After Mars comes a wide space in the heavens, which for a long while was thought to be quite empty of worlds. But it is not empty. Instead of one big planet a great many tiny ones are there, journeying round the Sun in company. Nearly three hundred and fifty are known to us, and fresh ones are still often found. When first discovered, about one hundred years ago, these small worlds were named ASTEROIDS, or Little Stars. Now they are known as PLANETOIDS, or Little Planets. This is the right name for them, since they are not stars but planets, or worlds. Only they are very, very tiny worlds. The biggest of them all is under 400 miles through; and most of them are much less. So if the Moon is pictured by a large grape, a pea would be far too big for most of the Planetoids. This belt of Planetoids comes after a broad gap of space, between it and Mars; and it is followed by another wide gap. Then we get to the pathway of JUPITER. Here indeed is a contrast. The Planetoids are the smallest worlds in the whole Polar System, and Jupiter is the largest. He is very, very far away; yet, as he shines in our sky, he is often the most splendid object we can see there, second only to Venus. Venus is very much _smaller_ than Jupiter; but Venus is also very much _nearer_ than Jupiter. Not one of all the other planets is as big as Jupiter. While our Earth is only eight thousand miles through, Jupiter is eighty-five thousand miles through. This makes a wonderful difference. Jupiter is very small beside the Sun, but he is very huge beside our little Earth. Jupiter’s speed is far slower than that of the inner planets. At his vast distance the pull of the Sun is much weaker, and so he does not go so fast. If Jupiter whirled round the Sun as fast as Mars does the Sun could not hold him in, and he would wander away and be lost. But if Mercury were to journey at Jupiter’s pace he could not keep away from the Sun, and he would, most likely, soon be destroyed. Jupiter does not travel alone. He has a family of moons; not one only, like our Earth, but five moons, the nearest of which has been quite lately found. These moons all journey with Jupiter round the Sun; and they also go round Jupiter as our Moon goes round the Earth. Beyond Jupiter, at a great distance, is another giant world, SATURN. Not quite so big as Jupiter, but not very far behind him in size. Saturn too has a family, not of five moons only but of eight moons. He also has three very wonderful rings, which shine in the sunlight. Neither rings nor moons can be seen without a very good telescope. Outside the pathway of Saturn lies that of URANUS, another huge world, though a good deal smaller than Saturn. Outside the pathway of Uranus travels the dim and distant NEPTUNE--so far as we know, the outermost world of the whole Solar System. Neptune is rather larger than Uranus.