Speaking of the distance of one planet from another we mean usually their _nearest_ distances, when they are both on one side of the Sun together. When they are on opposite sides of the Sun they are very much farther apart. The moons belonging to these planets are really like planets, or worlds, travelling with the bigger worlds. Some of them are not so very little, either. Mars’ moons are most tiny; but one of Jupiter’s moons, as you heard, is larger than Mercury. Mercury, however, being the nearest planet to the Sun, is a much more important world than a far-off moon of Jupiter can be. Each moon, whether of Jupiter, of Saturn, or of any other planet, travels, like our Moon, in a pathway of its own round the Sun. And as it goes it curves backwards and forwards, so as to face in turn each side of the large world with which it journeys. The pull of a great body like Jupiter is very strong; and the moons in consequence travel very fast round Jupiter--the nearest going most rapidly, the farthest off most slowly. It is the same again with Saturn’s eight moons. QUESTIONS. 1. What is the size of Saturn? Almost as large as Jupiter. 2. Does Saturn spin on his axis? Yes, in about ten hours, like Jupiter. 3. How long is Saturn’s year? Nearly thirty of our years. 4. Is Saturn like Jupiter in make? Saturn is very light, even lighter than Jupiter; not so heavy as water. Saturn, too, has faint bands of color. 5. What is Saturn’s state believed to be? Half-cooled, and very stormy, with great masses of cloud. 6. How many moons has Saturn? Eight moons, and also three rings. 7. How do the rings shine? Like the moons, on one side, by reflected sunlight. 8. When was Uranus discovered? About one hundred years ago. 9. By whom? By Herschel. 10. How long is the year of Uranus? Eighty-four of our years. 11. How many moons has Uranus? Four moons are known. 12. What size are these two outer planets, Uranus and Neptune? Much larger than Venus or the Earth, much smaller than Jupiter or Saturn. 13. How far is Neptune from the Sun? Eighty times the distance of Mercury, or twenty-eight hundred millions of miles. 14. How fast does Neptune travel? Some three miles each second. 15. What is the length of Neptune’s year? About one hundred and sixty-five of our years. 16. How many moons has Neptune? Only one has been seen. 17. Are Uranus and Neptune light or heavy in make? About as light in make as water. CHAPTER XVII. LONG-TAILED COMETS. A good deal has been said about empty gaps in the sky between and around the pathways of the worlds. But those gaps are at least not always quite empty. Comets, with long bright tails, flash through the darkness by hundreds, perhaps thousands. Meteors travel in vast swarms, by millions of millions. Each comet gives forth a radiant shining, and each little meteor is bright in the sunlight. I am going to tell you about Comets first, and then about Meteors.