The earth and moon form a system that is in a way unique. No satellite in the solar system is so large in proportion to its primary as is our own moon. Seen from the distance of Venus or Mars, the two bodies would apparently form a _double star_. The diameter of the moon is one-fourth that of the earth. Satellite III of Jupiter far exceeds our own moon in actual size but its diameter is only about four-hundredths of the diameter of the planet about which it revolves. The diameter of Titan, the largest satellite of the Saturnian system, bears the same ratio to the diameter of Saturn. Moreover, all the nearer satellites of Jupiter and Saturn lie nearly in the equatorial planes of these planets, but the orbit of the moon is inclined at a high angle to the plane of the earth's equator. It is not difficult to believe that the satellites of Jupiter or Saturn were at some time thrown off from the equatorial belts of their primaries, just as the planets themselves may have been ejected from the equatorial belt of the sun, but we cannot so readily believe that our own satellite was formed from the earth in a similar manner. The moon's orbit lies nearly in the plane of the sun's equator, however, and it is conceivable that both earth and moon were simultaneously ejected from the equatorial zone of the sun, the two nuclei being so close together that the smaller one remained under the gravitational control of the larger. The difficulties in the way of believing that the moon once formed a part of the earth are very great. It can be shown mathematically that if the two bodies at one time formed a single mass it would have been impossible for the moon to break away from the earth, unless the force that caused the separation were sufficient to hurl the moon to a greater distance than two and a half times the earth's radius. The mathematician, Roche, found out by computation that a satellite could not remain intact within this distance of the planet, but would be broken up into small fragments under the effects of the tides raised by the larger body. If, then, the moon had originally been ejected from the earth to a less distance than two and one-half radii of the earth (2.44 to be exact) it would have been disintegrated into small particles, or moonlets, under the tidal strains exerted upon it by the earth and would have been gradually distributed about the earth in the form of a meteoric ring which, in the course of ages, would be absorbed by the earth, just as Saturn is now gradually absorbing its rings.