At first it was natural enough to suppose that the big bright stars of what we call the first magnitude were the nearest to us, and the less bright the next nearest, and so on down to the tiny ones, only revealed by the telescope, which would be the furthest away of all; but research has shown that this is not correct. Some of the brightest stars may be comparatively near, and some of the smallest may be near also. The size is no test of distance. So far as we have been able to discover, the star which seems nearest is a first magnitude one, but some of the others which outshine it must be among the infinitely distant ones. Thus we lie in the centre of a jewelled universe, and cannot tell even the size of the jewels which cover its radiant robe.
I say 'lie,' but that is really not the correct word. So far as we have been able to find out, there is no such thing as absolute rest in the universe—in fact, it is impossible; for even supposing any body could be motionless at first, it would be drawn by the attraction of its nearest neighbours in space, and gradually gain a greater and greater velocity as it fell toward them. Even the stars we call 'fixed' are all hurrying along at a great pace, and though their distance prevents us from seeing any change in their positions, it can be measured by suitable instruments. Our sun is no exception to this universal rule. Like all his compeers, he is hurrying busily along somewhere in obedience to some impulse of which we do not know the nature; and as he goes he carries with him his whole cortège of planets and their satellites, and even the comets. Yes, we are racing through space with another motion, too, besides those of rotation and revolution, for our earth keeps up with its master attractor, the sun. It is difficult, no doubt, to follow this, but if you think for a moment you will remember that when you are in a railway-carriage everything in that carriage is really travelling along with it, though it does not appear to move. And the whole solar system may be looked at as if it were one block in movement. As in a carriage, the different bodies in it continue their own movements all the time, while sharing in the common movement. You can get up and change your seat in the train, and when you sit down again you have not only moved that little way of which you are conscious, but a great way of which you are not conscious unless you look out of the window. Now in the case of the earth's own motion we found it necessary to look for something which does not share in that motion for purposes of comparison, and we found that something in the sun, who shows us very clearly we are turning on our axis. But in the case of the motion of the solar system the sun is moving himself, so we have to look beyond him again and turn to the stars for confirmation. Then we find that the stars have motions of their own, so that it is very difficult to judge by them at all. It is as if you were bicycling swiftly towards a number of people all walking about in different directions on a wide lawn. They have their movements, but they all also have an apparent movement, really caused by you as you advance toward them; and what astronomers had to do was to separate the true movements of the stars from the false apparent movement made by the advance of the sun. This great problem was attacked and overcome, and it is now known with tolerable certainty that the sun is sweeping onward at a pace of about twelve miles a second toward a fixed point. It really matters very little to us where he is going, for the distances are so vast that hundreds of years must elapse before his movement makes the slightest difference in regard to the stars. But there is one thing which we can judge, and that is that though his course appears to be in a straight line, it is most probably only a part of a great curve so huge that the little bit we know seems straight.
When we speak of the stars, we ought to keep quite clearly in our minds the fact that they lie at such an incredible distance from us that it is probable we shall never learn a great deal about them. Why, men have not even yet been able to communicate with the planet Mars, at its nearest only some thirty-five million miles from us, and this is a mere nothing in measuring the space between us and the stars. To express the distances of the stars in figures is really a waste of time, so astronomers have invented another way. You know that light can go round the world eight times in a second; that is a speed quite beyond our comprehension, but we just accept it. Then think what a distance it could travel in an hour, in a day; and what about a year? The distance that light can travel in a year is taken as a convenient measure by astronomers for sounding the depths of space. Measured in this way light takes four years and four months to reach us from the nearest star we know of, and there are others so much more distant that hundreds—nay, thousands—of years would have to be used to convey it. Light which has been travelling along with a velocity quite beyond thought, silently, unresting, from the time when the Britons lived and ran half naked on this island of ours, has only reached us now, and there is no limit to the time we may go back in our imaginings. We see the stars, not as they are, but as they were. If some gigantic conflagration had happened a hundred years ago in one of them situated a hundred light-years away from us, only now would that messenger, swifter than any messenger we know, have brought the news of it to us. To put the matter in figures, we are sure that no star can lie nearer to us than twenty-five billions of miles. A billion is a million millions, and is represented by a figure with twelve noughts behind it, so—1,000,000,000,000; and twenty-five such billions is the least distance within which any star can lie. How much farther away stars may be we know not, but it is something to have found out even that. On the same scale as that we took in our first example, we might express it thus: If the earth were a greengage plum at a distance of about three hundred of your steps from the sun, and Neptune were, on the same scale, about three miles away, the nearest fixed star could not be nearer than the distance measured round the whole earth at the Equator!
All this must provoke the question, How can anyone find out these things? Well, for a long time the problem of the distances of the stars was thought to be too difficult for anyone to attempt to solve it, but at last an ingenious method was devised, a method which shows once more the triumph of man's mind over difficulties. In practice this method is extremely difficult to carry out, for it is complicated by so many other things which must be made allowance for; but in theory, roughly explained, it is not too hard for anyone to grasp. The way of it is this: If you hold up your finger so as to cover exactly some object a few feet distant from you, and shut first one eye and then the other, you will find that the finger has apparently shifted very considerably against the background. The finger has not really moved, but as seen from one eye or the other, it is thrown on a different part of the background, and so appears to jump; then if you draw two imaginary lines, one from each eye to the finger, and another between the two eyes, you will have made a triangle. Now, all of you who have done a little Euclid know that if you can ascertain the length of one side of a triangle, and the angles at each end of it, you can form the rest of the triangle; that is to say, you can tell the length of the other two sides. In this instance the base line, as it is called—that is to say the line lying between the two eyes—can easily be measured, and the angles at each end can be found by an instrument called a sextant, so that by simple calculation anyone could find out what distance the finger was from the eye. Now, some ingenious man decided to apply this method to the stars. He knew that it is only objects quite near to us that will appear to shift with so small a base line as that between the eyes, and that the further away anything is the longer must the base line be before it makes any difference. But this clever man thought that if he could only get a base line long enough he could easily compute the distance of the stars from the amount that they appeared to shift against their background. He knew that the longest base line he could get on earth would be about eight thousand miles, as that is the diameter of the earth from one side to the other; so he carefully observed a star from one end of this immense base line and then from the other, quite confident that this plan would answer. But what happened? After careful observations he discovered that no star moved at all with this base line, and that it must be ever so much longer in order to make any impression. Then indeed the case seemed hopeless, for here we are tied to the earth and we cannot get away into space. But the astronomer was nothing daunted. He knew that in its journey round the sun the earth travels in an orbit which measures about one hundred and eighty-five millions of miles across, so he resolved to take observations of the stars when the earth was at one side of this great circle, and again, six months later, when she had travelled to the other side. Then indeed he would have a magnificent base line, one of one hundred and eighty-five millions of miles in length. What was the result? Even with this mighty line the stars are found to be so distant that many do not move at all, not even when measured with the finest instruments, and others move, it may be, the breadth of a hair at a distance of several feet! But even this delicate measure, a hair's-breadth, tells its own tale; it lays down a limit of twenty-five billion miles within which no star can lie!
This system which I have explained to you is called finding the star's parallax, and perhaps it is easier to understand when we put it the other way round and say that the hair's-breadth is what the whole orbit of the earth would appear to have shrunk to if it were seen from the distance of these stars!
Many, many stars have now been examined, and of them all our nearest neighbour seems to be a bright star seen in the Southern Hemisphere. It is in the constellation or star group called Centaurus, and is the brightest star in it. In order to designate the stars when it is necessary to refer to them, astronomers have invented a system. To only the very brightest are proper names attached; others are noted according to the degree of their brightness, and called after the letters of the Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet' comes, you know, from the first two letters of this Greek series. As this particular star is the brightest in the constellation Centaurus, it is called Alpha Centauri; and if ever you travel into the Southern Hemisphere and see it, you may greet it as our nearest neighbour in the starry universe, so far as we know at present.
CHAPTER XI
THE CONSTELLATIONS
From the very earliest times men have watched the stars, felt their mysterious influence, tried to discover what they were, and noted their rising and setting. They classified them into groups, called constellations, and gave such groups the names of figures and animals, according to the positions of the stars composing them. Some of these imaginary figures seem to us so wildly ridiculous that we cannot conceive how anyone could have gone so far out of their way as to invent them. But they have been long sanctioned by custom, so now, though we find it difficult to recognize in scattered groups of stars any likeness to a fish or a ram or a bear; we still call the constellations by their old names for convenience in referring to them.