observer, although this may not happen exactly in the way just described; and the two observers will sometimes see different sides of the cover. But unless the observer who shifts his place goes far from the middle of the room, both observers will always see the same side of the cover except about those times when its edge is turned towards the stationary observer.

215. Accordingly, a terrestrial observer has the edge of Saturn's ring turned towards him more frequently than it is turned towards the centre of the Sun. He is sometimes carried by the Earth's motion from a place in view of that side of the ring on which the Sun is shining at the time to a place in view of its dark side; and the Earth's motion may bring him back again to some place in view of the bright side of the ring before the Sun has ceased to shine on it. Sometimes, too, the Earth may come opposite to the edge of the ring, and at once be carried back, by its movement in its orbit, to the side of the ring on which the Sun is shining; but when the dark side of the ring is in view from the Earth, the Earth will soon come up to the plane of the ring and pass through it, so as to have the bright side in view; it cannot in this case just come opposite to the edge of the ring and fall back again, as it can when it is on the same side of the ring with the Sun. All these facts may be readily shown by the experiment described above. About the time when the edge of Saturn's ring is turned towards the Sun, it may be turned in the course of a year once only, twice, or three times, towards the Earth.

216. When the dark side of the ring is in view, it appears as a black line crossing the planet; and on such occasions the sunlight reflected from the outer and inner edges of the rings A and B enables us to see traces of the ring on each side of Saturn, at least in places where two such reflections come nearly together. The seven inner satellites of Saturn, when the ring has its edge turned nearly towards us, may sometimes be seen between us and the ring or beyond it, looking like beads on or close beside the fine thread of light

to which the ring is then reduced. But at other times, since we look at the planes of their orbits, as well as at the ring, from one side or the other, and not edgewise, we see these satellites in various places around the planet; and their paths, as well as that of the outer satellite, do not appear straight like those of the satellites of Jupiter, and seldom carry them between Saturn and the Sun or Earth. The ring is of course most conspicuous at a time about half-way between the times when its edge is turned towards the Sun. Whenever its bright side is in view, its shape appears to us elliptical, owing to the effects of perspective, as we should call it in a picture. The parts of the ring called its ansæ are those about the ends of this seeming ellipse, one of them being on each side of Saturn. Ring C, which is somewhat transparent, is most easily seen at each ansa; but it appears as a dusky line where the inner edge of ring B comes between us and the main body of Saturn, or the ball, as it is often called. When the edge of the ring is turned away from us as much as it ever can be, ring A appears all round the ball; but the rings B and C always come between us and part of the ball, and are themselves partly hidden behind it.

217. How the ring of Saturn can keep its place about the planet is a question which it has always puzzled mathematicians to answer in conformity to the ordinary laws of motion. First it appeared that the ring could not be supposed to be made of solid matter all in one piece unless the masses of different parts of it were different, and unless at the same time the whole ring revolved rapidly round the ball; and this revolution of the ring was thought to be shown by observation as well as by calculation. But later inquirers satisfied themselves that even this explanation was not enough to show why the ring did not break to pieces, or fall down in some way upon the ball. They suggested, then, that the ring might be liquid. Still later, other mathematicians have satisfied themselves that the ring can neither be solid and in one piece, nor yet liquid, if it is to keep its place; and they conclude that it must be made up of a great number of small

satellites, each of which revolves around Saturn in an orbit of its own, but is always so near other little bodies like itself that it does not appear separate from them to observers at a distance. This theory is said to account for the division between rings A and B by considerations founded on the laws of motion and the observed place of one of the eight distinct satellites of Saturn. It will also explain the dusky appearance of ring C by the supposition that the small satellites nearest Saturn are not so closely crowded together as those beyond them; and the fact that ring B is brighter than ring A may probably be due to the great number of satellites which we should expect to find in the middle part of the ring. Recent spectroscopic observations show that the ball of Saturn has an atmosphere, but leave it doubtful whether the ring has one. Without the aid of any instrument, Saturn may be seen as a bright star.

218. Uranus is much smaller than Jupiter or Saturn; its diameter, however, is about four times that of the Earth. Its mass is only fifteen times as great as the Earth's, so that its density is about the same as that of Jupiter. Neptune is somewhat larger and heavier than Uranus, but not so dense. The time in which Uranus goes once round the Sun is over eighty years; Neptune occupies over one hundred and sixty years in one revolution. Nothing is known of the rotations of Uranus and Neptune, or of any difference in length between different diameters of either. Some observers have thought that they perceived spots on Uranus, but their conclusions with respect to its rotation do not agree with each other.

219. Uranus is known to have four satellites, and Neptune one. Other satellites have been suspected to accompany these planets, which have also been thought to have rings like the ring of Saturn.

220. The known satellite of Neptune is somewhat nearer Neptune than the Moon is to the Earth, and goes round Neptune once in about six days. Of the known satellites of Uranus, that which is farthest from the planet is about half

as far again from it as the Moon is from the Earth, and goes round Uranus once in less than a fortnight. The planes of the orbits of all these satellites differ considerably from the plane of the ecliptic; the orbit of Neptune's satellite would have to make about one-tenth of a rotation round the line of its nodes as an axis in order to be brought into the plane of the ecliptic, and more than twice as much rotation would be needed for that purpose by the orbits of the satellites of Uranus. An observer near Uranus, then, placed with his feet towards the plane of the ecliptic, would see the planet's satellites passing nearly over his head (140); so that the words direct and retrograde do not apply very well to their movement. But they would not pass exactly over the head of our supposed observer; and if he was on the northern side of the plane of the ecliptic, he could perceive their movement to be from his left to his right when they passed in front of him; they are therefore said to have a retrograde movement in their orbits. The movement of Neptune's satellite can be observed only with much difficulty; but it seems to be retrograde, like the movement of the satellites of Uranus.

221. Uranus can barely be seen, and Neptune not at all, without a telescope. Both these planets seem to have atmospheres; and it has been thought that some of the light we receive from them may proceed from incandescent matter belonging to them, and may not be merely sunlight reflected

to us.

222. Jupiter and Saturn, also, are considered by some astronomers as probably hot bodies; but the light they send us seems not to be regarded by observers with the spectroscope as any thing more than sunlight. But, as we have seen, it is likely that this light comes from clouds brightly lighted up by the Sun.

9

CHAPTER VI.

NEBULÆ, COMETS, AND METEORS.

223. ANY cluster of stars too faint to be distinguished separately will appear to us, if it can be seen at all, as a whitish speck in the sky, which looks like one of the objects called nebulæ; and clusters of faint stars are actually called nebulæ until we are able to make them out to be clusters. But it is now generally believed that many of the cloud-like objects seen with the help of telescopes are not clusters of stars, but collections of luminous gases or clouds of fine particles of solid or liquid matter. When an object of this kind shines by light of its own, and is so far from us that it seems to move very slowly, if at all, it is always called a nebula. To be visible at all, it must be larger than a star, since its light is not brilliant; and some nebulæ certainly stretch over vast spaces in the universe; but no one can tell how large these spaces are until we have learned the distance from the Earth of the nebulæ which fill them. It is more difficult to determine the distance of a nebula than that of a star, because a nebula is an indistinct object, so that its place among the stars, as seen from the Earth, cannot be exactly made out; and the distances even of the stars are yet almost wholly unknown. But if some of the nebulæ are no farther from the Earth than the nearest of the stars whose distances are known, even then each of them must be large enough to reach over spaces much greater than that between the Sun and the Earth.

224. The spectroscope shows us that some nebulæ are composed of gases, perhaps unlike any which are known to chemists; other nebulæ seem to be partly solid or liquid; but perhaps they are wholly gaseous, their gases being for some reason so dense in places as to give out light like that