Notwithstanding the imposing breadth of these rings, they are less than a hundred miles in thickness. They are, in fact, nothing more than an untold number of tiny satellites revolving about Saturn in the same plane and close enough together to appear, at the distance they are from us, as if they were one body. Just how close they are together, and how they appear when near by, we do not yet know. It was first shown by mechanical laws that they _must_ be composed of separate bodies; the spectroscope shows that they _are_; and it has recently been thought that they have even been _seen_ to be so through a telescope. Being all in the same plane, they form a flat, broad, thin ring, so thin that when the edge of the ring is turned toward us we cannot see them at all. We never see them at their full breadth. If we did, Saturn would be much brighter at times than he ever is. The plane in which they revolve is the plane of Saturn’s equator; and the axis of Saturn, with the rings, has a tilt of twenty-seven degrees in his orbit. The result of this is that at the time of Saturn’s equinoxes the edge of the rings is turned toward us, and they practically disappear. Half-way between the equinoxes they are open again as far as they ever are to our view. This is why Saturn alternates in brightness. The times of his equinoxes occur every fourteen and eight-tenths years, and he is then alternately in Leo and Aquarius and is least bright. The times at which the rings are most open occur at intervals of the same length, and he is then alternately in Scorpio and Taurus and at his brightest. [Illustration: SATURN AND ITS RINGS Photographed at Mt. Wilson by E. E. Barnard, the six exposures being made on one plate.] It is believed that Saturn’s rings were never a part of the planet, but are mere particles of cosmic materials which happened to be left over, and which he has gathered up by his force of gravity and compelled to revolve about him. Saturn, more fortunate than Jupiter, has escaped the unimaginative naming of his moons by number, though one would think that, having such a numerous offspring, a shortage in names would be more likely to occur in his than in any other planet family. They all have names more or less connected with the great god whose name the planet bears, and are, in order of their distance from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Japetus, Phœbe, and Themis. The largest and brightest of them all is Titan. It is larger than our moon, which is one of the large moons in the solar system, or than Mercury, and is not much smaller than Mars. It is more than three-quarters as large as all the other moons of Saturn put together. Naturally, it was the first to be discovered, and was under observation as long ago as 1655. Rhea and Japetus are next in size, and were discovered in 1671–72; Dione and Tethys were both discovered in 1684, and Enceladus and Mimas in 1789.