CHAPTER I. Astronomy Before History

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.

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