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 CHAPTER I

HE MOON is usually spoken of as the satellite of the Earth, but in point of fact it is more properly its twin. It is more attracted toward the Sun than toward the Earth, and if, when situated between the two, it were suddenly stopped in its orbit, it would leave the Earth, never to return to it, and drop directly into the Sun. Although the Moon appears to us to revolve about the Earth, yet an outsider would see that it should more properly be said to revolve about the Sun, since owing to the relative weakness of the Earth's attraction the Moon's path in space is always concave toward the Sun. The disturbance it produces in the orbit of its primary also far exceeds that produced by any of the other satellites. In fact, both bodies, while keeping relatively close together, really revolve about the Sun once in a year, following very closely an elliptical path called the ecliptic, the centre of each being alternately inside and outside of this path owing to their mutual attractions. As seen from the nearer planets, they must appear as a most beautiful double star. Thus from Venus, when it lies between us and the Sun, the Earth would appear two or three times as bright as Venus does to us when at its brightest. The Moon would appear about as bright as Jupiter, and the distance between the Moon and ourselves would never at any time appear greater than does the diameter of the Moon as seen from the Earth. To the assumed astronomer on Venus, the double planet, as he might properly call us, must be by far the most beautiful and interesting object in the heavens, and he would be able to see to advantage, what we unfortunately never can, that unknown country—the other side of the Moon. The diameter of the Moon is 2,163 miles (3,481 kilometers), or rather more than one-quarter of that of the Earth, while the next largest satellite in proportion to its primary measures only about one-twentieth of its diameter. In the figures, the circles represent the relative size of the Earth and Moon as compared with that of Jupiter and its largest satellite. The first pair are, for convenience, drawn on ten times the scale of the last.

The origin of the Moon is quite unlike that of the other satellites of the solar system. 3