the next page :
 
 ASTRONOMY BEFORE THE TELESCOPE 23 
 
 Epoch Time Interval 
 
 Winter Solstice 1910 Dec. 22 d. 5li. 12 m. p.m. 
 
 89 d. Oh. 42m. 
 Spring Equinox 1911 Mar. 21 „ 5 „ 54 „ p.m. 
 
 92 „ 19 „ 41 „ 
 
 Summer Solstice 1911 June 22 „ 1 „ 35 „ p.m. 
 
 93 „ 14 „ 43 „ 
 
 Autumn Equinox 1911 Sept. 24 „ 4 „ 18 „ a.m. 
 
 89 „ 18 „ 36 „ 
 
 Winter Solstice 1911 Dec. 22 „ 10 „ 54 „ p.m. 
 
 so that the winter half of the year is shorter than the 
 summer half; 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 it. 
 
 The motion of the Moon is more irregular still, as we 
 can see by taking out from the almanac the times of 
 new and full moon : 
 
 
 
 New Moon 
 
 
 
 
 Interval to Full Moon 
 
 Dec. 191C 
 
 Id. 
 
 9h. 
 
 10-7 
 
 in. 
 
 P.M. 
 
 14 d. 
 
 13 h. 
 
 54*4 m 
 
 j) 
 
 » 
 
 31 
 
 j> 
 
 4 
 
 » 
 
 21'2 
 
 » 
 
 P.M. 
 
 14 „ 
 
 6 
 
 » 
 
 4-8 „ 
 
 Jan. 1911 
 
 30 
 
 V 
 
 9 
 
 )) 
 
 44-7 
 
 M 
 
 A.M. 
 
 14 „ 
 
 
 
 » 
 
 52-8 „ 
 
 March 
 
 !> 
 
 1 
 
 » 
 
 
 
 11 
 
 31-1 
 
 » 
 
 A.M. 
 
 13 „ 
 
 23 
 
 >> 
 
 274 „ 
 
 » 
 
 )> 
 
 30 
 
 )> 
 
 
 
 )> 
 
 37-8 
 
 >J 
 
 P.M. 
 
 14 „ 
 
 1 
 
 >) 
 
 58-8 „ 
 
 April 
 
 If 
 
 28 
 
 )» 
 
 10 
 
 )) 
 
 25-0 
 
 » 
 
 P.M. 
 
 14 „ 
 
 7 
 
 >> 
 
 44-7 „ 
 
 May- 
 
 )> 
 
 28 
 
 »> 
 
 6 
 
 JJ 
 
 24-4 
 
 » 
 
 A.M. 
 
 14 „ 
 
 15 
 
 >) 
 
 26-3 „ 
 
 June 
 
 5) 
 
 26 
 
 >» 
 
 1 
 
 IJ 
 
 19-7 
 
 » 
 
 P.M. 
 
 14 „ 
 
 23 
 
 >) 
 
 33 7 „ 
 
 July 
 
 )) 
 
 25 
 
 }> 
 
 8 
 
 11 
 
 120 
 
 )> 
 
 P.M. 
 
 15 „ 
 
 6 
 
 » 
 
 42-7 „ 
 
 Aug. 
 
 >) 
 
 24 
 
 )> 
 
 4 
 
 11 
 
 143 
 
 » 
 
 A.M. 
 
 15 „ 
 
 11 
 
 » 
 
 42-4 „ 
 
 Sept. 
 
 » 
 
 22 
 
 M 
 
 2 
 
 >) 
 
 37-4 
 
 JJ 
 
 P.M. 
 
 15 „ 
 
 13 
 
 »> 
 
 337 „ 
 
 Oct. 
 
 J> 
 
 22 
 
 M 
 
 4 
 
 » 
 
 9 3 
 
 » 
 
 A.M. 
 
 15 „ 
 
 11 
 
 >> 
 
 38-8 „ 
 
 Nov. 
 
 )5 
 
 20 
 
 » 
 
 8 
 
 1) 
 
 49-4 
 
 » 
 
 P.M. 
 
 15 „ 
 
 6 
 
 )> 
 
 2-5 „ 
 
 Dec. 
 
 »> 
 
 20 
 
 n 
 
 3 
 
 n 
 
 40-3 
 
 }> 
 
 P.M. 
 
 14 „ 
 
 21 
 
 » 
 
 49-4 „
 
 24 
 
 SCIENCE OF THE STARS 
 
 
 
 Full Moon 
 
 
 
 Interval to New Moon 
 
 Dec. 
 
 1910 16 d 
 
 11 h. 
 
 5*1 m. a.m. 
 
 15 d. 
 
 5h. 
 
 16-1 m. 
 
 Jan. 
 
 1911 14 
 
 )> 
 
 10 „ 
 
 26 
 
 „ P.M. 
 
 15 
 
 » 
 
 11 
 
 )> 
 
 18-7 „ 
 
 Feb. 
 
 >) 
 
 13 
 
 )> 
 
 10 „ 
 
 37-5 
 
 „ A.M. 
 
 15 
 
 5) 
 
 13 
 
 » 
 
 53-6 „ 
 
 March. „ 
 
 14 
 
 >) 
 
 11 ii 
 
 58-5 
 
 „ P.M. 
 
 15 
 
 )) 
 
 12 
 
 )> 
 
 39-3 „ 
 
 April 
 
 » 
 
 13 
 
 )> 
 
 2» 
 
 36-6 
 
 „ P.M. 
 
 15 
 
 )) 
 
 7 
 
 » 
 
 48-4 „ 
 
 May 
 
 5) 
 
 13 
 
 ?) 
 
 6„ 
 
 9-7 
 
 „ A.M. 
 
 15 
 
 » 
 
 
 
 )> 
 
 14-7 „ 
 
 June 
 
 » 
 
 11 
 
 >) 
 
 9„ 
 
 50-7 
 
 „ P.M. 
 
 14 
 
 » 
 
 15 
 
 >> 
 
 29-0 „ 
 
 July 
 
 )> 
 
 11 
 
 >> 
 
 o„ 
 
 53-4 
 
 „ P.M. 
 
 14 
 
 )) 
 
 7 
 
 )> 
 
 18-6 „ 
 
 Aug. 
 
 5! 
 
 10 
 
 !) 
 
 2„ 
 
 547 
 
 „ A.M. 
 
 14 
 
 » 
 
 1 
 
 j) 
 
 19-6 „ 
 
 Sept. 
 
 » 
 
 8 
 
 55 
 
 3„ 
 
 56-7 
 
 „ P.M. 
 
 13 
 
 » 
 
 22 
 
 )> 
 
 40-7 „ 
 
 Oct. 
 
 ?> 
 
 8 
 
 )> 
 
 4 „ 
 
 11-1 
 
 „ A.M. 
 
 13 
 
 » 
 
 23 
 
 n 
 
 58-2 „ 
 
 Nov. 
 
 » 
 
 6 
 
 !> 
 
 3 „ 
 
 48-1 
 
 „ P.M. 
 
 14 
 
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 1-3 „ 
 
 Dec. 
 
 5) 
 
 6 
 
 » 
 
 2 „ 
 
 51-9 
 
 „ A.M. 
 
 14 
 
 5) 
 
 12 
 
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 48-4 „ 
 
 Jan. 
 
 1912 
 
 4 
 
 )) 
 
 1 „ 
 
 297 
 
 „ P.M. 
 
 14 
 
 » 
 
 21 
 
 }> 
 
 40-3 „ 
 
 The astronomer who dealt with this difficulty was 
 Hipparchus (about 190-120 B.C.), who was born at Nicaea, 
 in Bithynia, but made most of his astronomical observa- 
 tions in Rhodes. He attempted to explain these irre- 
 gularities in the motions of the Sun and Moon by sup- 
 posing that though they really moved uniformly in 
 their orbits, yet the centre of their orbits was not the 
 centre of the Earth, but was situated a little distance 
 from it. This point was called " the excentric," and the 
 line from the excentric to the Earth was called " the line 
 of apsides." 
 
 But when he tried to deal with the movements of 
 the planets, he found that there were not enough good 
 observations available for him to build up any satis- 
 factory theory. 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 was. 
 
 His great successor was Claudius Ptolemy of
 
 ASTRONOMY BEFORE THE TELESCOPE 25 
 
 Alexandria, who carried the work of astronomical ob- 
 servation from about a.d. 127 to 150. He was, however, 
 much greater as a mathematician than as an observer, 
 and he worked out a very elaborate scheme, by which 
 he was able to represent the motions of the planets 
 with considerable accuracy. The system was an ex- 
 tremely complex one, but its principle may be repre- 
 sented as follows : If we suppose that a planet is 
 moving round the Earth in a circle at a uniform rate, 
 and we tried to compute the place of the planet on this 
 assumption for regular intervals of time, we should find 
 that the planet gradually got further and further away 
 from the predicted place. Then after a certain time 
 the error would reach a maximum and begin to 
 diminish, until the error vanished and the planet was 
 in the predicted place at the proper time. The error 
 would then begin to fall in the opposite direction, and 
 would increase as before to a maximum, subsequently 
 diminishing again to zero. This state of things might 
 be met by supposing that the planet was not itself 
 carried by the circle round the earth, but by an epi- 
 cycle — i.e. 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 represented. But still there would be 
 smaller errors going through their own period, and these, 
 again, would have to be met by imagining that the first 
 epicycle carried a second, and it might be that the second 
 carried a third, and so on. 
 
 The Ptolemaic system was more complicated than 
 this brief summary would suggest, but it is not pos- 
 sible here to do more than indicate the general prin- 
 ciples upon which it was founded, and the numerous 
 other systems or modifications of them produced in the
 
 26 SCIENCE OF THE STARS 
 
 five centuries from Eudoxus to Ptolemy must be left 
 unnoticed. The point to be borne in mind is that one 
 fundamental assumption underlay them all, an assump- 
 tion fundamental to all science — the assumption that 
 like causes must always produce like effects. It was 
 apparent to the ancient astronomers that the stars — 
 that is to say, the great majority of the heavenly 
 bodies — do move round the Earth in circles, and with a 
 perfect uniformity of motion, and it seemed inevitable 
 that, if one body moved round another, it should thus 
 move. For if the revolving body came nearer to the 
 centre at one time and receded at another, if it moved 
 faster at one time and slower at another, then, the cause 
 remaining the same, the effect seemed to be different. 
 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 Nature. 
 
 For more than 1300 years the Ptolemaic system re- 
 mained without serious challenge, and the next great 
 name that it is necessary to notice is that of Coper- 
 nicus (1473-1543). Copernicus was a canon of Frauen- 
 burg, and led the quiet, retired life of a student. The 
 great work which made him immortal, De Revolutionibus, 
 was the result of many years' meditation and work, and 
 was not printed until he was on his deathbed. 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 
 unseen. 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 
 cast in the midst of the starry sphere ; and, as the
 
 ASTRONOMY BEFORE THE TELESCOPE 27 
 
 stars are at an immeasurable distance, the latter con- 
 ception was much the simpler. Extending the idea of 
 the Earth's motion further, the supposition that, 
 instead of the Sun revolving round a fixed Earth in 
 a year, the Earth revolved round a fixed Sun, made 
 at once an immense simplification in the planetary 
 motions. The reason became obvious why Mercury 
 and Venus were seen first on one side of the Sun and 
 then on the other, and why neither of them could move 
 very far from the Sun ; their orbits were within the 
 orbit, of the Earth. The stationary points and retro- 
 gressions of the planets were also explained ; for, as the 
 Earth was a planet, and as the planets moved in orbits 
 of different sizes, the outer planets taking a longer time 
 to complete a revolution than the inner, it followed, of 
 necessity, that the Earth in her motion would from 
 time to time be passed by the two inner planets, and 
 would overtake the three outer. The chief of the 
 Ptolemaic epicycles were done away with, and all the 
 planets moved continuously in the same direction round 
 the Sun. But no planet's motion could be represented 
 by uniform motion in a single circle, and Copernicus 
 had still to make use of systems of epicycles to account 
 for the deviations from regularity in the planetary 
 motions round the Sun. The Earth having been aban- 
 doned as the centre of the universe, a further sacrifice 
 had to be made : the principle of uniform motion in a 
 circle, which had seemed so necessary and inevitable, 
 had also to be given up. 
 
 For the time came when the instruments for measuring 
 the positions of the stars and planets had been much 
 improved, largely due to Tycho Brahe (1546-1601), a 
 Dane of noble birth, who was the keenest and most 
 careful observer that astronomy had yet produced.
 
 28 SCIENCE OF THE STARS 
 
 His observations enabled his friend and pupil, Johann 
 Kepler (1571-1630), to subject the planetary move- 
 ments to a far more searching examination than had 
 yet been attempted, and he discovered that the Sun is 
 in the plane of the orbit of each of the planets, and 
 also in its line of apsides — that is to say, the line join- 
 ing the two points of the orbit which are respectively 
 nearest and furthest from the Sun. Copernicus had 
 not been aware of either of these two relations, but 
 their discovery greatly strengthened the Copernican 
 theory. 
 
 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 him, 
 until at length he was led to discard the circle and try 
 a different curve — the oval or ellipse. Now the pro- 
 perty of a circle is that every point of it is situated at 
 the same distance from the centre, but in an ellipse 
 there are two points within it, the " foci," and the sum 
 of the distances of any point on the circumference from 
 these two foci is constant. If the two foci are at a 
 great distance from each other, then the ellipse is very 
 long and narrow ; if the foci are close together, the 
 ellipse differs very little from a circle ; and if we imagine 
 that the two foci actually coincide, the ellipse becomes 
 a circle. When Kepler tried motion in an ellipse in- 
 stead of motion in a circle, he found that it represented 
 correctly the motions of all the planets without any 
 need for epicycles, and that in each case the Sun occu- 
 pied one of the foci. And though the planet did not 
 move at a uniform speed in the ellipse, yet its motion 
 was governed by a uniform law, for the straight line 
 joining the planet to the Sun, the " radius vector," passed 
 over equal areas of space in equal periods of time.
 
 ASTRONOMY BEFORE THE TELESCOPE 29 
 
 These two discoveries are known as Kepler's First 
 and Second Laws. His Third Law connects all the 
 planets together. It was known that the outer planets 
 not only take longer to revolve round the Sun than the 
 inner, but that their actual motion in space is slower, 
 and Kepler found that this actual speed of motion is 
 inversely as the square root of its distance from the 
 Sun ; or, if the square of the speed of a planet be 
 multiplied by its distance from the Sun, we get the 
 same result in each case. This is usually expressed by 
 saying that the cube of the distance is proportional to 
 the square of the time of revolution. Thus the varying 
 rate' of motion of each planet in its orbit is not only 
 subject to a single law, but the very different speeds of 
 the different planets are also all subject to a law that 
 is the same for all. 
 
 Thus the whole of the complicated machinery of 
 Ptolemy had been reduced to three simple laws, which 
 at the same time represented the facts of observation 
 much better than any possible development of the 
 Ptolemaic mechanism. On his discovery of his third 
 law Kepler had written : " The book is written to be 
 read either now or by posterity — I care not which ; it 
 may well wait a century for a reader, as God has waited 
 6000 years for an observer." Twelve years after his 
 death, on Christmas Day 1642 (old style), near Grantham, 
 in Lincolnshire, the predestined " reader " was born. 
 The inner meaning of Kepler's three laws was brought 
 to light by Isaac Newton.
 
 CHAPTER III 
 
 THE LAW OF GRAVITATION 
 
 The fundamental thought which, recognised or not, had 
 lain at the root of the Ptolemaic system, as indeed it 
 lies at the root of all science, was that " like causes 
 must always produce like effects." 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 motion. 
 For, if there be any force tending to alter the distance 
 of the planet from that centre, it seemed inevitable that 
 sooner or later it should either reach that centre or be 
 indefinitely removed from it. If there be no such force, 
 then the planet's distance from that centre must remain 
 invariable, and if it move at all, it must move in a 
 circle ; move uniformly, because there is no force either 
 to hasten or retard it. Uniform motion in a circle 
 seemed a necessity of nature. 
 
 But all this system, logical and inevitable as it had 
 once seemed, had gone down before the assault of 
 observed facts. The great example of uniform circular 
 motion had been the daily revolution of the star 
 sphere ; but this was now seen to be only apparent, 
 the result of the rotation of the Earth. The planets 
 revolved round the Sun, but the Sun was not in the 
 centre of their motion ; they moved, not in circles, but 
 in ellipses ; not at a uniform speed, but at a speed 
 which diminished with the increase of their distance from 
 
 30
 
 THE LAW OF GRAVITATION 31 
 
 the Sun. There was need, therefore, for an entire 
 revision of the principles upon which motion was sup- 
 posed to take place. 
 
 The mistake of the ancients had been that they 
 supposed that continued motion demanded fresh appli- 
 cations of force. They noticed that a ball, set rolling, 
 sooner or later came to a stop ; that a pendulum, set 
 swinging, might swing for a good time, but eventually 
 came to rest ; and, as the forces that were checking 
 the motion — that is to say, the friction exercised by the 
 ground, the atmosphere, and the like — did not obtrude 
 themselves, they were overlooked. 
 
 Newton brought out into clear statement the true 
 conditions of motion. A body once moving, if acted 
 upon by no force whatsoever, must continue to move 
 forward in a straight line at exactly the same speed, 
 and that for ever. It does not require any maintaining 
 force to keep it going. If any change in its speed or 
 in its direction takes place, that change must be due 
 to the introduction of some further force. 
 
 This principle, that, if no force acts on a body in 
 motion, it will continue to move uniformly in a straight 
 line, is Newton's First Law of Motion. His Second 
 lays it down that, if force acts on a body, it produces a 
 change of motion proportionate to the force applied, 
 and in the same direction. And the Third Law states 
 that when one body exerts force upon another, that 
 second body reacts with equal force upon the first. 
 The problem of the motions of the planets was, there- 
 fore, not what kept them moving, but what made 
 them deviate from motion in a straight line, and deviate 
 by different amounts. 
 
 It was quite clear, from the work of Kepler, that the 
 force deflecting the planets from uniform motion in a
 
 32 SCIENCE OF THE STARS 
 
 straight line lay in the Sun. The facts that the Sun 
 lay in the plane of the orbits of all the planets, that 
 the Sun was in one of the foci of each of the planetary 
 ellipses, that the straight line joining the Sun and 
 planet moved for each planet over equal areas in equal 
 periods of time, established this fact clearly. But the 
 amount of deflection was very different for different 
 planets. Thus the orbit of Mercury is much smaller 
 than that of the Earth, and is travelled over in a much 
 shorter time, 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 time, Mercury falling 
 towards the Sun by about 159 miles, whilst the fall of 
 the Earth is only about 23*9 miles. The force drawing 
 Mercury towards the Sun is therefore 6*66 times that 
 drawing the Earth, but 6*66 is the square of 2*58, and 
 the Earth is 2*58 times as far from the Sun as Mercury. 
 Similarly, the fall in an hour of Jupiter towards the Sun 
 is about 0*88 miles, so that the force draAving the Earth 
 is 27 times that drawing Jupiter towards the Sun. 
 But 27 is the square of 5*2, and Jupiter is 5-2 times 
 as far from the Sun as the Earth. Similarly with the 
 other planets. The force, therefore, which deflects the 
 planets from motion in a straight line, and compels 
 them to move round the Sun, is one which varies in- 
 versely as the square of the distance. 
 
 But the Sun is not the only attracting body of which 
 we know. The old Ptolemaic system was correct to a 
 small extent ; the Earth is the centre of motion for the 
 Moon, which revolves round it at a mean distance of 
 238,800 miles, and in a period of 27 d. 7 h. 43 m. Hence 
 the circumference of her orbit is 1,500,450 miles, and 
 the length of the straight line which she would travel
 
 THE LAW OF GRAVITATION 33 
 
 in one second of time, if not deflected by the Earth, is 
 2828 feet. In this distance the deviation of a circle 
 from a straight line is one inch divided by 1866. But 
 we know from experiment that a stone let fall from a 
 height of 193 inches above the Earth's surface will 
 reach the ground in exactly one second of time. The 
 force drawing the stone to the Earth, therefore, is 
 193 x 18*66 ; i.e. 3601 times as great as that drawing 
 the Moon. But the stone is only -^ of a mile from 
 the Earth's surface, while the Moon is 238,800 miles 
 away — more than 78 million times as far. The force, 
 therefore would seem not to be diminished in the 
 proportion that the distance is increased — much less 
 in the proportion of its square. 
 
 But Newton proved that a sphere of uniform density, 
 or made up of any number of concentric shells of uni- 
 form density, attracted a body outside itself, just as if 
 its entire mass was concentrated at its centre. The 
 distance of the stone from the Earth must therefore 
 be measured, not from the Earth's surface, but from 
 its centre ; in other words, we must consider the stone 
 as being distant from the Earth, not some 16 feet, 
 but 3963 miles. This is very nearly one-sixtieth of the 
 Moon's distance, and the square of 60 is 3600. The 
 Earth's pull upon the Moon, therefore, is almost exactly 
 in the inverse square of the distance as compared with 
 its pull on the stone. 
 
 Kepler's book had found its "reader." His three 
 laws were but three particular aspects of Newton's 
 great discovery that the planets moved under the influ- 
 ence of a force, lodged in the Sun, which varied inversely 
 as the square of their distances from it. But Newton's 
 work went far beyond this, for he showed that the 
 same law governed the motion of the Moon round the
 
 34 SCIENCE OF THE STARS 
 
 Earth and the motions of the satellites revolving round 
 the different planets, and also governed the fall of 
 bodies upon the Earth itself. It was universal through- 
 out the solar system. The law, therefore, is stated as 
 of universal application. " Every particle of matter in 
 the universe attracts every other particle with a force 
 varying inversely as the square of the distance between 
 them, and directly as the product of the masses of the 
 two particles." And Newton further proved that if a 
 body, projected in free space and moving with any 
 velocity, became subject to a central force acting, like 
 gravitation, inversely as the square of the distance, it 
 must revolve in an ellipse, or in a closely allied curve. 
 
 These curves are what are known as the " conic 
 sections " — that is, they are the curves found when a 
 cone is cut across in different directions. Their rela- 
 tion to each other may be illustrated thus. If we have 
 a very powerful light emerging from a minute hole, 
 then, if we place a screen in the path of the beam of 
 light, and exactly at right angles to its axis, the light 
 falling on the screen will fill an exact circle. If we 
 turn the screen so as to be inclined to the axis of the 
 beam, the circle will lengthen out in one direction, and 
 will become an ellipse. If we turn the screen still 
 further, the ellipse will lengthen and lengthen, until at 
 last, when the screen has become parallel to one of the 
 edges of the beam of light, the ellipse will only have 
 one end ; the other will be lost. For it is clear that 
 that edge of the beam of light which is parallel to the 
 screen can never meet it. The curve now shown on 
 the screen is called a parabola, and if the screen is turned 
 further yet, the boundaries of the light falling upon it 
 become divergent, and we have a fourth curve, the 
 hyperbola. Bodies moving under the influence of
 
 THE LAW OF GRAVITATION 35 
 
 gravitation can move in any of these curves, but only 
 the circle and ellipse are closed orbits. A particle 
 moving in a parabola or hyperbola can only make one 
 approach to its attracting body ; after such approach 
 it continually recedes from it. As the circle and para- 
 bola are only the two extreme forms of an ellipse, the 
 two foci being at the same point for the circle and at 
 an infinite distance apart for the parabola, we may 
 regard all orbits urder gravitation as being ellipses of 
 one form or another. 
 
 From his great demonstration of the law of gravita- 
 tion; Newton went on to apply it in many directions. 
 He showed that the Earth could not be truly spherical 
 in shape, but that there must be a flattening of its 
 poles. He showed also that the Moon, which is exposed 
 to the attractions both of the Earth and of the Sun, 
 and, to a sensible extent, of some of the other planets, 
 must show irregularities in her motion, which at that 
 time had not been noticed. The Moon's orbit is in- 
 clined to that of the Earth, cutting its plane in two 
 opposite points, called the " nodes." It had long been 
 observed that the position of the nodes travelled round 
 the ecliptic once in about nineteen years. Newton was 
 able to show that this was a consequence of the Sun's 
 attraction upon the Moon. And he further made a 
 particular application of the principle thus brought out, 
 for, the Earth not being a true sphere, but flattened 
 at the poles and bulging at the equator, the equatorial 
 belt might be regarded as a compact ring of satellites 
 revolving round the Earth's equator. This, therefore, 
 would tend to retrograde precisely as the nodes of a 
 single satellite would, so that the axis of the equatorial 
 belt of the Earth — in other words, the axis of the Earth 
 — must revolve round the pole of the ecliptic. Conse- 
 
 c
 
 36 SCIENCE OF THE STARS 
 
 quently the pole of the heavens appears to move amongst 
 the stars, and the point where the celestial equator 
 crosses the equator necessarily moves with it. This is 
 what we know as the " Precession of the Equinoxes," 
 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 accomplished, namely, the grouping of 
 the stars into constellations by the astronomers of the 
 prehistoric age. 
 
 The publication of Newton's great work, the Prin- 
 cipia (The Mathematical Principles of Natural Philo- 
 sophy), in which he developed the Laws of Motion, the 
 significance of Kepler's Three Planetary Laws, and the 
 Law of "Universal Gravitation, took place in 1687, and 
 was due to his friend Edmund Halley, to whom he 
 had confided many of his results. That he was the 
 means of securing the publication of the Principia is 
 Halley's highest claim to the gratitude of posterity, 
 but his own work in the field which Newton had opened 
 was of great importance. Newton had treated comets 
 as moving in parabolic orbits, and Halley, collecting all 
 the observations of comets that were available to him, 
 worked out the particulars of their orbits on this 
 assumption, and found that the elements of three were 
 very closely similar, and that the interval between their 
 appearances was nearly the same, the comets having 
 been seen in 1531, 1607, and 1682. On further con- 
 sulting old records he found that comets had been 
 observed in 1456, 1378, and 1301. He concluded that 
 these were different appearances of the same object, 
 and predicted that it would be seen again in 1758, or, 
 according to a later and more careful computation, in 
 1759. As the time for its return drew near, Clairaut
 
 THE LAW OF GRAVITATION 37 
 
 computed with the utmost care the retardation which 
 would be caused to the comet by the attractions of 
 Jupiter and Saturn. The comet made its predicted 
 nearest approach to the Sun on March 13, 1759, just 
 one month earlier than Clairaut had computed. But 
 in its next return, in 1835, the computations effected 
 by Pontecotjlant were only two days in error, so 
 carefuUy had the comet been followed during its un- 
 seen journey to the confines of the solar system and 
 back again, during a period of seventy-five years. 
 Pontecoulant's exploit was outdone at the next return 
 by Drs. Cowell and Crommelin, of Greenwich Obser- 
 vatory, who not only computed the time of its peri- 
 helion passage — that is to say, its nearest approach to 
 the Sun— for April 16, 1910, but followed the comet 
 back in its wanderings during all its returns to the year 
 240 B.C. Halley's Comet, therefore, was the first comet 
 that was known to travel in a closed orbit and to return 
 to the neighbourhood of the Sun. Not a few small or 
 telescopic comets are now known to be " periodic," but 
 Halley's is the only one which has made a figure to the 
 naked eye. Notices of it occur not a few times in 
 history ; it was the comet " like a flaming sword " 
 which Josephus described as having been seen over 
 Jerusalem not very long before the destruction by 
 Titus. It was also the comet seen in the spring of the 
 year when William the Conqueror invaded England, 
 and was skilfully used by that leader as an omen of his 
 coming victory. 
 
 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 
 times. On March 13, 1781, Sir William Herschel
 
 38 SCIENCE OF THE STARS 
 
 detected a new object, which he at first supposed to 
 be a comet, but afterwards recognised as a planet far 
 beyond the orbit of Saturn. This planet, to which the 
 name of Uranus was finally given, had a mean distance 
 from the Sun nineteen times that of the Earth, and a 
 diameter four times as great. This was a second addi- 
 tion to the solar system, but it was a discovery by 
 sight, not by deduction. 
 
 The first day of the nineteenth century, January 1, 
 1801, was signalised by the discovery of a small planet 
 by Piazzi. The new object was lost for a time, but it 
 was redetected on December 31 of the same year. 
 This planet lay between the orbits of Mars and Jupiter — 
 a region in which many hundreds of other small bodies 
 have since been found. The first of these " minor planets " 
 was called Ceres; the next three to be discovered are 
 known as Pallas, Juno, and Vesta. Beside these four, 
 two others are of special interest : one, Eros, which 
 oomes nearer the Sun than the orbit of Mars — indeed 
 at some oppositions it approaches the Earth within 
 13,000,000 miles, and is therefore, next to the Moon, our 
 nearest neighbour in space ; the other, Achilles, moves 
 at a distance from the Sun equal to that of Jupiter. 
 
 Ceres is much the largest of all the minor planets; 
 indeed is larger than all the others put together. Yet 
 the Earth exceeds Ceres 4000 times in volume, and 
 7000 times in mass, and the entire swarm of minor 
 planets, all put together, would not equal in total volume 
 one-fiftieth part of the Moon. 
 
 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 attempted, 
 and this had an important bearing on the next great 
 event in the development of gravitational astronomy.
 
 THE LAW OF GRAVITATION 39 
 
 The movements of Uranus soon gave rise to difficulties. 
 It was found impossible, satisfactorily, to reconcile the 
 earlier and later observations, and in the tables of 
 Uranus, published by Botjvard in 1821, the earlier 
 observations were rejected. But the discrepancies be- 
 tween the observed and calculated places for the planet 
 soon began to reappear and quickly increase, and the 
 suggestion was made that these discrepancies were due 
 to an attraction exercised by some planet as yet un- 
 known. Thus Mrs. Somerville in a little book on the 
 connection of the physical sciences, published in 1836, 
 wrote, " Possibly it (that is, Uranus) may be subject to 
 disturbances from some unseen planet revolving about 
 the Sun beyond the present boundaries of our system. 
 If, after the lapse of years, the tables formed from a 
 combination of numerous observations should still be 
 inadequate to represent the motions of Uranus, the 
 discrepancies may reveal the existence, nay, even the 
 mass and orbit of a body placed for ever beyond the 
 sphere of vision." In 1843 John C. Adams, who had 
 just graduated as Senior Wrangler at Cambridge, pro- 
 ceeded to attack the problem of determining the posi- 
 tion, orbit, and mass of the unknown body by winch 
 on this assumption Uranus was disturbed, from the 
 irregularities evident in the motion of that planet. 
 The problem was one of extraordinary intricacy, but 
 by September 1845 Adams had obtained a first solution, 
 which he submitted to Airy, the Astronomer Royal. 
 As, however, he neglected to reply to some inquiries 
 made by Airy, no search for the new planet was in- 
 stituted in England until the results of a new and 
 independent worker had been published. The same 
 problem had been attacked by a well-known and very 
 gifted French mathematician, U. J. J. Leverrier, and
 
 40 SCIENCE OF THE STARS 
 
 in June 1846 he published his position for the unseen 
 planet, which proved to be in close accord with that 
 which Adams had furnished to Airy nine months 
 before. On this Airy stirred up Challis, the Director 
 of the Cambridge Observatory, which then possessed 
 the most powerful telescope in England, to search for 
 the planet, and Challis commenced to make charts, 
 which included more than 3000 stars, in order to make 
 sure that the stranger should not escape his net. 
 Leverrier, on the other hand, communicated his result 
 to the Berlin Observatory, where they had just received 
 some of the star charts prepared by Dr. Bremiker in 
 connection with the search for minor planets. The 
 Berlin observer, Dr. Galle, had therefore nothing to do 
 but to compare the stars in the field, upon which he 
 turned his telescope, with those shown on the chart ; a 
 star not in the chart would probably be the desired 
 stranger. He found it, therefore, on the very first 
 evening, September 23, 1846, within less than four 
 diameters of the Moon of the predicted place. The 
 same object had been observed by Challis at Cambridge 
 on August 4 and 12, but he was deferring the reduction 
 of his observations until he had completed his scrutiny 
 of the zone, and hence had not recognised it as different 
 from an ordinary star. 
 
 This discovery of the planet now known as Neptune, 
 which had been disturbing the movement of Uranus, 
 has rightly been regarded as the most brilliant triumph 
 of gravitational astronomy. It was the legitimate 
 crown of that long intellectual struggle which had com- 
 menced more than 2000 years earlier, when the first 
 Greek astronomers set themselves to unravel the appar- 
 ently aimless wanderings of the planets in the assured 
 faith that they would find them obedient unto law.
 
 THE LAW OF GRAVITATION 41 
 
 But of what use was all this effort ? What is the good 
 of astronomy ? The question is often asked, but it is 
 the question of ignorance. The use of astronomy is 
 the development which it has given to the intellectual 
 powers of man. Directly the problem of the planetary 
 motions was first attempted, it became necessary to 
 initiate mathematical processes in order to deal with it, 
 and the necessity for the continued development of 
 mathematics has been felt in the same connection right 
 down to the present day. When the Greek astronomers 
 first began their inquiries into the planetary movements 
 they hoped for no material gain, and they received 
 none. They laboured ; we have entered into their 
 labours. But the whole of our vast advances in 
 mechanical and engineering science — advances which 
 more than anything else differentiate this our present 
 age from ah 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 astronomy, and to follow after 
 knowledge, not for any material rewards which she had 
 to offer, but for her own sake.
 
 CHAPTER IV 
 
 ASTRONOMICAL MEASUREMENTS 
 
 The old proverb has it that " Science is measurement," 
 and of none of the sciences is this so true as of the 
 science of astronomy. Indeed the measurement of 
 time by observation of the movements of the heavenly 
 bodies was the beginning of astronomy. The move- 
 ment of the Sun gave the day, which was reckoned to 
 begin either at sunrise or at sunset. The changes of 
 the Moon gave the month, and in many languages the 
 root meaning of the word for Moon is '" measurer." 
 The apparent movement of the Sun amongst the stars 
 gave a yet longer division of time, the year, which 
 could be determined in a number of different ways, 
 either from the Sun alone, or from the Sun together 
 with the stars. A very simple and ancient form of 
 instrument for measuring this movement of the Sun was 
 the obelisk, a pillar with a pointed top set up on a level 
 pavement. Such obelisks were common in Egypt, and 
 one of the most celebrated, known as Cleopatra's Needle, 
 now stands on the Thames Embankment. As the Sun 
 moved in the sky, the shadow of the pillar moved on 
 the pavement, and midday, or noon, was marked when 
 the shadow was shortest. The length of the shadow at 
 noon varied from day to day ; it was shortest at mid- 
 summer, and longest at midwinter, i.e. at the summer 
 and winter solstices. Twice in the year the shadow of 
 the pillar pointed due west at sunrise, and due east at 
 
 42
 
 ASTRONOMICAL MEASUREMENTS 43 
 
 sunset — that is to say, the shadow at the beginning of 
 the day was in the same straight line as at its end. 
 These two days marked the two equinoxes of spring 
 and autumn. 
 
 The obelisk was a simple means of measuring the 
 height and position of the Sun, but it had its draw- 
 backs. The length of the shadow and its direction did 
 not vary by equal amounts in equal times, and if the 
 pavement upon which the shadow fell was divided by 
 marks corresponding to equal intervals of time for one 
 day of the year, the marks did not serve for all other 
 days. 
 
 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 heavens, round 
 which all the stars appeared to revolve — then the shadow 
 of the wall moved on the pavement in the same manner 
 every day, and the pavement if marked to show the 
 hours for one day would show them for any day. The 
 sundials still often found in the gardens of country 
 houses or in churchyards are miniatures of such an 
 instrument. 
 
 But the Greek astronomers devised other and better 
 methods for determining the positions of the heavenly 
 bodies. Obelisks or dials were of use only with the 
 Sun and Moon which cast shadows. To determine the 
 position of a star, " sights " like those of a rifle were 
 employed, and these were fixed to circles which were 
 carefully divided, generally into 360 " degrees." As 
 there are 365 days in a year, and as the Sun makes a 
 complete circuit of the Zodiac in this time, it moves 
 very nearly a degree in a day. The twelve Signs of 
 the Zodiac are therefore each 30° in length, and each
 
 44 SCIENCE OF THE STARS 
 
 takes on the average a double-hour to rise or set. 
 While the Sun and Moon are each about half a degree 
 in diameter, i.e. about one-sixtieth of the length of 
 a Sign, and therefore take a double-minute to rise or 
 set. Each degree of a circle is therefore divided into 
 60 minutes, and each minute may be divided into 60 
 seconds. 
 
 As the Sun or Moon are each about half a degree, or, 
 more exactly, 32 minutes in diameter, it is clear that, 
 so long as astronomical observations were made by the 
 unaided sight, a minute of arc (written 1') was the 
 smallest division of the circle that could be used. A 
 cord or wire can indeed be detected when seen pro- 
 jected against a moderately bright background if its 
 thickness is a second of arc (written 1") — a sixtieth of 
 a minute — but the wire is merely perceived, not pro- 
 perly defined. 
 
 Tycho Brahe had achieved the utmost that could be 
 done by the naked eye, 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 
 8 minutes of arc — that is to say, of a quarter the appar- 
 ent diameter of the Moon — that made Kepler finally 
 give up all attempts to explain the planetary move- 
 ments on the doctrine of circular orbits and to try 
 movements in an ellipse. But a contemporary of 
 Kepler, as gifted as he was himself, but in a different 
 direction, was the means of increasing the observing 
 power of the astronomer. Galileo Galilei (1564- 
 1642), of a noble Florentine family, was appointed 
 Lecturer in Mathematics at the University of Pisa. 
 Here he soon distinguished himself by his originality of 
 thought, and the ingenuity and decisiveness of his ex- 
 periments. Up to that time it had been taught that of
 
 ASTRONOMICAL MEASUREMENTS 45 
 
 two bodies the heavier would fall to the ground more 
 quickly than the lighter. Galileo let fall a 100-lb. 
 weight and a 1-lb. weight from the top of the Leaning 
 Tower, and both weights reached the pavement together. 
 By this and other ingenious experiments he laid a firm 
 foundation for the science of mechanics, and he dis- 
 covered the laws of motion which Newton afterwards 
 formulated. He heard that an instrument had been 
 invented in Holland which seemed to bring distant 
 objects nearer, and, having himself a considerable know- 
 ledge of optics, it was not long before he made himself 
 a little telescope. He fixed two spectacle glasses, one 
 for long and one for short sight, in a little old organ- 
 pipe, and thus made for himself a telescope which 
 magnified three times. Before long he had made 
 another which magnified thirty times, and, turning it 
 towards the heavenly bodies, he discovered dark moving 
 spots upon the Sun, mountains and valleys on the 
 Moon, and four small satellites revolving round Jupiter. 
 He also perceived that Venus showed " phases " — that is 
 to say, she changed her apparent shape just as the 
 Moon does — and he found the Milky Way to be com- 
 posed of an immense number of small stars. These 
 discoveries were made in the years 1609-11. 
 
 A telescope consists in principle of two parts — an 
 object-glass, to form an image of the distant object, 
 and an eye-piece, to magnify it. The rays of light from 
 the heavenly body fall on the object-glass, and are so 
 bent out of their course by it as to be brought together 
 in a point called the focus. The " light - gathering 
 power " of the telescope, therefore, depends upon the 
 size of the object-glass, and is proportional to its area. 
 But the size of the image depends upon the focal length 
 of the telescope, i.e. upon the distance that the focus
 
 46 SCIENCE OF THE STARS 
 
 is from the object-glass. Thus a small disc, an inch in 
 diameter — such as a halfpenny — will exactly cover the 
 full Moon if held up nine feet away from the eye ; and 
 necessarily the image of the full Moon made by an 
 object-glass of nine-feet focus will be an inch in diameter. 
 The eye-piece is a magnifying-glass or small microscope 
 applied to this image, 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 permit. 
 
 This little image of the Moon, planet, or group of stars 
 lent itself to measurement. A young English gentle- 
 man, Gascoigne, who afterwards fell at the Battle of 
 Marston Moor, devised the " micrometer " for this pur- 
 pose. The micrometer usually has two frames, each 
 carrying one or more very thin threads — usually spider's 
 threads — and the frames can be moved by very fine 
 screws, the number of turns or parts of a turn of each 
 screw being read off on suitable scales. By placing one 
 thread on the image of one star, and the other on the 
 image of another, the apparent separation of the two 
 can be readily and precisely measured. 
 
 Within the last thirty years photography has im- 
 mensely increased the ease with which astronomical 
 measurements can be made. The sensitive photographic 
 plate is placed in the focus of the telescope, and the 
 light of Sun, Moon, or stars, according to the object to 
 which the telescope is directed, makes a permanent 
 impression on the plate. Thus a picture is obtained, 
 which can be examined and measured in detail at any 
 convenient time afterwards ; a portion of the heavens 
 is, as it were, brought actually down to the astronomer's 
 study. 
 
 It was long before this great advance was effected.
 
 ASTRONOMICAL MEASUREMENTS 47 
 
 The first telescopes were very imperfect, for the rays of 
 different colour proceeding from any planet or star 
 came to different foci, so that the image was coloured, 
 diffused, and ill-defined. The first method by which 
 this difficulty was dealt with was by making telescopes 
 of enormously long focal length ; 80, 100, or 150 feet 
 were not uncommon, but these were at once cumber- 
 some and unsteady. Sir Isaac Newton therefore dis- 
 carded the use of object-glasses, and used curved 
 mirrors in order to form the image in the focus, and 
 succeeded in making two telescopes on this principle of 
 reflection. Others followed in the same direction, and 
 a century later Sir William Herschel was most 
 skilful and successful in making " reflectors," his largest 
 being 40 feet in focal length, and thus giving an image 
 of the Moon in its focus of nearly 4| inches diameter. 
 
 But in 1729 Chester Moor Hall found that by 
 combining two suitable lenses together in the object- 
 glass he could get over most of the colour difficulty, 
 and in 1758 the optician Dollond began to make 
 object-glasses that were almost free from the colour 
 defect. From that time onward the manufacture of 
 " refractors," as object-glass telescopes are called, has im- 
 proved ; the glass has been made more transparent 
 and more perfect in quality, and larger in size, and the 
 figure of the lens improved. The largest refractor now 
 in use is that of the Yerkes Observatory, Wisconsin, 
 U.S.A., and is 40 inches in aperture, with a focal length 
 of 65 feet, so that the image of the Moon in its focus 
 has a diameter of more than 7 inches. At present this 
 seems to mark the limit of size for refractors, and the 
 difficulty of getting good enough glass for so large a 
 lens is very great indeed. Reflectors have therefore 
 come again into favour, as mirrors can be made larger
 
 48 SCIENCE OF THE STARS 
 
 than any object-glass. Thus Lord Rosse's great tele- 
 scope was 6 feet in diameter; and the most powerful 
 telescope now in action is the great 5-foot mirror of the 
 Mt. Wilson Observatory, California, with a focal length, 
 as sometimes used, of 150 feet. Thus its light-gathering 
 power is about 60,000 times that of the unaided eye, 
 and the full Moon in its focus is 17 inches in diameter ; 
 such is the enormous increase to man's power of sight, 
 and consequently to his power of learning about the 
 heavenly bodies, which the development of the telescope 
 has afforded to Mm. 
 
 The measurement of time was the first purpose for 
 which men watched the heavenly bodies ; a second 
 purpose was the measurement of the size of the Earth. 
 If at one place a star was observed to pass exactly over- 
 head, and if at another, due south of it, the same star 
 was observed to pass the meridian one degree north of 
 the zenith, then by measuring the distance between the 
 two places the circumference of the whole Earth would 
 be known, for it would be 360 times that amount. In 
 this way the size of the Earth was roughly ascertained 
 2000 years before the invention of the telescope. But 
 with the telescope measures of much greater precision 
 could be made, and hence far more difficult problems 
 could be attacked. 
 
 One great practical problem was that of finding out 
 the position of a ship when out of sight of land. The 
 ancient Phoenician and Greek navigators had mostly 
 confined themselves to coasting voyages along the shores 
 of the Mediterranean Sea, and therefore the quick recog- 
 nition of landmarks was the first requisite for a good 
 sailor. But when, in 1492, Columbus had brought a 
 new continent to fight, and long voyages were freely 
 taken across the great oceans, it became an urgent
 
 ASTRONOMICAL MEASUREMENTS 49 
 
 necessity for the navigator to find out his position when 
 he had been out of sight of any landmark for weeks. 
 
 This necessity was especially felt by the nations of 
 Western Europe, the countries facing the Atlantic with 
 the New World on its far-distant other shore. Spain, 
 France, England, and Holland, all were eager com- 
 petitors for a grasp on the new lands, and therefore 
 were earnest in seeking a solution of the problem of 
 navigation. 
 
 The latitude of the ship could be found out by ob- 
 serving the height of the Sun at noon, or of the Pole 
 Star at night, or in several other ways. But the longi- 
 tude was more difficult. As the Earth turns on its 
 axis, different portions of its surface are brought in 
 succession under the Sun, and if we take the moment 
 when the Sun is on the meridian of any place as its 
 noon, as twelve o'clock for that place, then the differ- 
 ence of longitude between any two places is essentially 
 the difference in their local times. 
 
 It was possible for the sailor to find out when it was 
 local noon for him, but how could he possibly find out 
 what time it was at that moment at the port from 
 which he had sailed, perhaps several weeks before ? 
 
 The Moon and stars supplied eventually the means 
 for giving this information. For the Moon moves 
 amongst the stars, as the hand of a clock moves 
 aii'ingst the figures of a dial, and it became possible 
 at length to predict for long in advance exactly where 
 amongst the stars the Moon would be, for any given 
 time, of any selected place. 
 
 When this method was first suggested, however, 
 neither the motion of the Moon nor the places of the 
 principal stars were known with sufficient accuracy, and 
 it was to remedy this defect, and put navigation upon
 
 50 SCIENCE OF THE STARS 
 
 a sound basis, that Charles II. founded Greenwich 
 Observatory in the year 1675, and appointed Flam- 
 steed the first Astronomer Royal. In the year 1767 
 Maskelyne, the fifth Astronomer Royal, brought out 
 the first volume of the Nautical Almanac, in which the 
 positions of the Moon relative to certain stars were 
 given for regular intervals of Greenwich time. Much 
 about the same period the problem was solved in 
 another way by the invention of the chronometer, by 
 John Harrison, a Yorkshire carpenter. The chrono- 
 meter was a large watch, so constructed that its rate 
 was not greatly altered by heat or cold, so that the 
 navigator had Greenwich time with him wherever he 
 went. 
 
 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 
 Zealand, and a number of islands in the Pacific. The 
 building up of the vast oceanic commerce of Great 
 Britain and of her great colonial empire, both in North 
 America and in the Southern Oceans, has arisen out 
 of the work of the Royal Observatory, Greenwich, and 
 has had a real and intimate connection with it. 
 
 To observe the motions of the Moon, Sun, and planets, 
 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 time. Other great national observatories have 
 been Copenhagen, founded in 1637 ; Paris, in 1667 ; 
 Berlin, in 1700 ; St. Petersburg, in 1725, superseded by 
 that of Pulkowa, in 1839; and Washington, in 1842; 
 while not a few of the great universities have also 
 efficient observatories connected with them. 
 
 Of the directly practical results of astronomy, the
 
 ASTRONOMICAL MEASUREMENTS 51 
 
 promotion of navigation stands in the first rank. But 
 the science has never been limited to merely utilitarian 
 inquiries, and the problem of measuring celestial dis- 
 tances has followed on inevitably from the measurement 
 of the Earth. 
 
 The first distance to be attacked was that of the 
 nearest companion to the Earth, i.e. the Moon. It 
 often happens on our own planet that it is required to 
 find the distance of an object beyond our reach. Thus 
 a general on the march may come to a river and need 
 to know exactly how broad it is, that he may prepare 
 the means for bridging it. Such problems are usually 
 solved on the following principle. Let A be the distant 
 object. Then if the direction of A be observed from 
 each of two stations, B and C, and the distance of B 
 from C be measured, it is possible to calculate the dis- 
 tances of A from B and from C. The application of 
 this principle to the measurement of the Moon's dis- 
 tance was made by the establishment of an observatory 
 at the Cape of Good Hope, to co-operate with that of 
 Greenwich. It is, of course, not possible to see Green- 
 wich Observatory from the Cape, or vice versa, but the 
 stars, being at an almost infinite distance, he in the 
 same direction from both observatories. What is re- 
 quired then is to measure the apparent distance of 
 the Moon from the same stars as seen from Greenwich 
 and as seen from the Cape, and, the distance apart of 
 the two observatories being known, the distance of the 
 Moon can be calculated. 
 
 This was a comparatively easy problem. The next 
 step in celestial measurement was far harder ; it was 
 to find the distance of the Sun. The Sun is 400 times 
 as far off as the Moon, and therefore it seems to be 
 practically in the same direction as seen from each of 
 
 D
 
 52 SCIENCE OF THE STARS 
 
 the two observatories, and, being so bright, stars cannot 
 be seen near it in the telescope. But by carefully 
 watching the apparent movements of the planets their 
 relative distances from the Sun can be ascertained, and 
 were known long before it was thought possible that 
 we should ever know their real distances. Thus Venus 
 never appears to travel more than 47° 15' from the 
 Sun. This means that her distance from the Sun is a 
 little more than seven-tenths of that of the Earth. 
 If, therefore, the distance of one planet from the 
 Sun can be measured, or the distance of one planet 
 from the Earth, the actual distances of all the planets 
 will follow. We know the proportions of the parts of 
 the solar system, and, if we can fix the scale of one of 
 the parts, we fix the scale of all. 
 
 It has been found possible to determine the distance 
 of Mars, of several of the " minor planets," and espe- 
 cially of Eros, a very small minor planet that sometimes 
 comes within 13,000,000 miles of the Earth, or seven 
 times nearer to us than is the Sun. 
 
 From the measures of Eros, we have learned that the 
 Sun is separated from us by very nearly 93,000,000 
 miles — an unimaginable distance. Perhaps the nearest 
 way of getting some conception of this vast interval is 
 by remembering that there are only 31,556,926 seconds 
 of time in a year. If, therefore, an express train, 
 travelling 60 miles an hour — a mile a minute — set out 
 for the Sun, and travelled day and night without cease, 
 it would take more than 180 years to accomplish the 
 journey. 
 
 But this astronomical measure has led on to one 
 more daring still. The earth is on one side of the Sun 
 in January, on the other in July. At these two dates, 
 therefore, we are occupying stations 186,000,000 miles
 
 ASTRONOMICAL MEASUREMENTS 53 
 
 apart, 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 
 stars. A few, a very few, do show a very slight differ- 
 ence. The nearest star to us is about 280,000 times as 
 far from us as the Sun ; this is Alpha Centauri, the 
 brightest star in the constellation of the Centaur, and 
 the third brightest star in the sky. Sirius, the brightest 
 star, ia twice this distance. Some forty or fifty stars 
 have had their distances roughly determined ; but the 
 stars in general far transcend all our attempts to plumb 
 their distances. But, from certain indirect hints, it is 
 generally supposed that the mass of stars in the Milky 
 Way are something like 300,000,000 times as far from 
 us as we are from our Sun. 
 
 Thus far, then, astronomy has led us in the direction 
 of measurement. It has enabled us to measure the 
 size of the Earth upon which we live, and to find out 
 the position of a ship in the midst of the trackless ocean. 
 It has also enabled us to cast a sounding-line into 
 space, to show how remote and solitary the earth moves 
 through the void, and to what unimaginable lengths 
 the great stellar universe, of which it forms a secluded 
 atom, stretches out towards infinity.
 
 CHAPTER V 
 
 THE MEMBERS OF THE SOLAR SYSTEM 
 
 Astronomical measurement has not only given us the 
 distances of the various planets from the Sun ; it has 
 also furnished us, as in the annexed table, with their 
 real diameters, and, as a consequence of the law of 
 gravitation, with their densities and weights, and the 
 force of gravity at their surfaces. 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 conditions. 
 
 The theory of Copernicus deprived the Earth of its 
 special position as the immovable centre of the uni- 
 verse, but raised it to the rank of a planet. It is there- 
 fore a heavenly body, yet needs no telescope to bring 
 it within our ken ; bad weather does not hide it from 
 us, but rather shows it to us under new conditions. 
 We find it to be a globe of land and water, covered by 
 an atmosphere in which float changing clouds ; we have 
 mapped it, and we find that the land and water are 
 always there, but their relations are not quite fixed ; 
 there is give and take between them. We have found 
 of what elements the land and water consist, and how 
 these elements combine with each other or dissociate. 
 In a word, the Earth is the heavenly body that we know 
 the best, and with it we must compare and contrast all 
 the others. 
 
 Before the invention of the telescope there were but 
 
 54
 
 MEMBERS OF THE SOLAR SYSTEM 55 
 
 two other heavenly bodies — the Sun and the Moon — 
 that appeared as orbs showing visible discs, and even 
 in their cases nothing could be satisfactorily made out 
 as to their conditions. Now each of the five planets 
 known to the ancients reveals to us in the telescope a 
 measurable disc, and we can detect significant details 
 on their surfaces. 
 
 The Moon is the one object in the heavens which 
 does not disappoint <* novice when he first sees it in the 
 telescope. Every detail is hard, clear-cut, and sharp ; 
 it i3 manifest that we are looking at a globe, a very 
 rough globe, with hills and mountains, plains and valleys, 
 the whole in such distinct relief that it seems as if it 
 might be touched. No clouds ever conceal its details, 
 no mist ever softens its outlines ; there are no half- 
 lights, its shadows are dead black, its high lights are 
 molten silver. Certain changes of illumination go on 
 with the advancing age of the Moon, as the crescent 
 broadens out to the half, the half to the full, and the 
 full, in its turn, wanes away ; but the lunar day is 
 nearly thirty times as long as that of the Earth, and 
 these changes proceed but slowly. 
 
 The full Moon, as seen by the naked eye, shows several 
 vague dark spots, which most people agree to fancy as 
 like the eyes, nose, and mouth of a broad, sorrowful 
 face. The ordinary astronomical telescope inverts the 
 image, 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 disc. But in the 
 upper part, near the left-hand corner of the underlip, 
 there is a bright, round spot, from which a number of 
 bright streaks radiate — suggesting a peeled orange with 
 its stalk, and the fines marking the sections radiating 
 from it. This bright spot has been called after the great
 
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 Uranus 
 Neptune .
 
 MEMBERS OF THE SOLAR SYSTEM 59 
 
 Danish astronomer, " Tycho," and is one of the most 
 conspicuous objects of the full Moon. 
 
 The contrasts of the Moon are much more pro- 
 nounced when she is only partly lit up. Then the 
 mountains and valleys stand out in the strongest relief, 
 and it becomes clear that the general type of formation 
 on the Moon is that of rings — rings of every conceivable 
 size, from the smallest point that the telescope can 
 detect up to some of the great dusky plains them- 
 selves, hundreds of miles in diameter. These rings are 
 so numerous that Gahleo described the Moon as look- 
 ing as full of " eyes " as a peacock's tail. 
 
 The " right eye " of the moonface, as we see it in the 
 sky, is formed by a vast dusky plain, nearly as large 
 as France and Germany put together, to which has 
 been given the name of the " Sea of Rains " (Mare 
 Imbrium), and just below this (as seen in the tele- 
 scope) is one of the most perfect and beautiful of all 
 the lunar rings — a great ring-plain, 56 miles in diameter, 
 called after the thinker who revolutionised men's ideas 
 of the solar system, " Copernicus." " Copernicus," like 
 " Tycho," is the centre of a set of bright streaks ; 
 and a neighbouring but smaller ring, bearing the 
 great name of " Kepler," stands in a like relation to 
 another set. 
 
 The most elevated region of the Moon is immediately 
 in the neighbourhood of the great " stalk of the orange," 
 " Tycho." Here the rings are crowded together as 
 closely as they can be packed ; more closely in many 
 places, for they intrude upon and overlap each other 
 in the most intricate manner. A long chain of fine 
 rings stretches from this disturbed region nearly to 
 the centre of the disc, where the great Alexandrian 
 astronomer is commemorated by a vast walled plain,
 
 60 SCIENCE OF THE STARS 
 
 considerably larger than the whole of Wales, and known 
 as " Ptolemaeus." 
 
 The distinctness of the lunar features shows at once 
 that the Moon is in an altogether different condition 
 from that of the Earth. Here the sky is continually 
 being hidden by cloud, and hence the details of the 
 surface of the Earth as viewed from any other planet 
 must often be invisible, and even when actual cloud is 
 absent there is a more permanent veil of dust, which 
 must greatly soften and confuse terrestrial outlines. 
 The clearness, therefore, with which we perceive the 
 lunar formations proves that there is little or no 
 atmosphere there. Nor is there any sign upon 
 it of water, either as seas or lakes or running 
 streams. 
 
 Yet the Moon shows clearly that in the past it has 
 gone through great and violent changes. The grada- 
 tion is so complete from the little craterlets, which 
 resemble closely, in form and size, volcanic craters on 
 the Earth, up to the great ring-plains, like "Coper- 
 nicus " or " Tycho," or formations larger still, that it 
 seems natural to infer not only that the smaller craters 
 were formed by volcanic eruption, like the similar ob- 
 jects with which we are acquainted on our own Earth, 
 but that the others, despite their greater sizes, had a 
 like origin. In consequence of the feebler force of 
 gravity on the Moon, the same explosive force there 
 would carry the material of an eruption much further 
 than on the Earth. 
 
 The darker, low-lying districts of the Moon give 
 token of changes of a different order. It is manifest 
 that the material of which the floors of these plains is 
 composed has invaded, broken down, and almost sub- 
 merged many of the ring-formations. Sometimes half
 
 MEMBERS OF THE SOLAR SYSTEM 61 
 
 of a ring has been washed away ; sometimes just the 
 outline of a ring can still be traced upon the floor of 
 the sea ; sometimes only a slight breach has been 
 made in the wall. So it is clear that the Moon was 
 once richer in the great crater-like formations than it 
 is to-day, but a lava-flood has overflowed at least 
 one-third of its area. More recent still are the bright 
 streaks, or rays, which radiate in all directions from 
 " Tycho," and from some of the other ring-plains. 
 
 It is evident from these different types of structure 
 on the Moon, and from the relations which they bear to 
 each other, that the lunar surface has passed through 
 several successive stages, and that its changes tended, 
 on the whole, to diminish in violence as time went on ; 
 the minute crater pits with which the surface is stippled 
 having been probably the last to form. 
 
 But the 300 years during which the Moon has been 
 watched with the telescope have afforded no trace of 
 any continuance of these changes. She has had a 
 stormy and fiery past ; but nothing like the events of 
 those bygone ages disturbs her serenity to-day. 
 
 And yet we must believe that change does take place 
 on the Moon even now, because during the 354 hours 
 of its long day the Sun beats down with full force on 
 the unprotected surface, and during the equally long 
 night that surface is exposed to the cold of outer space. 
 Every part of the surface must be exposed in turn to 
 an extreme range of temperature, and must be cracked, 
 torn, and riven by alternate expansion and contraction. 
 Apart from this slow, wearing process, and a very few 
 rather doubtful cases in which a minute alteration of 
 some surface detail has been suspected, our sister planet, 
 the Moon, shows herself as changeless and inert, with- 
 out any appreciable trace of air or water or any sign
 
 62 SCIENCE OF THE STARS 
 
 of life — a dead world, with all its changes and activities 
 in the past. 
 
 Mars, after the Moon, is the planet whose surface we 
 can study to best advantage. Its orbit lies outside 
 that of the Earth, so that when it is nearest to us it 
 turns the same side to both the Sun and Earth, and we 
 see it fully illuminated. Mercury and Venus, on the 
 contrary, when nearest us are between us and the 
 Sun, and turn their dark sides to us. When fully 
 illuminated they are at their greatest distance, and 
 appear very small, and, being near the Sun, are observed 
 with difficulty. These three are intermediate in size 
 between the Moon and the Earth. 
 
 In early telescopic days it was seen that Mars was 
 an orange-coloured globe with certain dusky markings 
 upon it, and that these markings slowly changed their 
 place — that, in short, it was a world rotating upon its 
 axis, and in a period not very different from that of 
 the Earth. The rotation period of Mars has indeed 
 been fixed to the one-hundredth part of a second of 
 time ; it is 24 h. 37 m. 22-67 s. And this has been 
 possible because some of the dusky spots observed in 
 the seventeenth century can be identified now in the 
 twentieth. Some of the markings on Mars, like our 
 own continents and seas, and like the craters on the 
 Moon, are permanent features ; and many charts of 
 the planet have been constructed. 
 
 Other markings are variable. Since the planet ro- 
 tates on its axis, the positions of its poles and equator 
 are known, its equator being inclined to its orbit at an 
 angle of 24° 50', while the angle in the case of the 
 Earth is 23° 27'. The times when its seasons begin 
 and end are therefore known ; and it is found that 
 the spring of its northern hemisnhere lasts 199 of our
 
 MEMBERS OF THE SOLAR SYSTEM 63 
 
 days, the summer 183, the autumn 147, and the winter 
 158. Round the pole in winter a broad white cap 
 forms, which begins to shrink as spring comes on, and 
 may entirely disappear in summer. No corresponding 
 changes have been observed on the Moon, but it is 
 easy to find an analogy to them on the Earth. Round 
 both our poles a great cap of ice and snow is spread — 
 a cap which increases in size as winter comes on, and 
 diminishes with the advance of summer — and it seems 
 a reasonable inference to suppose that the white polar 
 caps of Mars are, like our own, composed of ice and 
 snow. ^ 
 
 From time to time indications have been observed 
 of the presence on Mars of a certain amount of cloud. 
 Familiar dark markings have, for a short time, been 
 interrupted, or been entirely hidden, by white bands, 
 and have recovered their ordinary appearance later. 
 With rotation on its axis and succession of seasons, 
 with atmosphere and cloud, with land and water, with 
 ice and snow, Mars would seem to be a world very 
 similar to our own. 
 
 This was the general opinion up to the year 1877, 
 when Schiapakelli announced that he had discovered 
 a number of very narrow, straight, dark fines on the 
 planet — fines to which he gave the name of " canali " — 
 that is, " channels." This word was unfortunately 
 rendered into English by the word " canals," and, as a 
 canal means a waterway artificially made, this mis- 
 translation gave the idea that Mars was inhabited by 
 intelligent beings, who had dug out the surface of the 
 planet into a network of canals of stupendous length 
 and breadth. 
 
 The chief advocate of this theory is Lowell, an 
 American observer, who has given very great attention
 
 64 SCIENCE OF THE STARS 
 
 to the study of the planet during the last seventeen 
 years. His argument is that the straight lines, the 
 canals, which he sees on the planet, and the round 
 dots, the " oases," which he finds at their intersections, 
 form a system so obviously wwnatural, that it must be 
 the work of design — of intelligent beings. The canals 
 are to him absolutely regular and straight, like fines 
 drawn with ruler and pen-and-ink, and the oases are 
 exactly round. But, on the one hand, the best ob- 
 servers, armed with the most powerful telescopes, have 
 often been able to perceive that markings were really 
 full of irregular detail, which Lowell has represented 
 as mere hard straight fines and circular dots, and, on 
 the other hand, the straight fine and the round dot are 
 the two geometric forms which all very minute objects 
 must approach in appearance. That we cannot see 
 irregularities in very small and distant objects is nc 
 proof at all that irregularities do not exist in them, 
 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 
 nearer. 
 
 Astronomers, therefore, 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 
 origin. And indeed the numerical facts that we know 
 about the planet render it almost impossible that there 
 should be any life upon it. 
 
 If we turn to the table, we see that in size, volume, 
 density, and force of gravity at its surface, Mars lies 
 between the Moon and the Earth, but is nearer the 
 Moon. This has an important bearing as to the ques- 
 tion of the planet's atmosphere. On the Earth we pass 
 through half the atmosphere by ascending a mountain
 
 MEMBERS OF THE SOLAR SYSTEM 65 
 
 that is three and a third miles in height ; on Mars we 
 should have to ascend nearly nine miles. If the atmo- 
 spheric pressure at the surface of Mars were as great 
 as it is at the surface of the Earth, his atmosphere 
 would be far deeper than ours and would veil the planet 
 more effectively. But we see the surface of Mars with 
 remarkable distinctness, almost as clearly, when its 
 greater distance is allowed for, as we see the Moon. 
 It is therefore accepted that the atmospheric pressure 
 at the surface of Mars must be very slight, probably 
 much less than at the top of our very highest moun- 
 tains, where there is eternal snow, and life is completely 
 absent. 
 
 But Mars compares badly with the Earth in another 
 respect. It receives less light and heat from the Sun 
 in the proportion of three to seven. This we may 
 express by saying that Mars, on the whole, is almost 
 as much worse off than the Earth as a point on the 
 Arctic Circle is worse off than a point on the Equator. 
 The mean temperature of the Earth is taken as about 
 60° of the Fahrenheit thermometer (say, 15° Cent.) ; the 
 mean temperature of Mars must certainly be consider- 
 ably below freezing-point, probably near 0° F. Here 
 on our Earth the boiling-point of water is 212°, and, 
 since the mean temperature is 60° and water freezes 
 at 32°, it is normally in the liquid state. On Mars it 
 must normally be in the solid state — ice, snow, or 
 frost, or the like. But with so rare an atmosphere 
 water will boil at a low temperature, and it is not im- 
 possible that under the direct rays of the Sun — that is 
 to say, at midday of the torrid zone of Mars — ice may 
 not only melt, but water boil by day, condensing and 
 freezing again during the night. Newcomb, the fore- 
 most astronomer of his day, concluded " that during
 
 66 SCIENCE OF THE STARS 
 
 the night of Mars, even in the equatorial regions, the 
 surface of the planet probably falls to a lower tem- 
 perature than any we ever experienced on our globe. 
 If any water exists, it must not only be frozen, but the 
 temperature of the ice must be far below the freez- 
 ing point. . . . The most careful calculation shows that 
 if there are any considerable bodies of water on our 
 neighbouring planet, they exist in the form of ice, and 
 can never be liquid to a depth of more than one or two 
 inches, and that only within the torrid zone and during 
 a few hours each day." With regard to the snow caps 
 of Mars, Newcomb thought it not possible that any 
 considerable fall of snow could ever take place. He 
 regarded the white caps as simply due to a thin deposit 
 of hoar frost, and it cannot be deemed wonderful that 
 such should gradually disappear, when it is remembered 
 that each of the two poles of Mars is in turn presented 
 to the Sun for more than 300 consecutive days. New- 
 comb's conclusion was : " Thus we have a kind of Martian 
 meteorological changes, very slight indeed, and seem- 
 ingly very different from those of our Earth, but yet 
 following similar lines on their small scale. For snowfall 
 substitute frostfall ; instead of (the barometer reading) 
 feet or inches say fractions of a millimetre, and instead 
 of storms or wind substitute little motions of an air 
 thinner than that on the top of the Himalayas, and we 
 shall have a general description of Martian meteorology." 
 
 We conclude, then, that Mars is not so inert a world 
 as the Moon, but, though some slight changes of climate 
 or weather take place upon it, it is quite unfitted for 
 the nourishment and development of the different forms 
 of organic fife. 
 
 Of Mercury we know very little. It is smaller than 
 Mars but larger than the Moon, but it differs from them
 
 MEMBERS OF THE SOLAR SYSTEM 67 
 
 both in that it is much nearer the Sun, and receives, 
 therefore, many times the light and heat, surface for 
 surface. We should expect, therefore, that water on 
 Mercury would exist in the gaseous state instead of in 
 the solid state as on Mars. The little planet reflects 
 the sunlight only feebly, and shows no evidence of 
 cloud. A few markings have been made out on its 
 surface, and the best observers agree that it appears to 
 turn the same face always to the Sun. This would 
 imply that the one hemisphere is in perpetual dark- 
 ness and cold, the other, exposed to an unimaginable 
 fiery heat. 
 
 Venus is nearly of the same size as the Earth, and 
 the conditions as to the arrangement of its atmosphere, 
 the force of gravity at its surface, must be nearly the 
 same as on our own world. But we know almost 
 nothing of the details of its surface ; the planet is very 
 bright, reflecting fully seven-tenths of the sunlight that 
 falls upon it. It would seem that, in general, we see 
 nothing of the actual details of the planet, but only 
 the upper surface of a very cloudy atmosphere. Owing 
 to the fact that Venus shows no fixed definite marking 
 that we can watch, it is still a matter of controversy as 
 to the time in which it rotates upon its axis. Schia- 
 parelli and some other observers consider that it rotates 
 in the same time as it revolves round the Sun. Others 
 believe that it rotates in a little less than twenty-four 
 hours. If this be so, and there is any body in the solar 
 system other than the Earth, which is adapted to be the 
 home of life, then the planet Venus is that one. 
 
 The Sun, like the Moon, presents a visible surface to 
 the naked eye, but one that shows no details. In the 
 telescope the contrast between it and the Moon is very 
 great, and still greater is the contrast which is brought 
 
 E
 
 68 SCIENCE OF THE STARS 
 
 out by the measurements of its size, volume, and weight. 
 But the really significant difference is that the Sun is 
 a body giving out light and heat, not merely reflecting 
 them. Without doubt this last difference is connected 
 most closely with the difference in size. The Moon is 
 cold, dead, unchanging, because it is a small world ; 
 the Sun is bright, fervent, and undergoes the most 
 violent change, because it is an exceedingly large world. 
 
 The two bodies — the Sun and Moon — appear to the eye 
 as being about the same size, but since the Sun is 400 
 times as far off as the Moon it must be 400 times the 
 diameter. That means that it has 400 times 400, or 
 160,000 times the surface and 400 times 400 times 400, 
 or 64,000,000 times the volume. The Sun and Moon, 
 therefore, stand at the very extremes of the scale. 
 
 The heat of the Sun is so great that there is some 
 difficulty in observing it in the telescope. It is not 
 sufficient to use a dark glass in order to protect the eye, 
 unless the telescope be quite a small one. Some means 
 have to be employed to get rid of the greater part of 
 the heat and light. 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 
 screen, or the sensitive plate may be substituted for 
 the screen, and a photograph obtained, which can be 
 examined at leisure afterwards. 
 
 As generally seen, the surface of the Sun appears 
 to be mottled all over by a fine irregular stippling. 
 This stippling, though everywhere present, is not very 
 strongly marked, and a first hasty glance might over- 
 look it. From time to time, however, dark spots are 
 seen, of ever-changing form and size. By watching 
 these, Galileo proved that the Sun rotated on its axis 
 in a little more than twenty-five days, and in the nine-
 
 MEMBERS OF THE SOLAR SYSTEM 69 
 
 teenth century Schwabe proved that the sunspots 
 were not equally large and numerous at all times, but 
 that there was a kind of cycle of a little more than 
 eleven years in average length. At one time the Sun 
 would be free from spots ; then a few small ones would 
 appear ; these would gradually become larger and more 
 numerous; then a decline would follow, and another 
 spotless period would succeed about eleven years after 
 the first. As a rule, the increase in the spots takes place 
 more quickly than the decline. 
 
 Most of the spot-groups last only a very few days, 
 but about one group in four lasts long enough to be 
 brought round by the rotation of the Sun a second 
 time ; in other words, it continues for about a month. 
 In a very few cases spots have endured for half a 
 year. 
 
 An ordinary form for a group of spots is a long 
 stream drawn out parallel to the Sun's equator, the 
 leading spot being the largest and best defined. It is 
 followed by a number of very small irregular and ill- 
 developed spots, and the train is brought up by a large 
 spot, sometimes even larger than the leader, but by no 
 means so regular in form or so well denned. The leading 
 spot for a short time moves forward much faster than 
 its followers, at a speed of about 8000 miles per day. 
 The small middle spots then gradually die out, or rather 
 seem to be overflowed by the bright material of the 
 solar surface, the " photosphere," as it is called ; the spot 
 in the rear breaks up a little later, and the leader, which 
 is now almost circular, is left alone, and may last in this 
 condition for some weeks. Finally, it slowly contracts 
 or breaks up, and the disturbance comes to an end. 
 This is the course of development of many long-lived 
 spot-groups, but all do not conform to the same type.
 
 70 SCIENCE OF THE STARS 
 
 The very largest spots are indeed usually quite different 
 in their appearance and history. 
 
 In size, 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 miles ; the great group of February 1905 
 had an area of 4,000,000,000 square miles, a thousand 
 times the area of Europe. 
 
 Closely associated with the maculce, as the spots were 
 called by the first observers, are the "faculse" — long, 
 branching lines of bright white light, bright as seen even 
 against the dazzling background of the Sun itself, and 
 looking like the long lines of foam of an incoming tide. 
 These are often associated with the spots ; the spots 
 are formed between their ridges, and after a spot- 
 group has disappeared the broken waves of faculse wall 
 sometimes persist in the same region for quite a long 
 time. 
 
 The faculse clearly rise above the ordinary solar sur- 
 face ; the spots as clearly are depressed a little below 
 it ; because from time to time we see the bright material 
 of the surface pour over spots, across them, and some- 
 times into them. But there is no reason to believe 
 that the spots are deep, in proportion either to the Sun 
 itself or even to their own extent. 
 
 Sunspots are not seen in all regions of the Sun. It is 
 very seldom that they are noted in a higher solar lati- 
 tude than 40°, the great majority of spots lying in the 
 two zones between 5° and 25° latitude on either side 
 of the equator. Faculse, on the other hand, though 
 most frequent in the spot zones, are observed much 
 nearer the two poles. 
 
 It is very hard to find analogies on our Earth for 
 sunspots and for their peculiarities of behaviour. Some
 
 MEMBERS OF THE SOLAR SYSTEM 71 
 
 of the earlier astronomers thought they were like 
 terrestrial volcanoes, or rather like the eruptions from 
 them. But if there were a solid nucleus to the Sun, 
 and the spots were eruptions from definite areas of the 
 nucleus, they would all give the same period of rotation. 
 But sunspots move about freely on the solar surface, 
 and the different zones of that surface rotate in different 
 times, the region of the equator rotating the most 
 quickly. This alone is enough to show that the Sun 
 is essentially not a solid body. Yet far down below 
 the photosphere something approaching to a definite 
 structure must already be forming. For there is a 
 well-marked progression in the zones of sunspots during 
 the eleven-year cycle. At a time when spots are few 
 and small, known as the sunspot minimum, they begin 
 to be seen in fairly high latitudes. As they get more 
 numerous, and many of them larger, they frequent the 
 medium zones. When the Sun is at its greatest activity, 
 known as the sunspot maximum, they are found from 
 the highest zone right down to the equator. Then the 
 decline sets in, but it sets in first in the highest zones, 
 and when the time of minimum has come again the 
 spots are close to the equator. Before these have all 
 died away, a few small spots, the heralds of a new 
 cycle of activity, begin to appear in high latitudes. 
 
 This law, called after Sporer, its discoverer, indicates 
 that the origin and source of sunspot activity he within 
 the Sun. At one time it was thought that sunspots 
 were due to some action of Jupiter — for Jupiter moves 
 round the Sun in 11*8 years, a period not very different 
 from the sunspot cycle — or to some meteoric stream. 
 But Sporer's Law could not be imposed by some influ- 
 ence from without. Still sunspots, once formed, may 
 be influenced by the Earth, and perhaps by other
 
 72 SCIENCE OF THE STARS 
 
 planets also, for Mrs. Walter Maunder has shown 
 that the numbers and areas of spots tend to be smaller 
 on the western half of the disc, as seen from the Earth, 
 than on the eastern, while considerably more groups 
 come into view at the east edge of the Sun than 
 pass out of view at the west edge, so that it would 
 appear as if the Earth had a damping effect upon the 
 spots exposed to it. 
 
 But the Sun is far greater than it ordinarily appears 
 to us. Twice every year, and sometimes oftener, the 
 Moon, when new, comes between the Earth and the 
 Sun, and we have an Eclipse of the Sun, the dark body 
 of the Moon hiding part, or all, of the greater light. 
 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 surface, and, as the Moon 
 moves more quickly than the Sun, the eclipse only 
 remains total for a very short time — seven minutes at 
 the outside, more usually only two or three. North or 
 south of that belt the Moon is projected, so as to 
 leave uncovered a part of the Sun north or south of 
 the Moon. A total eclipse, therefore, is rare at any 
 particular place, and if a man were able to put himself 
 in the best possible position on each occasion, it would 
 cost him thirty years to secure an hour's accumulated 
 duration. 
 
 Eclipses of the Moon are visible over half the world 
 at one time, for there is a real loss to the Moon of her 
 light. Her eclipses are brought about when, in her 
 orbit, she passes behind the Earth, and the Earth, being 
 between the Sun and the Moon, cuts off from the latter 
 most of the light falling upon her; not quite all; a 
 small portion reaches her after passing through the 
 thickest part of the Earth's atmosphere, so that the
 
 MEMBERS OF THE SOLAR SYSTEM 73 
 
 Moon in an eclipse looks a deep copper colour, much as 
 she does when rising on a foggy evening. 
 
 Total eclipses of the Sun have well repaid all the 
 efforts made to observe them. 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 
 hidden ; to watch the colours fade away from the 
 landscape and a deathlike, leaden hue pervade all 
 nature, and then to see a silvery, star-like halo, flecked 
 with bright little rose-coloured flames, flash out round 
 the black disc that has taken the place of the Sun. 
 
 These rose-coloured flames are the solar " prominences," 
 and the halo is the " corona," and it is to watch these 
 that astronomers have made so many expeditions hither 
 and thither during the last seventy years. The " pro- 
 minences," or red flames, can be observed, without an 
 eclipse, by means of the spectroscope, but, as the work 
 of the spectroscope is to form the subject of another 
 volume of this series, it is sufficient to add here that 
 the prominences are composed of various glowing gases, 
 chiefly of hydrogen, calcium, and helium. 
 
 These and other gases form a shell round the Sun, 
 about 3000 miles in depth, to which the name " chromo- 
 sphere " has been given. It is out of the chromosphere 
 that the prominences arise as vast irregular jets and 
 clouds. Ordinarily they do not exceed 40 or 50 thousand 
 miles in height, but occasionally they extend for 200 
 or even 300 thousand miles from the Sun. Their 
 changes are as remarkable as their dimensions ; huge 
 jets of 50 or 100 thousand miles have been seen to 
 form, rise, and disappear within an hour or less, and 
 movements have been chronicled of 200 or 300 miles 
 in a single second of time. 
 
 Prominences are largest and most frequent when
 
 74 SCIENCE OF THE STARS 
 
 sunspots and faculae are most frequent, and fewest 
 when those are fewest. The corona, too, varies with 
 the sunspots. At the time of maximum the corona 
 sends forth rays and streamers in all directions, and 
 looks like the conventional figure of a star on a gigantic 
 scale. At minimum the corona is simpler in form, and 
 shows two great wings, east and west, in the direction 
 of the Sun's equator, and round both of his poles a 
 number of small, beautiful jets like a crest of feathers. 
 
 Some of the streamers or wings of the corona have 
 been traced to an enormous distance from the Sun. 
 Mrs. Walter Maunder photographed one ray of the 
 corona of 1898 to a distance of 6 millions of miles. 
 Langley, in the clear air of Pike's Peak, traced the 
 wings of the corona of 1878 with the naked eye to 
 nearly double tins distance. 
 
 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 
 Sun, i.e. the enormous amount of light and heat which 
 it is continuaUy giving off. Here we can only put 
 together figures which by their vastness escape our 
 understanding. Sunlight is to moonlight as 600,000 is 
 to 1, so that if the entire sky were filled up with full 
 moons, they would not give us a quarter as much light 
 as we derive from the Sun. The intensity of sunlight 
 exceeds by far any artificial light ; it is 150 times as 
 bright as the calcium light, and three or four times as 
 bright as the brightest part of the electric arc fight. 
 The amount of heat radiated by the Sun has been ex- 
 pressed in a variety of different ways ; C. A. Yotjng very 
 graphically by saying that if the Sun were encased in 
 a shell of ice 64 feet deep, its heat would melt the shell 
 in one minute, and that if a bridge of ice could be
 
 MEMBERS OF THE SOLAR SYSTEM 75 
 
 formed from the Earth to the Sun, 2h miles square in 
 section and 93 millions of miles long, and the entire 
 solar radiation concentrated upon it, in one second the 
 ice would be melted, in seven more dissipated into 
 vapour. 
 
 The Earth derives from the Sun not merely light 
 and heat, but, by transformation of these, almost every 
 form of energy manifest upon it ; the energy of the 
 growth of plants, the vital energy of animals, are only 
 the energy received from the Sun, changed in its 
 expression. 
 
 The question naturally arises, " If the Sun, to which 
 the Earth is indebted for nearly everything, passes 
 through a change in its activity every eleven years or 
 so, how is the Earth affected by it ? " It would seem 
 at first sight that the effect should be great and mani- 
 fest. A sunspot, like that of February 1905, one 
 thousand times as large as Europe, into which worlds 
 as large as our Earth might be poured, like peas into a 
 saucer, must mean, one might think, an immense fall- 
 ing off of the solar heat. 
 
 Yet it is not so. For even this great sunspot was 
 but small as compared with the Sun as a whole. Had 
 it been dead black, it would have stopped out much 
 less than 1 per cent, of the Sun's heat ; and even the 
 darkest sunspot is really very bright. And the more 
 spots there are, the more numerous and brighter are 
 the faculse ; so that we do not know certainly which of 
 the two phases, maximum or minimum, means the 
 greater radiation. If the weather on the Earth answers 
 to the sunspot cycle, the connection is not a simple 
 one ; as yet no connection has been proved. Thus 
 two of the worst and coldest summers experienced in 
 England fell the one in 1860, the other in 1879, i.e. at
 
 76 SCIENCE OF THE STARS 
 
 maximum and minimum respectively. So, too, the hot 
 summers of 1893 and 1911 were also, the one at 
 maximum and the other at minimum ; and ordinary 
 average years have fallen at both the phases just the 
 same. 
 
 Yet there is an answer on the part of the Earth to 
 these solar changes. The Earth itself is a kind of 
 magnet, possessing a magnetism of which the intensity 
 and direction is always changing. To watch these 
 changes, very sensitive magnets are set up, and a slight 
 daily to-and-fro swing is noticed in them ; this swing is 
 more marked in summer than in winter, but it is also 
 more marked at times of the sunspot maximum than 
 at minimum, showing a dependence upon the solar 
 activity. 
 
 Yet more, from time to time the magnetic needle 
 undergoes more or less violent disturbance ; in extreme 
 cases the electric telegraph communication has been 
 disturbed all over the world, as on September 25, 1909, 
 when the submarine cables ceased to carry messages 
 for several hours. In most cases when such a " magnetic 
 storm " occurs, there is an unusually large or active 
 spot on the Sun. The writer was able in 1904 to 
 further prove that such " storms " have a marked 
 tendency to recur when the same longitude of the Sun 
 is presented again towards the Earth. Thus in 
 February 1892, when a very large spot was on the 
 Sun, a violent magnetic storm broke out. The spot 
 passed out of sight and the storm ceased, but in the 
 following month, when the spot reached exactly the 
 same apparent place on the Sun's disc, the storm broke 
 out again. Such magnetic disturbances are therefore 
 due to streams of particles driven off from limited areas 
 of the Sun, probably in the same way that the long,
 
 MEMBERS OF THE SOLAR SYSTEM 77 
 
 straight rays of the corona are driven off. Such streams 
 of particles, shot out into space, do not spread out 
 equally in all directions, like the rays of light and heat, 
 but are limited in direction, and from time to time 
 they overtake the Earth in its orbit, and, striking it, 
 cause a magnetic storm, which is felt all over the Earth 
 at practicaDy the same moment. 
 
 Jupiter is, after the Sun, much the largest member 
 of the solar system, and it is a peculiarly beautiful 
 object in the telescope. Even a small instrument shows 
 the little disc striped with many delicately coloured 
 bands or belts, broken by white clouds and dark streaks, 
 like a " windy sky " at sunset. And it changes while 
 being watched, for, though 400,000,000 miles away from 
 us, it rotates so fast upon its axis that its central 
 markings can actually be seen to move. 
 
 This rapid rotation, in less than ten hours, is the 
 most significant fact about Jupiter. For different spots 
 have different rotation periods, even in the same lati- 
 tude, proving that we are looking down not upon any 
 solid surface of Jupiter, but upon its cloud envelope — 
 an envelope swept by its rapid rotation and by its winds 
 into a vast system of parallel currents. 
 
 One object on Jupiter, the great " Bed Spot," has been 
 under observation since 1878, and possibly for 200 years 
 before that. It is a large, oval object fitted in a frame 
 of the same shape. The spot itself has often faded and 
 been lost since 1878, but the frame has remained. The 
 spot is in size and position relative to Jupiter much as 
 Australia is to the Earth, but while Australia moves 
 solidly with the rest of the Earth in the daily rota- 
 tion, neither gaining on South America nor losing 
 on Africa, the Red Spot on Jupiter sees many 
 other spots and clouds pass it by, and does not even
 
 78 SCIENCE OF THE STARS 
 
 retain the same rate of motion itself from one year to 
 another. 
 
 No other marking on Jupiter is so permanent as this. 
 From time to time great round white clouds form in a 
 long series as if shot up from some eruption below, and 
 then drawn into the equatorial current. From time to 
 time the belts themselves change in breadth, in colour, 
 and complexity. Jupiter is emphatically the planet of 
 change. 
 
 And such change means energy, especially energy in 
 the form of heat. If Jupiter possessed no heat but 
 that it derived from the Sun, it would be colder than 
 Mars, and therefore an absolutely frozen globe. But 
 these rushing winds and hurrying clouds are evidences 
 of heat and activity — a native heat much above that 
 of our Earth. While Mars is probably nearer to the 
 Moon than to the Earth in its condition, Jupiter has 
 probably more analogies with the Sun. 
 
 The one unrivalled distinction of Saturn is its Ring. 
 Nothing like this exists elsewhere in the solar system. 
 Everywhere else we see spherical globes ; this is a flat 
 disc, but without its central portion. It surrounds the 
 planet, lying in the plane of its equator, but touches it 
 nowhere, a gap of 7000 miles intervening. It appears 
 to be circular, and is 42,000 miles in breadth. 
 
 Yet it is not, as it appears to be, a flat continuous 
 surface. It is in reality made up of an infinite number 
 of tiny satellites, mere dust or pebbles for the most 
 part, but so numerous as to look from our distance like 
 a continuous ring, or rather like three or four concentric 
 rings, for certain divisions have been noticed in it — an 
 inner broad division called after its discoverer, Casstni, 
 and an outer, fainter, narrower one discovered by 
 Encke. The innermost part of the ring is dusky, fainter
 
 MEMBERS OF THE SOLAR SYSTEM 79 
 
 than the planet or the rest of the ring, and is known as 
 the " crape-ring." 
 
 Of Saturn itself we know little ; it is further off and 
 fainter than Jupiter, and its details are not so pro- 
 nounced, but in general they resemble those of Jupiter. 
 The planet rotates quickly — in 10 h. 14 m. — its mark- 
 ings run into parallel belts, and are diversified by spots 
 of the same character as on Jupiter. Saturn is pro- 
 bably possessed of no small amount of native heat. 
 
 Uranus and Neptune are much smaller bodies than 
 Jupiter and Saturn, though far larger than the Earth. 
 But their distance from the Earth and Sun makes their 
 discs small and faint, and they show little in the tele- 
 scope beyond a hint of " belts '' like those of Jupiter ; 
 so that, as with that planet, the surfaces that they 
 show are almost certainly the upper surfaces of a shell 
 of cloud. 
 
 In general, therefore, 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 change ; that smaller bodies are less hot and less 
 active, until we come down to the smallest, which are 
 cold, inert, and dead. Our own Earth, midway in the 
 series, is itself cold, but is placed at such a distance 
 from the Sun as to receive from it a sufficient but not 
 excessive supply of fight and heat, 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 life. 
 
 The smallest members of the solar system are known 
 as Meteors. These are often no more than pebbles 
 or particles of dust, moving together in associated orbits 
 round the Sun. They are too small and too scattered 
 to be seen in open space, and become visible to us only
 
 80 SCIENCE OF THE STARS 
 
 when their orbits intersect that of the earth, and the 
 earth actually encounters them. They then rush into 
 our atmosphere at a great speed, and become highly 
 heated and luminous as they compress the air before 
 them ; so highly heated that most are vapourised and 
 dissipated, but a few reach the ground. As they are 
 actually moving in parallel paths at the time of one of 
 these encounters, they appear from the effect of per- 
 spective to diverge from a point, hence called the 
 " radiant." Some showers occur on the same date of 
 every year ; thus a radiant in the constellation Lyra is 
 active about April 21, giving us meteors, known as 
 the " Lyrids " ; and another in Perseus in August, 
 gives us the " Perseids." Other radiants are active 
 at intervals of several years ; the most famous of all 
 meteoric showers, that of the " Leonids," from a radiant 
 in Leo, was active for many centuries every thirty-third 
 year ; and another falling in the same month, November, 
 came from a radiant in Andromeda every thirteen years. 
 In these four cases and in some others the meteors 
 have been found to be travelling along the same path 
 as a comet. It is therefore considered that meteoric 
 swarms are due to the gradual break up of comets ; 
 indeed the comet of the Andromeda shower, known 
 from one of its observers as " Biela's," was actually 
 seen to divide into two in December 1845, and has not 
 been observed as a comet since 1852, though the showers 
 connected with it, giving us the meteors known as the 
 " Andromedes," have continued to be frequent and rich. 
 Meteors, therefore, are the smallest, most insignificant, 
 of all the celestial bodies ; and the shining out of a 
 meteor is the last stage of its history — its death ; after 
 death it simply goes to add an infinitesimal trifle to the 
 dust of the earth.
 
 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 2700 B.C. arranged the stars into con- 
 stellations, for it was the first step towards distin- 
 guishing one star from another. When one star began 
 to be known as " the star in the eye of the Bull," and 
 another as " the star in the shoulder of the Giant," 
 the heavens ceased to display an indiscriminate crowd 
 of twinkling fights ; each star began to possess indi- 
 viduality. 
 
 The next step was taken when Hipparchus made 
 his catalogue of stars (129 B.C.), not only giving its 
 name to each star, but measuiing and fixing its place 
 — a catalogue represented to us by that of Claudius 
 Ptolemy (a.d. 137). 
 
 The third step was taken when Bradley, the third 
 Astronomer Royal, made, at Greenwich, a catalogue of 
 more than 3000 star-places determined with the tele- 
 scope. 
 
 A century later Argelander made the great Bonn 
 Zone catalogue of 330,000 stars, and now a great 
 photographic catalogue and chart of the entire heavens 
 have been arranged between eighteen observatories of 
 different countries. This great chart when complete 
 will probably present 30 millions of stars in position 
 and brightness. 
 
 81
 
 82 SCIENCE OF THE STARS 
 
 The question naturally arises, " Why so many stars ? 
 What conceivable use can be served by catalogues of 
 30 millions or even of 3000 stars ? " And so far as 
 strictly practical purposes are concerned, the answer 
 must be that there is none. Thus Maskelyne, the 
 fifth Astronomer Royal, restricted his observations to 
 some thirty-six stars, which were all that he needed 
 for his Nautical Almanac, and these, with perhaps a 
 few additions, would be sufficient for all purely practical 
 ends. 
 
 But there is in man a restless, resistless passion for 
 knowledge — for knowledge for its own sake — that is 
 always compelling him to answer the challenge of the 
 unknown. The secret hid behind the hills, or across the 
 seas, has drawn the explorer in all ages ; and the secret 
 hid behind the stars has been a magnet not less powerful. 
 So catalogues of stars have been made, and made again, 
 and enlarged and repeated ; instruments of ever-increas- 
 ing delicacy have been built in order to determine the 
 positions of stars, and observations have been made 
 with ever-increasing care and refinement. It is know- 
 ledge for its own sake that is longed for, knowledge 
 that can only be won by infinite patience and care. 
 
 The chief instrument used in making a star catalogue 
 is called a transit circle ; two great stone pillars are set 
 up, each carrying one end of an axis, and the axis carries 
 a telescope. The telescope can turn round like a wheel, 
 in one direction only ; it points due north or due south. 
 A circle carefully divided into degrees and fractions of 
 a degree is attached to the telescope. 
 
 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 telescope, and at 
 that moment the observer points the telescope to the
 
 THE SYSTEM OF THE STARS 83 
 
 star, and notes the time by his clock when the star 
 crossed the spider's threads, which are fitted in the 
 focus of his eye-piece. He also notes the angle at 
 which the telescope was inclined to the horizon by 
 reading the divisions of his circle. 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 star. 
 
 " But why should catalogues be repeated ? When 
 once the position of a star has been observed, why 
 trouble to observe it again ? Will not the record serve 
 in perpetuity ? " 
 
 The ai.s.vers to these questions have been given by 
 star catalogues themselves, or have come out in the 
 process of making them. The Earth rotates on its axis 
 and revolves round the Sun. But that axis also has a 
 rolling motion of its own, and gives rise to an apparent 
 motion of the stars called Precession. Hipparchus dis- 
 covered this effect while at work on his catalogue, and 
 our knowledge of the amount of Precession enables us 
 to fix the date when the constellations were designed. 
 
 Similarly, Bradley discovered two further apparent 
 motions of the stars — Aberration and Nutation. Of 
 these, the first arises from the fact that the light coming 
 from the stars moves with an inconceivable speed, but 
 does not cross from star to Earth instantly ; it takes 
 an appreciable, even a long, time to make the journey. 
 But the Earth is travelling round the Sun, and there- 
 fore continually changing its direction of motion, and 
 in consequence there is an apparent change in the 
 direction in which the star is seen. The change is very 
 small, for though the Earth moves 18J miles in a second, 
 light travels 10,000 times as fast. Stars therefore are 
 deflected from their true positions by Aberration, by 
 
 F
 
 84 SCIENCE OF THE STARS 
 
 an extreme amount of 20 - 47" of arc, that being the 
 angle shown by an object that is slightly more distant 
 than 10,000 times its diameter. 
 
 The axis of the Earth not only rolls on itself, but it 
 does so with a slight staggering, nodding motion, due 
 to the attractions of the Sun and Moon, known as 
 Nutation. And the axis does not remain fixed in the 
 solid substance of the Earth, but moves about irre- 
 gularly in an area of about 60 feet in diameter. The 
 positions of the north and south poles are therefore not 
 precisely fixed, but move, producing what is known as 
 the Variation of Latitude. Then star-places have to 
 be corrected for the effect of our own atmosphere, 
 i.e. refraction, and for errors of the instruments by which 
 their places are determined. And when all these have 
 been allowed for, the result stands out that different 
 stars have real movement of their own — their Proper 
 Motions. 
 
 No stars are really " fixed " ; 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 motion. But nothing is fixed. 
 The Earth on which we stand has many different 
 motions ; the stars are all in headlong flight. 
 
 And from this motion of the stars it has been learned 
 that the Sun too moves. When Copernicus overthrew 
 the Ptolemaic theory and showed that the Earth moves 
 round the Sun, it was natural that men should be 
 satisfied to take this as the centre of all tilings, fixed 
 and immutable. It is not so. Just as a traveller 
 driving through a wood sees the trees in front appar- 
 ently open out and drift rapidly past him on either 
 hand, and then slowly close together behind him, so 
 Sir William Herschel showed that the stars in one
 
 THE SYSTEM OF THE STARS 85 
 
 part of the heavens appear to be opening out, or slowly 
 moving apart, while in the opposite part there seems 
 to be a slight tendency for them to come together, 
 and in a belt midway between the two the tendency 
 is for a somewhat quicker motion toward the second 
 point. And the explanation is the same in the one 
 case as in the other — the real movement is with the 
 observer. The Sun with all its planets and smaller 
 attendants is rushing onward, onward, towards a point 
 near the borders of the constellations Lyra and Her- 
 cules, at the rate of about twelve miles per second. 
 
 Part of the Proper Motions of the stars are thus only 
 apparent, being due to the actual motion of the Sun — 
 the " Sun's Way," as it is called — but part of the Proper 
 Motions belong to the stars themselves ; they are really 
 in motion, and this not in a haphazard, random manner. 
 For recently Kapteyn and other workers in the same 
 field have brought to light the fact of Star-Drift, i.e. that 
 many of the stars are travelling in associated com- 
 panies. This may be illustrated by the seven bright 
 stars that make up the well-known group of the 
 " Plough," or " Charles's Wain," as country people call 
 it. For the two stars of the seven that are furthest 
 apart in the sky are moving together in one direction, 
 and the other five in another. 
 
 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 so. Quite a distinct 
 and important department of astronomy has arisen 
 dealing with the continual observation and measure- 
 ment of these objects. For many double stars are in 
 motion round each other in obedience to the law of 
 gravitation, and their orbits have been computed.
 
 86 SCIENCE OF THE STARS 
 
 Some of these systems contain three or even four 
 members. But in every case the smaller body shines 
 by its own light ; we have no instance in these double 
 stars of a sun attended by a planet ; in each case it 
 is a sun with a companion sun. The first double star 
 to be observed as such w r as one of the seven stars of the 
 Plough. It is the middle star in the Plough handle, 
 and has a faint star near it that is visible to any 
 ordinarily good sight. 
 
 Star catalogues and the work of preparing them have 
 brought out another class — Variable Staes. As the 
 places of stars are not fixed, so neither are their bright- 
 nesses, and some change their brightness quickly, even 
 as seen by the naked eye. One of these is called Algol, 
 i.e. the Demon Star, and is in the constellation Perseus. 
 The ancient Greeks divided all stars visible to the 
 naked eye into six classes, or " magnitudes," according to 
 their brightness, the brightest stars being said to be 
 of the first magnitude, those not quite so bright of the 
 second, and so on. Algol is then usually classed as a 
 star of the second magnitude, and for two days and a 
 half it retains its brightness unchanged. Then it begins 
 to fade, and for four and a half hours its brightness 
 declines, until two-thirds of it has gone. No further 
 change takes place for about twenty minutes, after which 
 the light begins to increase again, and in another four 
 and a half hours it is as bright as ever, to go through 
 
 the same changes again after another interval of tw T o 
 days and a half. 
 
 Algol is a double star, but, unlike those stars that we 
 know under that name, the companion is dark, but is 
 nearly as large as its sun, and is very close to it, moving 
 round it in a little less than three days. At one point 
 of its orbit it comes between Algol and the Earth,
 
 THE SYSTEM OF THE STARS 87 
 
 and Algol suffers, from our point of view, a partial 
 eclipse. 
 
 There are many other cases of variable stars of this 
 kind in which the variation is caused by a dark com- 
 panion moving round the bright star, and eclipsing it 
 once in each revolution ; and the diameters and dis- 
 tances of some of these have been computed, showing 
 that in some cases the two stars are almost in contact. 
 In some instances th* companion is a dull but not a 
 dark star ; it gives a certain amount of light. When 
 this k the case there is a fall of light twee in the period — 
 once when the fainter star partly eclipses the brighter, 
 once when the brighter star partly eclipses the fainter. 
 
 But not all variable stars are of this kind. There is 
 a star in the constellation Cetus which is sometimes of 
 the second magnitude, at which brightness it may remain 
 for about a fortnight. Then it will gradually diminish 
 in brightness for nine or ten weeks, until it is lost to the 
 unassisted sight, and after six months of invisibility it 
 reappears and increases during another nine or ten 
 weeks to another maximum. " Mira," i.e. wonderful 
 star, as this variable is called, is about 1000 times as 
 bright at maximum as at minimum, but some maxima 
 are fainter than others ; neither is the period of varia- 
 tion always the same. It is clear that variation of this 
 kind cannot be caused by an eclipse, and though many 
 theories have been suggested, the "long-period variables," 
 of which Mira is the type, as yet remain without a 
 complete explanation. 
 
 More remarkable still are the " New Stars " — stars 
 that suddenly burst out into view, and then quickly 
 fade away, as if a beacon out in the stellar depths 
 had suddenly been fired. One of these suggested 
 to Hipparchus the need for a catalogue of the
 
 88 SCIENCE OF THE STARS 
 
 stars ; another, the so-called " Pilgrim Star," in the 
 year 1572 was the means of fixing the attention of 
 Tycho Brahe upon astronomy ; a third in 1604 was 
 observed and fully described by Kepler. The real 
 meaning of these " new," or " temporary," stars was 
 not understood until the spectroscope was applied to 
 astronomy. They will therefore be treated in the 
 volume of this series to be devoted to that subject. 
 It need only be mentioned here that their appearance 
 is evidently due to some kind of collision between 
 celestial bodies, producing an enormous and instan- 
 taneous development of light and heat. 
 
 These New Stars do not occur in all parts of the 
 heavens. Even a hasty glance at the sky will show 
 that the stars are not equally scattered, but that a 
 broad belt apparently made up of an immense number 
 of very small stars divides them into two parts. 
 
 The Milky Way, or Galaxy, as this belt is called, 
 bridges the heavens at midnight, early in October, like 
 an enormous arch, resting one foot on the horizon in 
 the east, and the other in the west, and passing through 
 the " Zenith," i.e. the point overhead. It is on this belt 
 of small stars — on the Milky Way — that New Stars are 
 most apt to break out. 
 
 The region of the Milky Way is richer in stars than 
 are the heavens in general. But it varies itself also 
 in richness in a remarkable degree. In some places the 
 stars, as seen on some of the wonderful photographs 
 taken by E. E. Barnard, seem almost to form a con- 
 tinuous wall ; in other places, close at hand, barren 
 spots appear that look inky black by contrast. And 
 the Star Clusters, stars evidently crowded together, are 
 frequent in the Milky Way. 
 
 And yet again beside the stars the telescope reveals
 
 THE SYSTEM OF THE STARS 89 
 
 to us the Nebttlje. Some of these are the Irregular 
 Nebulae — wide-stretching, cloudy, diffused masses of 
 filmy light, like the Great Nebula in Orion. Others 
 are faint but more defined objects, some of them with 
 small circular discs, and looking like a very dim 
 Uranus, or even like Saturn — that is to say, like a 
 planet with a ring round its equator. This class are 
 therefore known as " Planetary Nebulae," and, when bright 
 enough to show traces of colour, appear green or greenish 
 blue. 
 
 These are, however, comparatively rare. Other of 
 these faint, filmy objects are known as the "White 
 Nebulae," and are now counted by thousands. They 
 affect the spiral form. Sometimes the spiral is seen 
 fully presented ; sometimes it is seen edgewise ; some- 
 times more or less foreshortened, but in general the 
 spiral character can be detected. And these White 
 Nebulae appear to shun the Galaxy as much as the 
 Planetary Nebulae and Star Clusters prefer it ; indeed 
 the part of the northern heavens most remote from the 
 Milky Way is simply crow r ded with them. 
 
 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 Way, and other classes 
 avoid it ; it points to the whole forming a single 
 growth, an essential unity. For there is but one belt 
 in the heavens, like the Mlky Way, a belt in which 
 small stars, New Stars, and Planetary Nebulae find their 
 favourite home ; and that belt encircles the entire 
 heavens ; and similarly that belt is the only region 
 from which the White Nebulae appear to be repelled. 
 The Milky Way forms the foundation, the strong and 
 buttressed wall of the celestial building ; the White 
 Nebulae close in the roof of its dome.
 
 90 SCIENCE OF THE STARS 
 
 And how vast may that structure be — how far is it 
 from wall to wall ? 
 
 That, as yet, we can only guess. But the stars whose 
 distances we can measure, the stars whose drifting we 
 can watch, almost infinitely distant as they are, carry 
 us but a small part of the way. Still, from little hints 
 gathered here and there, we are able to guess that, 
 though the nearest star to us is nearly 300,000 times 
 as far as the Sun, yet we must overpass the distance of 
 that star 1000 times before we shall have reached the 
 further confines of the Galaxy. Nor is the end in sight 
 even there. 
 
 This is, in briefest outline, the Story of Astronomy. 
 It has led us from a time when men were acquainted 
 with only a few square miles of the Earth, and knew 
 nothing of its size and shape, or of its relation to the 
 moving lights which shone down from above, on to 
 our present conception of our place in a universe of 
 suns of which the vastness, glory, and complexity sur- 
 pass our utmost powers of expression. The science 
 began in the desire to use Sun, Moon, and stars as 
 timekeepers, but as the exercise of ordered sight and 
 ordered thought brought knowledge, knowledge began 
 to be desired, not for any advantage it might bring, 
 but for its own sake. And the pursuit itself has brought 
 its own reward in that it has increased men's powers, 
 and made them keener in observation, clearer in reason- 
 ing, surer in inference. The pursuit indeed knows no 
 ending ; the questions to be answered that lie before 
 us are now more numerous than ever they have been, 
 and the call of the heavens grows more insistent : 
 
 " LIFT UP YOUR EYES ON HIGH."
 
 BOOKS TO READ 
 
 Popular General Descriptions : — 
 
 Sir R. S. Ball— Star-Land. (Cassell.) 
 
 Agnes Gibeme. — Sun, Moon and Stars. (Seeley.) 
 
 W. T. Lynn. — Celestial Motions. (Stanford.) 
 
 A. & W. Maunder. — The Heavens and their Story. (Culley.) 
 
 Simon Newcomb. — Astronomy for Everybody. (Isbister.) 
 
 For Beginners in Observation : — 
 
 W. F. Denning. — Telescopic Work for Starlight Evenings. 
 
 (Taylor & Francis.) 
 E. \V. Maunder. — Astronomy without a Telescope. (Thacker.) 
 Arthur P. Norton. — A Star Atlas and Telescopic Handbook. 
 
 (Gall & Inglis.) 
 Garrett P. Serviss. — Astronomy with an Opera-Glass. 
 
 (Appleton.) 
 
 Star-Atlases : — 
 
 Rev. J. Gall. — An Easy Guide to the Constellations. (Gall 
 
 and Inglis.) 
 E. M'Clure and H. J. Klein. — Star-Atlas. (Society for 
 
 Promoting Christian Knowledge.) 
 R A. Proctor. — Nevj Star Atlas. (Longmans.) 
 
 Astronomical Instruments and Methods : — 
 
 Sir G. B. Airy. — Popular Astronomy ; Lectures delivered at 
 
 Ipsicich. (Macmillan.) 
 E. W. Maunder. — Royal Observatory, Greenwich; its History 
 
 and Work. (Religious Tract Society.)
 
 92 BOOKS TO READ 
 
 General Text-Books: — 
 
 Clerke, Fowler & Gore. — Concise Astronomy. (Hutchinson.) 
 Simon Newcomb. — Popular Astronomy. (Macmillan.) 
 C. A. Young. — Manual of Astronomy. (Ginn.) 
 
 Special Subjects : — 
 
 Rev. E. Ledger. — The Sun; its Planets and Satellites. 
 
 (Stanford.) 
 C. A. Young.— The Sun. (Kegan Paul.) 
 Mrs. Todd. — Total Eclipses. (Sampson Low.) 
 Nasmyth and Carpenter. — The Moon. (John Murray.) 
 Percival Lowell. — Mars. (Longmans.) 
 Ellen M. Clerke. — Jupiter. (Stanford.) 
 R. A. Proctor. — Saturn and its System. (Longmans.) 
 W. T. Lynn. — Remarkable Comets. (Stanford.) 
 E. W. Maunder.— The Astronomy of the Bible. (Hodder 
 
 and Stoughton.) 
 
 Historical : — 
 
 "W. W. Bryant.— History of Astronomy. (Methuen.) 
 Agnes M. Clerke. — History of Astronomy in the Nineteenth 
 
 Century. (A. & C. Black.) 
 George Forbes.— History of Astronomy. (Watts.) 
 
 Biographical : — 
 
 Sir R. S. Ball. — Great Astronomers. (Isbister.) 
 
 Agnes M. Clerke. — The Herschels and Modem Astronomy. 
 
 (Cassell.) 
 Sir O. Lodge. — Pioneers of Science. (Macmillan.)
 
 INDEX 
 
 Aberration, 83 
 
 "Achilles" (Minor planet), 38 
 
 Adams, John C, 39 
 
 Airy, 39 
 
 " Algol,'- 86 
 
 "AndrcLM-des" (Meteors), 80 
 
 Apsides, 24, 28 
 
 Argelander, 81 
 
 Barnard, E. E., 88 
 "Bear," The, 14 
 Biela's Comet, 80 
 Bouvard, 39 
 Bradley, 81, 83 
 Bremiker, 40 
 
 Catalogues (star), 81-83 
 Centauri, Alpha, 53 
 "Ceres" (Minor planet), 38 
 Challis, 40 
 Charles II., 50 
 Chromosphere, 73 
 Chronometer, 50 
 Clairaut, 36 
 Columbus, 48 
 Comets, 36 
 Comet, Halley's, 37 
 
 Biela's, 80 
 
 Conic Sections, 34 
 Constellations, the, 15 
 
 date of, 16 
 
 Cook, Oapt., 50 
 
 93 
 
 Copernicus, 26, 54, 84 
 
 " Copernicus " (Lunar crater), 
 
 59, 60 
 Corona, 73 
 Cowell, 37 
 Crommelin, 37 
 
 Degrees, 43 
 Dollond, 47 
 Double stars, 85 
 
 Earth, form of, 16 
 
 size of, 17, 33 
 
 Eclipses, 72 
 
 Ecliptic, 21 
 
 Ellipse, 28 
 
 Epicycle, 25 
 
 Eratosthenes, 17 
 
 " Eros " (Minor planet), 38, 52 
 
 Eudoxus, 21 
 
 Excentric, 24 
 
 Eye -piece, 45 
 
 Facul-s:, 70 
 Flamsteed, 50 
 
 Galileo, 44 
 Galle, 40 
 Gascoigne, 46 
 Gravitation, Law of, 34 
 
 Hall, Chester Moor, 47 
 Halley, 36
 
 94 INDEX 
 
 Halley's Comet, 37 
 HarrisoD, John, 50 
 Hersohel, Sir W., 37, 47, 84 
 Hipparchus, 24, 81, 83, 87 
 Hyperbola, 34 
 
 Job, Book of, 12, 14 
 "Juno " (Minor planet), 38 
 Jupiter, 18, 32, 77-78 
 
 Kapteyn, 85 
 
 Kepler, 28, 44, 88 
 
 Kepler's Laws, 29 
 
 " Kepler " (Lunar crater), 59 
 
 Langley, 74 
 
 Latitude, Variation of, 84 
 
 " Leonids " (Meteors), 80 
 
 Leverrier, 39 
 
 Lowell, 63, 64 
 
 " Lyrids " (Meteors), 80 
 
 Magnetic Storm, 76 
 Magnetism, Earth's, 76 
 Magnitudes of stars, 86 
 " Mare Imbrium," 59 
 Mars, 18, 52, 62-66 
 
 Canals of, 63 
 
 Maskelyne, 50, 82 
 
 Maunder, Mrs. Walter, 72, 74 
 
 Mercury, 17, 18, 27, 32, 66-67 
 
 Meteors, 79, 80 
 
 Micrometer, 46 
 
 Milky Way, 53, 88 
 
 Minor Planets, 38, 52 
 
 Minutes of arc, 44 
 
 "Mira," 87 
 
 Moon, 11, 14, 21, 32, 33, 49, 55-62 
 
 distance of, 51 
 
 " Nautical Almanac" 50, 82 
 
 Navigation, 49 
 Nebulas, 89 
 Neptune, 40, 79 
 Newcomb, 65 
 New stars, 87 
 Newton, 29, 31, 47 
 Newton's Laws of motion, 31 
 Nodes, 35 
 Nutation, 83, 84 
 
 "OASES of Mars," 64 
 Obelisks, 42 
 Object glass, 45 
 Observatories, Berlin, 50 
 
 ■ ■ Copenhagen, 50 
 
 Greenwich, 50 
 
 Mt. Wilson, 48 
 
 Paris, 50 
 
 Pulkowa, 50 
 
 St. Petersburg, 50 
 
 Washington, 50 
 
 Yerkes, 47 
 
 " Pallas " (Minor planet), 38 
 
 Parabola, 34 
 
 "Perseids" (Meteors), 80 
 
 rhotography, 46 
 
 Photosphere, 69 
 
 "Pilgrim" star, 88 
 
 Piazzi, 38 
 
 Planets, 17 
 
 Pole of the Heavens, 13 
 
 Ponte"coulant, 37 
 
 Precession of the Equinoxes, 36, 
 
 83 
 " Principia," 36 
 Prominences, 73 
 " Ptolemaeus " (Lunar crater), 60 
 Ptolemy, 24, 81 
 
 Radiant Points, 80
 
 INDEX 
 
 95 
 
 Radius Vector, 28 
 Reflectors, 47 
 Refractors, 47 
 
 SATUKN, 18, 78-79 
 
 Schiaparelli, 63 
 
 Schwabe, 69 
 
 Seconds of arc, 44 
 
 Sirius, 53 
 
 Solar System, Tables of, 56-58 
 
 Somerville, Mrs., 39 
 
 Spheres, Planetary, 21 
 
 Sporer, 71 
 
 Sp6rei*'«.Law, 71 
 
 Star c;;tclogues, 81-83 
 
 clusters, 88 
 
 drift, 85 
 
 Stars, fixed, 84 
 
 proper motions of, 84 
 
 Sun, 11, 12, 14, 21, 32, 67-77 
 
 distance of, 51 
 
 dials, 43 
 
 Sun spots, 69 
 
 spot maximum, 71 
 
 minimum, 71 
 
 "Sun's Way," 85 
 
 Telescope, Invention of, 45 
 Transit Circle, 82 
 Tycho Brahe, 27, 44, 88 
 " Tycho " (Lunar crater), 59, 60, 
 61 
 
 Uranus, 38, 79 
 
 Variable stars, 86 
 
 , Long period, 87 
 
 Venus, 18, 27, 67 
 "Vesta" (Minor planet), 38 
 
 Young, C. A., 74 
 
 Zenith, 17, 88 
 
 Zodiac, Signs of, 14, 15, 16, 43 
 
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