Star-land: Being Talks With Young People About the Wonders of the Heavens

by Robert S. Ball

Fig. 25.—The Sun seems to revolve around the Earth.

If you look out at Christmas time, towards the south, you will see the Belt of Orion and the Dog Star in a splendid portion of the heavens. These stars you will see every winter in the same place. But you may look in vain for them in summer. No doubt you can see stars in the summer evenings, but they will be totally different from those that adorned the skies in winter. Each season has its own constellations. This simple fact was known to the ancients, and we shall find its explanation full of meaning. Let us select four well-known constellations which will best answer our purpose. They lie in a circle round the heavens. They are Orion, Virgo, Scorpio, and Pisces. I am supposing that you are looking out at midnight towards the south. In December you will see Orion; in March, Virgo; in June, Scorpio; and in September, Pisces; and then next December you will be looking at Orion again. See what this proves. At midnight, of course, the sun is at the other side of the earth, so that if I am looking at Orion in midwinter the sun must be behind my back. Look at our little picture ([Fig. 25]). The earth is in the middle, and the sun must be on the opposite side to Orion. That is, the sun must be somewhere about the position I have marked at A. In March we see Virgo in the south at midnight, when, of course, the sun is at the other side of the earth; so that the sun must be somewhere at B. In June Scorpio is seen, so that the sun must be at the other side, at C. That is to say, in midsummer the sun is in that part of the sky where Orion is situated. If, therefore, on a bright June day we could see the stars, we should find Orion in the south. But, of course, the light of the sun makes Orion invisible. We can, however, see the stars by our telescopes, and on rare occasions an eclipse of the sun will occur, by which he is temporarily extinguished, and then we can see the stars without the help of a telescope, even though it is daytime.

Fig. 26.—The Earth, however, really revolves around the Sun.

Thus it would seem as if the sun were first at A and then at B, C, and D, and then began to go round again. I say it would seem as if the sun had these movements, and the ancients thought there was no doubt about the matter. Even after it was plain that the earth turned round on its axis so as to give the changes of day and night, it was still thought necessary to suppose that the sun went round the earth once in the year, in order to explain how the changes in the stars during the different seasons were produced.

Here is another case in which we must be careful to distinguish between what appears to be true and what is actually the case. Everything that we undoubtedly see would be just as well explained by supposing that the sun remained at rest, and that the earth revolved around it, as in [Fig. 26]. If, for instance, the earth were at A in midwinter, then the sun is on the opposite side to Orion, and of course at midnight we shall be able to see Orion. So in spring the earth is at B, and we see Virgo, and similarly in summer we have Scorpio, and in autumn Pisces. Thus all that is actually visible could be fully accounted for by regarding the sun as fixed in the centre, and the earth as travelling round it from A to B, to C and to D respectively, and completing the journey in a twelvemonth. Which idea are we to adopt? Shall we say that the earth goes round the sun, or the sun goes round the earth?

I remember an old college story, which I cannot help giving you at this place. It may serve to lighten what I fear you must otherwise have thought rather a tedious part of our subject. There were three students brought up for examination in astronomy, and they showed a lamentable ignorance of the subject, but the examiner, being a kind-hearted man, wished, if possible, to pass them; and so he proposed to the three youths the very simplest question that he could think of. Accordingly, addressing the first student, he said: “Now tell me, does the earth go round the sun, or the sun go round the earth?” “It is—the earth—goes round the sun.” “What do you say?” he inquired, turning rather suddenly on the next, who gasped out: “Oh, sir—of course—it is the sun goes round the earth.” “What do you say?” he shouted at the third unhappy victim. “Oh, sir, it is—sometimes one way, sir, and sometimes the other!”

But which is it? Well, we must remember that the earth is comparatively a very little body and the sun a very big one, so it is not at all surprising to learn that the earth goes round the sun, which remains, practically speaking, at rest in the centre. Thus our great earth and all it contains are continually bound in what is very nearly a circular course round the great luminary. You will find it instructive to work out this little sum. How fast is the earth moving, or how far do we go in a second? We are about 93,000,000 miles from the sun, and the great circle that we go round has a diameter twice as great as this—that is, about 186,000,000 miles. The circumference of a circle is nearly three and one-seventh times its diameter, and accordingly the whole length of the voyage in the year is about 585,000,000 miles. This has to be accomplished in 365 days, so that the daily run must be about 1,600,000 miles. We divide this by 24, to find the distance journeyed each hour, which we find to be about 67,000 miles; and we must divide this again by 60 to find the length covered in a minute, and by 60 again for the progress made each second. It is truly startling to find that, night and day, this great earth has to travel more than eighteen miles every second in order to get round its mighty path in the allotted time.

I began this lecture about forty minutes ago, and I think from what I have said you will be able to calculate a result that will, I dare say, astonish you. In these forty minutes we have moved about 45,000 miles. No doubt my lecture commenced in this hall, and in your presence; but can I truly say I began it here? Well, no; I began it not here, but at a place 45,000 miles away; but we have all been travelling together, and the journey has been so very smooth and free from all jolts, that we never thought anything about the motion.

I am sure many of those to whom I am now speaking have read accounts of voyages in the Arctic regions. You have been told of the sufferings of the crews during the long winters, amid the ice and snow; and you have heard how, during that dismal period, there is total darkness, for the sun never rises for weeks and months together. On the other hand, these northern regions often present a more cheerful picture. During midsummer, the long darkness of winter is atoned for by perpetual sunshine. At midnight there is still the full brilliance of day, and the sun, though low, no doubt, has not passed below the horizon. Even in the northerly parts of Europe we can see the midnight sun. Lord Dufferin, in his delightful narrative of a cruise, entitled “Letters from High Latitudes,” gives an interesting illustration of the perplexities arising from endless daylight. It appears that everything went on happily until the fatal moment when the yacht crossed the Arctic circle. Then it was that dire tribulation arose among the poultry. A fine cock was the cause of the trouble. Knowing his duty, he always liked to be particular about performing the important task of crowing at sunrise. This he could do regularly, so long as the yacht remained in reasonable latitudes, where the sun behaved properly. But when they crossed the Arctic circle, the cock was confronted with a wholly new experience. The sun never set in the evening, and consequently never had to rise in the morning. What was the distracted bird to do? He did everything. He burst into occasional fits of terrific crowing at all sorts of hours, then he gave up crowing altogether, but finding that did not mend matters, he took to crowing incessantly. Exhaustion was succeeded by delirium, and rather than live any longer in a universe where the sun was capable of pranks so heartless, the indignant fowl flung himself from the vessel and perished in the Arctic Ocean.