“Certainly; the shadow of a globe must have a circular outline. But the shadow of the earth, although it finally diminishes to a point, is, at the moon’s distance, still about 5,700 miles in diameter, or more than two and a half times the diameter of the moon. In consequence of the motion of the earth in its orbit around the sun, its shadow constantly moves eastward, like a great pencil of darkness sweeping straight across the heavens, but invisible to us except when the moon, traveling eastward faster than the shadow, overtakes and passes through it. This does not by any means happen at every full moon, because, for a reason which I shall explain presently, the moon usually passes either above or below the shadow of the earth, and thus escapes an eclipse. When an eclipse does occur it lasts a long time because the shadow is moving in the same direction as the moon. The moon must pass entirely through it before the eclipse ends. On this occasion the moon will be in the shadow more than three hours, and during an hour and a half she will be totally immersed. We shall have plenty of time, then, to observe the phenomenon, and after you have satisfied your curiosity a little by watching the slow advance of the shadow movement across the moon, we can return to our diagram and finish its explanation before the eclipse becomes total.” Accordingly, after having watched the progress of the eclipse for half an hour, during which time the shadow began perceptibly to diminish the moonlight in the park, we returned to the lamplight and the diagram on the table.