These two moons are very small, the outer one perhaps five or six miles in diameter, and the inner one about seven; therefore from Mars the outer one, Deimos, cannot look much more than a brilliant star, and the inner one would be but a fifth part the apparent width of our own moon. So Mars is not very well off, after all. Still, there is great variety, for it must be odd to see the same moon appearing three times in the day, showing all the different phases as it goes from new to full, even though it is small!
Such wonderful discoveries have already been made that it is not too much to say that perhaps some day we may be able to establish some sort of communication with Mars, and if it be inhabited by any intelligent beings, we may be able to signal to them; but it is almost impossible that any contrivance could bridge the gulf of airless space that separates us, and it is not likely that holiday trips to Mars will ever become fashionable!
CHAPTER VI
FOUR LARGE WORLDS
I have told you about the four lesser worlds of which our earth is one, and you know that beyond Mars, the last of them, there lies a vast space, in which are found the asteroids, those strange small planets circling near to each other, like a swarm of bees. After this there comes Jupiter, who is so enormous, so superb in size compared with us, that he might well serve as the sun of a little system of his own. You remember that we represented him by a football, while the earth was only a greengage plum. But Jupiter himself is far less in comparison with the sun than we are in comparison with him. He differs from the planets we have heard about up to the present in that he seems to glow with some heat that he does not receive from the sun. The illumination which makes him appear as a star to us is, of course, merely reflected sunlight, and what we see is the external covering, his envelope of cloud.
There is every reason to believe that the great bulk of Jupiter is still at a high temperature. We know that in the depths of the earth there is still plenty of heat, which every now and then makes its presence felt by bursting up through the vents we call volcanoes, the weak spots in the earth's crust; but our surface long ago cooled, for the outside of any body gets cool before the inside, as you may have found if ever you were trying to eat hot porridge, and circled round the edge of the plate with a spoon. A large body cools more slowly than a small one, and it is possible that Jupiter, being so much larger than we are, has taken longer to cool. One reason we have for thinking this is that he is so very light compared with his size—in other words, his density is so small that it is not possible he could be made of materials such as the earth is made of.
As I said, when we study him through telescopes we see just the exterior, the outer envelope of cloud, and as we should expect, this changes continually, and appears as a series of belts, owing to the rotation of the planet. Jupiter's rotation is very rapid; though he is so much greater than the earth, he takes less than half the time the earth does to turn round—that is to say, only ten hours. His days and nights of five hours each seem short to us, accustomed to measure things by our own estimates. But we must remember that everything is relative; that is to say, there is really no such thing as fast or slow; it is all by comparison. A spider runs fast compared with a snail, but either is terribly slow compared with an express train; and the speed of an express train itself is nothing to the velocity of light.
In the same way there is nothing absolutely great or small; it is all by comparison. We say how marvellous it is that a little insect has all the mechanism of life in its body when it is so tiny, but if we imagine that insect magnified by a powerful microscope until it appears quite large, the marvel ceases. Again, imagine a man walking on the surface of the earth as seen from a great distance through a telescope: he would seem less than an insect, and we might ask how could the mechanism of life be compressed into anything so small? Thus, when we say enormous or tiny we must always remember we are only speaking by the measurements of our own standards.
There is nothing very striking about Jupiter's orbit. He takes between eleven and twelve of our years to get round the sun, so you see, though his day is shorter, his year is longer than ours. And this is not only because his path is much larger, but because by the law of gravity the more distant a planet is from the sun the more slowly it travels, so that while the earth speeds over eighteen miles Jupiter has only done eight. Of course, we must be careful to remember the difference between rotation and revolution. Jupiter rotates much quicker than the earth—that is to say, he turns round more quickly—but he actually gets over the ground more slowly. The sun appears much smaller to him than it does to us, and he receives considerably less light and heat. There are various spots on his surface, and one remarkable feature is a dark mark, which is called the 'great red spot.' If as we suppose what we see of the planet is merely the cloudy upper atmosphere, we should not expect to find anything permanent there, for the markings would change from day to day, and this they do with this exception—that this spot, dark red in colour, has been seen for many years, turning as the planet turned. It was first noticed in 1878, and was supposed to be some great mountain or excrescence peeping up through the clouds. It grew stronger and darker for several years, and then seemed to fade, and was not so easily seen, and though still remaining it is now pale. But, most startling to say, it has shifted its position a little—that is, it takes a few seconds longer to get round the planet than it did at first. A few seconds, you will say, but that is nothing! It does not seem much, but it shows how marvellously accurate astronomers are. Discoveries of vast importance have been made from observing a few seconds' discrepancy in the time the heavenly bodies take in their journeys, and the fact that this spot takes a little longer in its rotation than it did at first shows that it cannot be attached to the body of the planet. It is impossible for it to be the summit of a mountain or anything of that sort. What can it be? No one has yet answered that question.