For a long time it was supposed that Mars had no moons, and when Dean Swift wrote 'Gulliver's Travels' he wanted to make the Laputans do something very clever, so he described their discovery of two moons attending Mars, and to make it quite absurd he said that when they observed these moons they found that one of them went round the planet in about ten hours. Now, as Mars takes more than twenty-four hours to rotate, this was considered ridiculous, for no moon known then took less time to go round its primary world than the primary world took to turn on its own axis. Our own moon, of course, takes thirty times as long--that is a month contains thirty days. Then one hundred and fifty years later this jest of Dean Swift's came true, for two moons were really discovered revolving round Mars, and one of them does actually take less time to complete its orbit than the planet does to rotate--namely, a little more than seven hours! So the absurdity in 'Gulliver's Travels' was a kind of prophecy! 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.