Page:Astronomy for Everybody.djvu/243

ing the planet like a border to the inner ring is the shadow of the ring on the planet.

Very interesting are rather rare occasions when the plane of the ring passes between the earth and the sun. Then the sun shines on one side of the ring while the other side is presented to us, though, of course, at a very small angle. The chances for observing Saturn at such times are rather few, especially in recent times. At both the last occasions, 1877 and 1892, this only happened for a few days, when the planet was not well situated for these observations. Nevertheless, in October, 1892, Barnard got a look at it from the Lick Observatory, and found that the rings were totally invisible, though their shadow could be seen on the planet. This shows that the rings are so thin that their edges are invisible in a powerful telescope.

When it became accepted that the laws of mechanics, as we learn them on the earth, govern the motions of the heavenly bodies, another riddle was presented by the rings of Saturn. What keeps the rings in place? What keeps the planet from running against the inner ring and producing, to modify Addison's verse, a "wreck of matter and crush of worlds" that would lay the whole beautiful structure in ruins? It was for a time supposed that a liquid ring might be proof against such a catastrophe, and then it was shown that such was not the case. Finally it was made clear that the rings could not be cohering bodies of any kind, but were merely clouds of