The Children's Book of Stars

by G. E. Mitton

JUPITER AND ONE OF HIS MOONS

When we get to the chapter on the sun, we shall find curiosities respecting the spots there as well.

Jupiter has seven moons, and four of these are comparatively large. They have the honour of having been the first heavenly bodies ever actually discovered, for the six large planets nearest the sun have been known so long that there is no record of their first discovery, and of course our own moon has always been known. Galileo, who invented the telescope, turned it on to the sky in 1610, when our King Charles I. was on the throne, and he saw these curious bodies which at first he could not believe to be moons. The four which he saw vary in size from two thousand one hundred miles in diameter to nearly three thousand six hundred. You remember our own moon is two thousand miles across, so even the smallest is larger than she. They go round at about the same level as the planet's Equator, and therefore they cross right in front of him, and go behind him once in every revolution. Since then the other three have been discovered in the band of Jupiter's satellites—one a small moon closer to him than any of the first set, and two others further out. It was by observation of the first four, however, that very interesting results were obtained. Mathematicians calculated the time that these satellites ought to disappear behind Jupiter and reappear again, but they found that this did not happen exactly at the time predicted; sometimes the moons disappeared sooner than they should have done, and sometimes later. Then this was discovered to have some relation to the distance of our earth from Jupiter. When he was at the far side of his immense orbit he was much more distant from us than when he was on the nearer side—in fact, the difference may amount to more than three hundred millions of miles. And it occurred to some clever man that the irregularities in time we noticed in the eclipses of the satellites corresponded with the distance of Jupiter from us. The further he drew away from us, the later were the eclipses, and as he came nearer they grew earlier. By a brilliant inspiration, this was attributed to the time light took to travel from them to us, and this was the first time anyone had been able to measure the velocity or speed of light. For all practical purposes, on the earth's surface we hold light to be instantaneous, and well we may, for light could travel more than eight times round the world in one second. It makes one's brain reel to think of such a thing. Then think how far Jupiter must be away from us at the furthest, when you hear that sometimes these eclipses were delayed seventeen minutes—minutes, not seconds—because it took that time for light to cross the gulf to us!

JUPITER AND HIS PRINCIPAL MOONS.

Sound is very slow compared with light, and that is why, if you watch a man hammering at a distance, the stroke he gives the nail does not coincide with the bang that reaches you, for light gets to you practically at once, and the sound comes after it. No sound can travel without air, as we have heard, therefore no sound reaches us across space. If the moon were to blow up into a million pieces we should see the amazing spectacle, but should hear nothing of it. Light travels everywhere throughout the universe, and by the use of this universal carrier we have learnt all that we know about the stars and planets. When the time that light takes to travel had been ascertained by means of Jupiter's satellites, a still more important problem could be solved—that was our own distance from the sun, which before had only been known approximately, and this was calculated to be ninety-two millions seven hundred thousand miles, though sometimes we are a little nearer and sometimes a little further away.

Jupiter is marvellous, but beyond him lies the most wonderful body in the whole solar system. We have found curiosities on our way out: we have studied the problem of the asteroids, of the little moon that goes round Mars in less time than Mars himself rotates; we have considered the 'great red spot' on Jupiter, which apparently moves independently of the planet; but nothing have we found as yet to compare with the rings of Saturn. May you see this amazing sight through a telescope one day!

Look at the picture of this wonderful system, and think what it would be like if the earth were surrounded with similar rings! The first question which occurs to all of us is what must the sky look like from Saturn? What must it be to look up overhead and see several great hoops or arches extending from one horizon to another, reflecting light in different degrees of intensity? It would be as if we saw several immense rainbows, far larger than any earthly rainbow, and of pure light, not split into colours, extending permanently across the sky, and now and then broken by the black shadow of the planet itself as it came between them and the sun. However, we must begin at the beginning, and find out about Saturn himself before we puzzle ourselves over his rings. Saturn is not a very great deal less than Jupiter, though, so small are the other planets in comparison, that if Saturn and all the rest were rolled together, they would not make one mass so bulky as Jupiter! Saturn is so light—in other words, his density is so small—that he is actually lighter than water. He is the lightest, in comparison with his size, of any of the planets. Therefore he cannot be made largely of solid land, as our earth is, but must be to a great extent, composed of air and gaseous vapour, like his mighty neighbour. He approaches at times as near to Jupiter as Jupiter does to us, and on these occasions he must present a splendid spectacle to Jupiter. He takes no less than twenty-nine and a half of our years to complete his stately march around the sun, and his axis is a little more bent than ours; but, of course, at his great distance from the sun, this cannot have the same effect on the seasons that it does with us. Saturn turns fast on his axis, but not so fast as Jupiter, and in turning his face, or what we call his surface, presents much the same appearance to us that we might expect, for it changes very frequently and looks like cloud belts.

The marvellous feature about Saturn is, of course, the rings. There are three of these, lying one within the other, and separated by a fine line from each other. The middle one is much the broadest, probably about ten thousand miles in width, and the inner one, which is the darkest, was not discovered until some time after the others. As the planet swings in his orbit the rings naturally appear very different to us at different times. Sometimes we can only see them edgewise, and then even in the largest telescope they are only like a streak of light, and this shows that they cannot be more than fifty or sixty miles in thickness. The one which is nearest to Saturn's surface does not approach him within ten thousand miles. Saturn has no less than ten satellites, in addition to the rings, so that his midnight sky must present a magnificent spectacle. The rings, which do not shine by their own light but by reflected sunlight, are solid enough to throw a shadow on the body of the planet, and themselves receive his shadow. Sometimes for days together a large part of Saturn must suffer eclipse beneath the encircling rings, but at other times, at night, when the rings are clear of the planet's body, so that the light is not cut off from them, they must appear as radiant arches of glory spanning the sky.