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immersed in the shadow has not entirely disappeared, but shines with a very faint light. If the whole body of the moon enters into the shadow, the eclipse is said to be total; if only a portion of her body dips into the shadow, it is called partial. If the eclipse is total, the light which illuminates the eclipsed moon will be very plainly seen, because it is not drowned out by the dazzling light of the uneclipsed portion. This light is of a dingy red colour, and arises from the refraction of the earth's atmosphere, which was described in a former chapter. In consequence of this, those rays of the sun which just graze the earth, or pass within a short distance of its surface, are bent out of their course and thrown into the shadow by refraction. Thus they fill the shadow and fall on the moon. The red colour is due to the same cause that makes the sun appear red at sunset, namely, the absorption of the green and blue rays by the atmosphere, which lets the red rays pass.

Two or three eclipses of the moon occur every year, of which one, at least, is nearly always total. But, of course, the eclipse will be visible only in that hemisphere of the earth on which the moon is shining at the time.

When the moon is eclipsed an observer on that body would see an eclipse of the sun by the earth. The cause of the phenomenon we have described would then be plain enough to him. The apparent size of the earth would be much larger than that of the moon as we see it. Its diameter would be between three and four times that of the sun. At first this immense body would be invisible when it approached the sun. What the observer would see would be the cutting off of the light of the sun by the