Page:Astronomy for Everybody.djvu/157

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V

Eclipses of the Moon

The reader is doubtless aware that an eclipse of the moon is caused by that body entering the shadow of the earth, and that an eclipse of the sun is caused by the moon passing between us and the sun. Taking this knowledge for granted, we shall explain the more interesting features of these phenomena and the laws of their recurrence.

The first question to be considered is: Why is there not an eclipse of the moon at every full moon, since the earth's shadow must always be in its place opposite the

Fig. 23.—The Moon in the Shadow of the Earth.

sun? The answer is that the moon commonly passes either above or below the shadow of the earth, and so fails to be eclipsed. This, again, arises from the fact that the orbit of the moon has a small inclination, about five degrees, to the plane of the ecliptic, in which the earth moves, and in which the centre of the shadow always lies. Returning to our former thought of the ecliptic being

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