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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-14 19:06:32 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-14 19:06:32 -0700 |
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diff --git a/48218-0.txt b/48218-0.txt new file mode 100644 index 0000000..1c8653a --- /dev/null +++ b/48218-0.txt @@ -0,0 +1,2835 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 48218 *** + + THE SCIENCE OF + THE STARS + + + BY E. WALTER MAUNDER, F.R.A.S. + + OF THE ROYAL OBSERVATORY, GREENWICH + + AUTHOR OF "ASTRONOMY WITHOUT A TELESCOPE" + "THE ASTRONOMY OF THE BIBLE," ETC. + + + + LONDON: T. C. & E. C. JACK + 67 LONG ACRE, W.C., AND EDINBURGH + NEW YORK: DODGE PUBLISHING CO. + + + + +{vii} + + CONTENTS + + CHAP. + + I. ASTRONOMY BEFORE HISTORY + II. ASTRONOMY BEFORE THE TELESCOPE + III. THE LAW OF GRAVITATION + IV. ASTRONOMICAL MEASUREMENTS + V. THE MEMBERS OF THE SOLAR SYSTEM + VI. THE SYSTEM OF THE STARS + INDEX + + + + +{9} + +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} 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 which 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 difficulties which might well +have seemed superhuman. The true spirit of the science can be far +better exemplified by the presentation of some of these difficulties, +and of the methods by which they have been overcome, than by many +volumes of picturesque description or of eloquent rhapsody. + +There was a time when men knew nothing of astronomy; 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 {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 intelligent, were mistaken; some, though wrong, +led eventually 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} 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 +reflection 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 direction 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 which 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 {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 connected 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 evidently 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} 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 continual 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 {15} value, and the +return of the moonlit portion of the month was eagerly looked for. + +These 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 observations 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 +Constellations+ 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 constellations 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} lived about 2700 B.C., and in North latitude 37° or +38°. The indication is only rough, but the amount of uncertainty 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 designed 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 inference as +clearly set forth as they would be to-day. {17} The same inference +followed, moreover, from the observation 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 overhead--at 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 +complicated 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} 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 consequence 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 companion +planets returned at no definite seasons. Nevertheless, 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 ingenuity 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 {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. + + + + +{20} + +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 towards 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, {21} and when once their attention was drawn to the +movements of the heavenly bodies, they devoted themselves 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. EUDOXUS 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 23-½° to the equator of the sky, so that the Sun is +23-½° north of the equator at midsummer and 23-½° south of the equator +at midwinter, for the poles of the sphere peculiar to the Sun were +supposed to be 23-½° 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} Sun. 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 mentioned 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 contrary 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: + +{23} + + _Epoch Time Interval_ + + Winter Solstice 1910 Dec. 22 d. 5 h. 12 m. P.M. 89 d. 0 h. 42 m. + Spring Equinox 1911 Mar. 21 " 5 " 54 " P.M. 92 " 19 " 41 " + Summer Solstice 191l 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. 1910 1 d. 9 h. 10.7 m. P.M. 14 d. 13 h. 54.4 m. + " " 31 " 4 " 21.2 " P.M. 14 " 6 " 4.8 " + Jan. 1911 30 " 9 " 44.7 " A.M. 14 " 0 " 52.8 " + March " 1 " 0 " 31.1 " A.M. 13 " 23 " 27.4 " + " " 30 " 0 " 37.8 " P.M. 14 " 1 " 58.8 " + April " 28 " 10 " 25.0 " P.M. 14 " 7 " 44.7 " + May " 28 " 6 " 24.4 " A.M. 14 " 15 " 26.3 " + June " 26 " 1 " 19.7 " P.M. 14 " 23 " 33.7 " + July " 25 " 8 " 12.0 " P.M. 15 " 6 " 42.7 " + Aug. " 24 " 4 " 14.3 " A.M. 15 " 11 " 42.4 " + Sept. " 22 " 2 " 37.4 " P.M. 15 " 13 " 33.7 " + Oct. " 22 " 4 " 9.3 " A.M. 15 " 11 " 38.8 " + Nov. " 20 " 8 " 49.4 " P.M. 15 " 6 " 2.5 " + Dec. " 20 " 3 " 40.3 " P.M. 14 " 21 " 49.4 " + +{24} + + _Full Moon Interval to New Moon_ + + Dec. 1910 16 d 11 h. 5.1 m. A.M. 15 d. 5 h. 16.1 m. + Jan. 1911 14 " 10 " 26.0 " P.M. 15 " 11 " 18.7 " + Feb. " 13 " 10 " 37.5 " A.M. 15 " 13 " 53.6 " + March " 14 " 11 " 58.5 " P.M. 15 " 12 " 39.3 " + April " 13 " 2 " 36.6 " P.M. 15 " 7 " 48.4 " + May " 13 " 6 " 9.7 " A.M. 15 " 0 " 14.7 " + June " 11 " 9 " 50.7 " P.M. 14 " 15 " 29.0 " + July " 11 " 0 " 53.4 " P.M. 14 " 7 " 18.6 " + Aug. " 10 " 2 " 54.7 " A.M. 14 " 1 " 19.6 " + Sept. " 8 " 3 " 56.7 " P.M. 13 " 22 " 40.7 " + Oct. " 8 " 4 " 11.1 " A.M. 13 " 23 " 58.2 " + Nov. " 6 " 3 " 48.1 " P.M. 14 " 5 " 1.3 " + Dec. " 6 " 2 " 51.9 " A.M. 14 " 12 " 48.4 " + Jan. 1912 4 " 1 " 99.7 " P.M. 14 " 21 " 40.3 " + + +The astronomer who dealt with this difficulty was HIPPARCHUS (about +190-120 B.C.), who was born at Nicæa, in Bithynia, but made most of his +astronomical observations in Rhodes. He attempted to explain these +irregularities in the motions of the Sun and Moon by supposing 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 satisfactory 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 {25} +Alexandria, who carried the work of astronomical observation 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 extremely complex one, but its principle +may be represented 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 ++epicycle+--_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 possible here to do more than indicate the +general principles upon which it was founded, and the numerous other +systems or modifications of them produced in the {26} 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 +assumption 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 remained without serious +challenge, and the next great name that it is necessary to notice is +that of COPERNICUS (1473-1543). Copernicus was a canon of Frauenburg, +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 east in the midst of the starry sphere; and, +as the {27} stars are at an immeasurable distance, the latter +conception 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 retrogressions 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 abandoned 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} His +observations enabled his friend and pupil, JOHANN KEPLER, (1571-1630), +to subject the planetary movements 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 joining 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 property 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 +instead 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 occupied 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. + +{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. + + + + +{30} + +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 {31} the Sun. There was need, +therefore, for an entire revision of the principles upon which motion +was supposed to take place. + +The mistake of the ancients had been that they supposed that continued +motion demanded fresh applications 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, therefore, 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} 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 +drawing 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 inversely +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 {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 18.66. 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 1/330 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 uniform 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 influence 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} 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 throughout 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 relation 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 {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 parabola 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 under gravitation as being ellipses of one form or +another. + +From his great demonstration of the law of gravitation, 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 +inclined 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. {36} +Consequently 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 _Principia_ (_The +Mathematical Principles of Natural Philosophy_), 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 +consulting 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 {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 PONTÉCOULANT were only two days in error, so +carefully had the comet been followed during its unseen journey to the +confines of the solar system and back again, during a period of +seventy-five years. Pontécoulant's exploit was outdone at the next +return by Drs. COWELL and CROMMELIN, of Greenwich Observatory, who not +only computed the time of its perihelion 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} 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 addition 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 comes +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. + +{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 BOUVARD in +1821, the earlier observations were rejected. But the discrepancies +between 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 +unknown. 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, proceeded to attack the problem of determining +the position, orbit, and mass of the unknown body by which 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 +instituted 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} 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 commenced +more than 2000 years earlier, when the first Greek astronomers set +themselves to unravel the apparently aimless wanderings of the planets +in the assured faith that they would find them obedient unto law. {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 all those which have +preceded it--are built upon our command of mathematics and our +knowledge of the laws of motion--a command and a knowledge which we owe +directly to their persevering attempts to advance the science of +astronomy, and to follow after knowledge, not for any material rewards +which she had to offer, but for her own sake. + + + + +{42} + +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 movement 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 {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 drawbacks. 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} 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 projected 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 properly +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 apparent diameter of +the Moon--that made Kepler finally give up all attempts to explain the +planetary movements on the doctrine of circular orbits and to try +movements in an 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 +experiments. Up to that time it had been taught that of {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 discovered 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 knowledge 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 composed 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} 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 gentleman, GASCOIGNE, who afterwards fell +at the Battle of Marston Moor, devised the "micrometer" for this +purpose. 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 immensely 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. {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 cumbersome and unsteady. Sir Isaac Newton +therefore discarded 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 4-½ 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 improved; 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} than any object-glass. Thus Lord Rosse's great +telescope 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 him. + +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 +overhead, 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 recognition of +landmarks was the first requisite for a good sailor. But when, in +1492, Columbus had brought a new continent to light, and long voyages +were freely taken across the great oceans, it became an urgent {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 +competitors 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 observing the height of +the Sun at noon, or of the Pole Star at night, or in several other +ways. But the longitude 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 +difference 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 amongst 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} a sound basis, that CHARLES II. founded Greenwich +Observatory in the year 1675, and appointed FLAMSTEED 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 +chronometer+ 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 {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 distances 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 distances of A from B and from C. The +application of this principle to the measurement of the Moon's distance +was made by the establishment of an observatory at the Cape of Good +Hope, to co-operate with that of Greenwich. It is, of course, not +possible to see Greenwich Observatory from the Cape, or vice versa, but +the stars, being at an almost infinite distance, lie in the same +direction from both observatories. What is required then is to measure +the apparent distance of the Moon from the same stars as seen from +Greenwich and as seen from the 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 {52} 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 especially 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 +{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 difference. 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, is +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 or 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. + + + + +{54} + +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 universe, but raised it to the rank of a +planet. It is therefore 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 {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 a +novice when he first sees it in the telescope. Every detail is hard, +clear-cut, and sharp; it is 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 lines marking the sections radiating from it. This +bright spot has been called after the great {56} + + Mean distance from Sun. Period Velocity + Class. Name. Earth's In millions of revolution. in orbit. Eccentricity. + distance of miles. In years. Miles per + =1. sec. + + Terrestrial Mercury 0.387 36.0 0.24 29.7 0.2056 + Planets Venus 0.723 67.2 0.62 21.9 0.0068 + Earth 1.000 92.9 1.00 18.5 0.0168 + Mars 1.524 141.5 1.88 15.0 0.0933 + + Minor Eros 1.458 135.5 1.76 15.5 0.2228 + Planets Ceres 2.767 257.1 4.60 11.1 0.0763 + Achilles 5.253 488.0 12.04 8.1 0.0509 + + Major Jupiter 5.203 483.3 11.86 8.1 0.0483 + Planets Saturn 9.539 886.6 29.46 6.0 0.0561 + Uranus 19.183 1781.9 84.02 4.2 0.0463 + Neptune 30.055 2791.6 164.78 3.4 0.0090 + +{57} + + Mean diameter. Surface. Volume. Mass. + Name. Symbol. In miles. [Earth]=1. [Earth]=1. [Earth]=1. [Earth]=1. + + Sun [Sun] 866400 109.422 11973. 1310130. 332000. + Moon [Moon] 2163 0.273 0.075 0.02 0.012 + + Mercury [Mercury] 3030 0.383 0.147 0.06 0.048 + Venus [Venus] 7700 0.972 0.945 0.92 0.820 + Earth [Earth] 7918 1.000 1.000 1.00 1.000 + Mars [Mars] 4230 0.534 0.285 0.15 0.107 + + Jupiter [Jupiter] 86500 10.924 119.3 1304. 317.7 + Saturn [Saturn] 73000 9.219 85.0 783. 94.8 + Uranus [Uranus] 31900 4.029 16.2 65. 14.6 + Neptune [Neptune] 34800 4.395 19.3 85. 17.0 + +{58} + + Light + Gravity. and heat Albedo; + Density. Fall in received _i.e._ re- + [Earth] Water [Earth] feet per from Sun. Time of rotation flecting + Name. =1. =1. =1. sec. [Earth]=1. on axis. power. + + d. h. m. + Sun 0.25 1.39 27.65 444.60 ... 25 4 48 ± ... + Moon 0.61 3.39 0.17 2.73 1.00 27 7 43 0.17 + + d. h. m. s. + Mercury 0.85 4.72 0.43 6.91 6.67 88 (?) 0.14 + Venus 0.89 4.94 0.82 13.19 1.91 23 21 23 (?) 0.76 + Earth 1.00 5.55 1.00 16.08 1.00 23 56 4 0.50 (?) + Mars 0.71 3.92 0.38 6.11 0.43 24 37 23 0.22 + + h. m. + Jupiter 0.24 1.32 2.65 42.61 0.037 9 55 ± 0.62 + Saturn 0.13 0.72 1.18 18.97 0.011 10 14 ± 0.72 + Uranus 0.22 1.22 0.90 14.47 0.003 9 30 (?) 0.60 + Neptune 0.20 1.11 0.89 14.31 0.001 (?) 0.52 + +{59} Danish astronomer, "Tycho," and is one of the most conspicuous +objects of the full Moon. + +The contrasts of the Moon are much more pronounced 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 themselves, hundreds of miles in diameter. +These rings are so numerous that Galileo described the Moon as looking +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 telescope) 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} +considerably larger than the whole of Wales, and known as "Ptolemæus." + +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 gradation 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 "Copernicus" 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 objects 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 +submerged many of the ring-formations. Sometimes half {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, without any appreciable trace of air +or water or any sign {62} 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 rotates 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 +hemisphere lasts 199 of our {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 SCHIAPARELLI +announced that he had discovered a number of very narrow, straight, +dark lines on the planet--lines 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 mistranslation 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} 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 +_un_natural, that it must be the work of design--of intelligent beings. +The canals are to him absolutely regular and straight, like lines drawn +with ruler and pen-and-ink, and the oases are exactly round. But, on +the one hand, the best observers, 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 lines and circular dots, and, on the other hand, the straight +line 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 no 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 +question of the planet's atmosphere. On the Earth we pass through half +the atmosphere by ascending a mountain {65} that is three and a third +miles in height; on Mars we should have to ascend nearly nine miles. +If the atmospheric 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 mountains, 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 considerably 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 impossible 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 foremost astronomer of his day, concluded +"that during {66} the night of Mars, even in the equatorial regions, +the surface of the planet probably falls to a lower temperature 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 +freezing 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. Newcomb's conclusion was: "Thus we have a kind of Martian +meteorological changes, very slight indeed, and seemingly 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 life. + +Of MERCURY we know very little. It is smaller than Mars but larger +than the Moon, but it differs from them {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 darkness 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. Schiaparelli 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 {68} 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 +overlook 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 {69} nineteenth 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 defined. 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} 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 _maculæ_, as the spots were called by the +first observers, are the "+faculæ+"--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 faculæ will sometimes persist in the same region for quite a +long time. + +The faculæ clearly rise above the ordinary solar surface; 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 sometimes 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 latitude than 40°, the great +majority of spots lying in the two zones between 5° and 25° latitude on +either side of the equator. Faculæ, 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 {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 SPÖRER, its discoverer, indicates that the +origin and source of sunspot activity lie 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 Spörer's Law +could not be imposed by some influence from without. Still sunspots, +once formed, may be influenced by the Earth, and perhaps by other {72} +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 {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 "prominences," 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 "+chromosphere+" 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} sunspots and faculæ +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 this 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 continually 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 +light. The amount of heat radiated by the Sun has been expressed in a +variety of different ways; C. A. YOUNG 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 {75} +formed from the Earth to the Sun, 2-½ 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 manifest. 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 falling 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 faculæ; 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} 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, {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 practically 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 latitude, 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 "+Red 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 rotation, 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} 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, CASSINI, and an +outer, fainter, narrower one discovered by ENCKE. The innermost part +of the ring is dusky, fainter {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 pronounced, but in general they +resemble those of Jupiter. The planet rotates quickly--in 10 h. 14 +m.--its markings run into parallel belts, and are diversified by spots +of the same character as on Jupiter. Saturn is probably 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 +telescope 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 light 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} 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 perspective +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. + + + + +{81} + +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 constellations, for it was the first step towards +distinguishing 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 lights; each star began to possess individuality. + +The next step was taken when Hipparchus made his catalogue of stars +(129 B.C.), not only giving its name to each star, but measuring 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 telescope. + +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. + +{82} + +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-increasing 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 knowledge 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 +{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 answers 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 discovered 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 therefore 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 18-½ miles in a second, light +travels 10,000 times as fast. Stars therefore are deflected from their +true positions by Aberration, by {84} 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 irregularly 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 things, fixed and +immutable. It is not so. Just as a traveller driving through a wood +sees the trees in front apparently 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 {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 +Hercules, 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 companies. 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 measurement 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} 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 was +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 STARS. As the places of stars are not fixed, so +neither are their brightnesses, 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 two 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, {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 companion moving round the bright star, +and eclipsing it once in each revolution; and the diameters and +distances of some of these have been computed, showing that in some +cases the two stars are almost in contact. In some instances the +companion is a dull but not a dark star; it gives a certain amount of +light. When this is the case there is a fall of light twice 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 variation 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} 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 instantaneous +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 continuous 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 {89} to us the +NEBULÆ. Some of these are the Irregular Nebulæ--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 Nebulæ+," 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 Nebulæ+," and are now counted by +thousands. They affect the spiral form. Sometimes the spiral is seen +fully presented; sometimes it is seen edgewise; sometimes more or less +foreshortened, but in general the spiral character can be detected. +And these White Nebulæ appear to shun the Galaxy as much as the +Planetary Nebula; and Star Clusters prefer it; indeed the part of the +northern heavens most remote from the Milky Way is simply crowded 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 Milky Way, a belt in which small stars, New Stars, +and Planetary Nebulæ find their favourite home; and that belt encircles +the entire heavens; and similarly that belt is the only region from +which the White Nebulæ appear to be repelled. The Milky Way forms the +foundation, the strong and buttressed wall of the celestial building; +the White Nebulæ close in the roof of its dome. + +{90} + +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 surpass 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 reasoning, 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." + + + + +{91} + +BOOKS TO READ + + +POPULAR GENERAL DESCRIPTIONS:-- + + Sir R. S. Ball.--_Star-Land_. (Cassell.) + Agnes Giberne.---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. W. 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.--_New Star Atlas_. (Longmans.) + + +ASTRONOMICAL INSTRUMENTS AND METHODS:-- + + Sir G. B. Airy.--_Popular Astronomy; Lectures delivered at + Ipswich_. (Macmillan.) + E. W. Maunder.--_Royal Observatory, Greenwich; its History + and Work_. (Religious Tract Society.) + +{92} + +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.) + E. 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 E. S. Ball.--_Great Astronomers_. (Isbister.) + Agnes M. Clerke.--_The Herschels and Modern Astronomy_. (Cassell.) + Sir O. Lodge.--_Pioneers of Science_. (Macmillan.) + + + + +{93} + +INDEX + + + ABERRATION, 83 + "Achilles" (Minor planet), 38 + Adams, John C., 39 + Airy, 39 + "Algol," 86 + "Andromedes" (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, Capt., 50 + 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 + + + FACULÆ, 70 + Flamsteed, 50 + + + GALILEO, 44 + Galle, 40 + Gascoigne, 46 + Gravitation, Law of, 34 + + + HALL, CHESTER MOOR, 47 + Halley, 36 + Halley's Comet, 37 + Harrison, John, 50 + Herschel, 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 + Nebulæ, 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 + Photography, 46 + Photosphere, 69 + "Pilgrim" star, 88 + Piazzi, 38 + Planets, 17 + Pole of the Heavens, 13 + Pontécoulant, 37 + Precession of the Equinoxes, 36, 83 + "_Principia_," 36 + Prominences, 73 + "Ptolemæus" (Lunar crater), 60 + Ptolemy, 24, 81 + + + RADIANT POINTS, 80 + Radius Vector, 28 + Reflectors, 47 + Refractors, 47 + + + SATURN, 18, 78-79 + Schiaparelli, 63 + Schwabe, 69 + Seconds of arc, 44 + Sirius, 53 + Solar System, Tables of, 56-58 + Somerville, Mrs., 89 + Spheres, Planetary, 21 + Spörer, 71 + Spörer's Law, 71 + Star catalogues, 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 + + + + + Printed by BALLANTYNE, HANSON & Co. + Edinburgh & London + + + + + * * * * * + + + + + "We have nothing but the highest praise for these + little books, and no one who examines them will have + anything else."--_Westminster Gazette_, 22nd June 1912. + + +THE PEOPLE'S BOOKS + +THE FIRST NINETY VOLUMES + +The volumes issued are marked with an asterisk + + +SCIENCE + + 1. The Foundations of Science . . . By W. C. D. Whetham, M.A., F.R.S. + 2. Embryology--The Beginnings of Life . . . By Prof. Gerald Leighton, M.D. + 3. Biology . . . By Prof. W. D. Henderson, M.A. + 4. Zoology: The Study of Animal Life . . . By Prof. E. W. MacBride, + M.A., F.R.S. + 5. Botany; The Modern Study of Plants . . . By M. C. Stopes, D.Sc., + Ph.D., F.L.S. + 6. Bacteriology . . . By W. E. Carnegie Dickson, M.D. + 7. The Structure of the Earth . . . By Prof. T. G. Bonney, F.R.S. + 8. Evolution . . . By E. S. Goodrich, M.A., F.R.S. + 9. Darwin . . . By Prof. W. Garstang, M.A., D.Sc. + 10. Heredity . . . By J. A. S. Watson, B.Sc. + 11. Inorganic Chemistry . . . By Prof. E. C. C. Baly, F.R.S. + 12. Organic Chemistry . . . By Prof. J. B. Cohen, B.Sc., F.R.S. + 13. The Principles of Electricity . . . By Norman K. Campbell, M.A. + 14. Radiation . . . By P. Phillips, D.Sc. + 15. The Science of the Stars . . . By E. W. Maunder, F.R.A.S. + 16. The Science of Light . . . By P. Phillips, D.Sc. + 17. Weather Science . . . By R. G. K. Lempfert, M.A. + 18. Hypnotism and Self-Education . . . By A. M. Hutchison, M.D. + 19. The Baby: A Mother's Book . . . By a University Woman. + 20. Youth and Sex--Dangers and Safeguards for Boys and Girls . . . + By Mary Scharlieb, M.D., M.S., and F. Arthur Sibly, M.A., LL.D. + 21. Marriage and Motherhood . . . By H. S. Davidson, M.B., F.R.C.S.E. + 22. Lord Kelvin . . . By A. Russell, M.A., D.Sc., M.I.E.E. + 23. Huxley . . . By Professor G. Leighton, M.D. + 24. Sir William Huggins and Spectroscopic Astronomy . . . + By E. W. Maunder, F.R.A.S., of the Royal Observatory, Greenwich. + 62. Practical Astronomy . . . By H. Macpherson, Jr., F.R.A.S. + 63. Aviation . . . By Sydney F. Walker, R.N. + 64. Navigation . . . By William Hall, R.N., B.A. + 65. Pond Life . . . By E. C. Ash, M.R.A.C. + 66. Dietetics . . . By Alex. Bryce, M.D., D.P.H. + +PHILOSOPHY AND RELIGION + + 25. The Meaning of Philosophy . . . By Prof. A. E. Taylor, M.A., F.B.A. + 26. Henri Bergson . . . By H. Wildon Carr, Litt.D. + 27. Psychology . . . By H. J. Watt, M.A., Ph.D., D.Phil. + 28. Ethics . . . By Canon Rashdall, D.Litt., F.B.A. + 29. Kant's Philosophy . . . By A. D. Lindsay, M.A. + 30. The Teaching of Plato . . . By A. D. Lindsay, M.A. + 67. Aristotle . . . By Prof. A. E. Taylor, M.A., F.B.A. + 68. Friedrich Nietzsche . . . By M. A. Mügge. + 69. Eucken: A Philosophy of Life . . . By A. J. Jones, M.A., B.Sc., Ph.D. + 70. The Experimental Psychology of Beauty . . . By C. W. Valentine, + B.A., D.Phil. + 71. The Problem of Truth . . . By H. Wildon Carr, Litt.D. + 31. Buddhism . . . By Prof. T. W. Rhys Davids, M.A., F.B.A. + 32. Roman Catholicism . . . By H. B. Coxon. Preface, Mgr. R. H. Benson. + 33. The Oxford Movement . . . By Wilfrid Ward. + 34. The Bible and Criticism . . . By W. H. Bennett, D.D., Litt.P., + and W. F. Adeney, D.D. + 35. Cardinal Newman . . . By Wilfrid Meynell. + 72. The Church of England . . . By Rev. Canon Masterman. + 73. Anglo-Catholicism . . . By A. E. Manning Foster. + 74. The Free Churches . . . By Rev. Edward Shillito, M.A. + 75. Judaism . . . By Ephraim Levine, M.A. + 76. Theosophy . . . By Annie Besant. + +HISTORY + + 36. The Growth of Freedom . . . By H. W. Nevinson. + 37. Bismarck and the Origin of the German Empire . . . + By Professor F. M. Powicke. + 38. Oliver Cromwell . . . By Hilda Johnstone, M.A. + 39. Mary Queen of Scots . . . By E. O'Neill, M.A. + 40. Cecil John Rhodes, 1853-1902 . . . By Ian D. Colvin. + 41. Julius Cæsar . . . By Hilary Hardinge. + 42. England in the Making . . . By Prof. F. J. C. Hearnshaw, M.A., LL.D. + 43. England in the Middle Ages . . . By E. O'Neill, M.A. + 44. The Monarchy and the People . . . By W. T. Waugh, M.A. + 45. The Industrial Revolution . . . By Arthur Jones, M.A. + 46. Empire and Democracy . . . By G. S. Veitch, M.A., Litt.D. + 61. Home Rule . . . By L. G. Redmond Howard. + Preface by Robert Harcourt, M.P. + 77. Nelson . . . By H. W. Wilson. + 78. Wellington and Waterloo . . . By Major G. W. Redway. + +SOCIAL AND ECONOMIC + + 47. Women's Suffrage . . . By M. G. Fawcett, LL.D. + 48. The Working of the British System + of Government to-day . . . By Prof. Ramsay Muir, M.A. + 49. An Introduction to Economic Science . . . By Prof H. O. Meredith. M.A. + 50. Socialism . . . By B. B. Kirkman, B.A. + 79. Mediæval Socialism . . . By Bede Jarrett, O.P., M.A. + 80. Syndicalism . . . By J. H. Harley, M.A. + 81. Labour and Wages . . . By H. M. Hallsworth, M.A., B.Sc. + 82. Co-operation . . . By Joseph Clayton. + 83. Insurance as a Means of Investment . . . By W. A. Robertson, F.F.A. + 92. The Training of the Child . . . By G. Spiller + +LETTERS + + 51. Shakespeare . . . By Prof. C. H. Herford, Litt.D. + 52. Wordsworth . . . By Rosaline Masson. + 53. Pure Gold--A Choice of Lyrics and Sonnets . . . by H. C. O'Neill + 54. Francis Bacon . . . By Prof. A. R. Skemp, M.A. + 55. The Brontës . . . By Flora Masson. + 56. Carlyle . . . By L. MacLean Watt. + 57. Dante . . . By A. G. Ferrers Howell. + 58. Ruskin . . . By A. Blyth Webster, M.A. + 59. Common Faults in Writing English . . . By Prof. A. R. Skemp, M.A. + 60. A Dictionary of Synonyms . . . By Austin K. Gray, B.A. + 84. Classical Dictionary . . . By Miss A. E. Stirling + 85. A History of English Literature . . . By A. Compton-Rickett, LL.D. + 86. Browning . . . By Prof. A. R. Skemp, M.A. + 87. Charles Lamb . . . By Flora Masson. + 88. Goethe . . . By Prof. C. H. Herford, Litt.D. + 89. Balzac . . . By Frank Harris + 90. Rousseau . . . By F. B. Kirkman, B.A. + 91. Ibsen . . . By Hilary Hardinge. + 93. Tennyson . . . By Aaron Watson + + + LONDON AND EDINBURGH: T. C. & E. C. JACK + NEW YORK: DODGE PUBLISHING CO. + + + + + +[Transcriber's Note: + +Italicized text is indicated with _underscores_. + +Bold text is indicated with +plus signs+. + +Numbers inside curly braces, e.g. {99} are page numbers.] + + + + + + + + + + + +End of Project Gutenberg's The Science of the Stars, by E. Walter Maunder + +*** END OF THE PROJECT GUTENBERG EBOOK 48218 *** |