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THE MOON’S CHANGES

earth remained at E. Now obviously, when the moonis at Mj, her darkened side is turned towards theearth, and she cannot be seen. She is as at 1, fig. 48.As she advances towards M 2 , the observer onthe earth E, and supposed to be standing on thehalf of the earth shown in the figure, sees the moonon the left of the sun,—that is, towards the east,*and he would clearly see the right or western side ofthe moon partly illuminated. The case, so far as thisillumination is concerned, is exactly the same asthough the moon at Mj had turned an eighth round onan axis upright to the plane of her motion, in such away as to bring into view the parts beyond her easternedge. Thus, the aspect of the moon is as shown at2, fig. 48. It is readily seen that when she is atM s , fig. 47, her aspect is as at 3, fig. 48 ; and so on.

All this is as explained in the text-books. Butthere are two points, even in this elementary matter,which may need a word or two of explanation.

First, as to the position of the lunar crescent. Wesee the moon in varying positions on the sky; and atfirst sight there appears to be no definite relationbetween her position and the position of her cusps orhorns. Indeed, this feature of her aspect has seemed sochangeful and capricious that it has even been regardedas a weather-token. In reality, however, there is asimple relation always fulfilled by the moon’s cusps.The line joining them is always at right angles to the

* The reader should here hold the plate so as to have E towardshim, and S and M , from him.