According to Professor Pickering, from whose magnificent work, entitled, "The Moon," much of the information in this chapter has been obtained, the moon, which is generally spoken of as a satellite of the earth, ought rather to be called the earth's twin planet. Although the moon appears to revolve in a small elliptical orbit around the earth it should properly be said to revolve around the sun; for, together with the earth, it revolves around the sun once every year. As seen from any of the planets that lie near the earth the earth and moon would appear as a very beautiful double star. In order the more readily to understand what will be said shortly concerning the origin of the moon, it may be mentioned that the moon's diameter is 2,163 miles, or a little more than one-fourth the diameter of our earth. You will, most probably, be surprised to learn that the origin of the moon is believed to be very different from the origin of the moons or satellites of Jupiter, Saturn, and the other planets. As we have already seen, according to the nebular hypothesis, all the planets except the earth probably had their moons formed from the rings that were left surrounding them when they shrunk on cooling to their present dimensions. Such a ring is still to be seen surrounding Saturn. Now it is believed that our moon was formed in a different manner. It was not thrown off from the earth while the latter was in a highly fluid or gaseous condition, but after the earth had shrunken to nearly its present size and, most probably, after a solid crust had been formed on its surface. In order that our earth should be able to violently throw off a large portion of its mass, it is only necessary that at the time this separation occurred, its motion of rotation on its axis was sufficiently great to enable it to make one complete revolution in rather less than three hours instead of in the twenty-four hours it now requires. At this velocity of rotation, objects would fly off the earth in the neighborhood of the equator, under the influence of the high centrifugal force. Let us, then, endeavor to see if it was at all probable that the earth ever did turn so rapidly on its axis.