The four larger planets apparently resemble our Sun in constitution more than they do the Earth, and their satellites were probably formed from rings left surrounding them when they cooled and contracted to their present dimensions. The well-known example of such a ring is the one at present surrounding the planet Saturn. This ring is composed of myriads of independent meteorites each moving in its own orbit. These may possibly in the future coalesce to form still another satellite to that already wellfurnished planet.
The satellites of Mars may have been formed in this manner, or they may be captured asteroids, taken from that great swarm of little bodies, at some remote epoch when the eccentricity of the planet's orbit was greater than it is at present, carrying it at times far beyond its present greatest distance from the Sun. Under these circumstances Mars would be moving very slowly, and should it chance to pass near an asteroid moving at about the same speed it would be quite able to disturb the orbit of the latter to such an extent that the little body would be unable to escape from it, and in future they would therefore continue to travel together around the Sun for all time. The largest satellite of Mars, Phobos, is about thirty miles in diameter, or about the average size of those asteroids at present known to us.
Our own Moon was created by still a third method, and of this we are more certain than of either of the others. While the origin given of the other satellites may be described as probable, in the case of our Moon we are fairly certain, and it is perhaps the strangest of the three. We have all of us read of those little animals living in the sea, mere shapeless masses of jelly, that, instead of swallowing their dinner, having no mouths or stomachs whatever, simply wrap themselves around it. If we watch them under the microscope we may see one of them slowly lengthen out, then break in two, and each part go swimming away by itself, a perfect animal. It was Professor George H. Darwin, the son of the great naturalist, who proved mathematically that the origin of the Moon was such that we may properly compare it to that of the little animals above described.
To understand this matter better, we must first consider the mutual influences that the Earth and Moon exert upon one another at the present time. We all know that the Moon produces a double tide upon the Earth, in virtue of which large masses of water are transported over its surface. One tide is on the side of the Earth toward the Moon, and the other tide is on the opposite side. The Earth revolves between these tides very
