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THE SUN, EARTH, AND MOON

having changed places. By 1919 each will have made an entire revolution through the year.

Let us imagine ourselves to be looking at the sun and earth from the moon when the latter is about to enter the earth's shadow. The earth, looking much larger than the sun, will be seen to approach it, and at length will begin to impinge on its disk and cut off a part of its light. The region within which this will occur is called the penumbra, and it is shown outside the shadow in the figure. So long as the moon is only in this region, an

Fig. 24.—Passage of the Moon through the Earths Shadow.

ordinary observer would not notice any diminution in its light, although such a diminution could be detected by exact photometric measurements. The moon is not said to be eclipsed until it begins to enter into the actual shadow, where the whole direct light of the sun is cut off.

How an Eclipse of the Moon Looks

If we watch the moon when an eclipse is about to begin, we shall see a small portion of her eastern edge gradually grow dim and finally disappear. As the moon advances in her orbit, more and more of her face thus disappears from view by entering into the shadow. If, however, we look very carefully, we shall see that the part

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