The moon's function in causing eclipses of the sun has already been briefly alluded to. In turn she is herself eclipsed, by passing behind the earth and into the long cone of shadow which our world casts behind it into space (Fig. 19). It is obvious that such eclipses can only happen when the moon is full. A total eclipse of the moon, though by no means so important as a solar eclipse, is yet a very interesting and beautiful sight. The faint shadow or penumbra is often scarcely perceptible as the moon passes through it; but the passage of the dark umbra over the various lunar formations can be readily traced, and is most impressive. Cases of 'black eclipses' have been sometimes recorded, in which the moon at totality has seemed actually to disappear as though blotted out of the heavens; but in general this is not the case. The lunar disc still remains visible, shining with a dull coppery light, something like the ashen light, but of a redder tone. This is due to the fact that our earth is not, like its satellite, a next to airless globe, but is possessed of a pretty extensive atmosphere. By this atmosphere those rays of the sun which would otherwise have just passed the edge of the world are caught and refracted so that they are directed upon the face of the eclipsed moon, lighting it up feebly. The redness of the light is due to that same atmospheric absorption of the green and blue rays which causes the body of the setting sun to seem red when viewed through the dense layer of vapours near the horizon. When the moon appears totally eclipsed to us, the sun must appear totally eclipsed to an observer stationed on the moon. A total solar eclipse seen from the moon must present features of interest differing to some extent from those which the similar phenomenon exhibits to us. The duration of totality will be much longer, and, in addition to the usual display of prominences and corona, there will be the strange and weird effect of the black globe of our world becoming gradually bordered with a rim of ruddy light as our atmosphere catches and bends the solar rays inwards upon the lunar surface.