“I should have told you that,” I replied. “Yes, it does represent the moon at a time when more of its surface, visible to us, is illuminated by the sun. In fact, we may regard it as a picture of the moon made about a day later than the other. But I must now tell you that these photographs were not all taken in regular succession, a day apart, or even two days apart. That was impracticable for reasons that I need not explain. Some of them were made at one season of the year and some at another. Yet taken together they form a sufficiently continuous series to enable us, with their aid, to follow the changing aspects of the moon during more than three weeks, or all that part of a lunation in which the moon is a conspicuous object in the sky.”[2] Footnote 2: In addition to what is said in the text concerning the photographs the reader should be informed that, in consequence of her “librations,” the moon does not, all the time, present _exactly_ the same surface toward the earth. If she did we should never see more than one half of her surface. In fact, however, at one time or another, we see, in all (but never at the same time), about fifty-nine per cent of her surface, leaving forty-one per cent which is forever invisible because never turned in our direction. The librations, or “balancings,” of the moon, which bring now one and now another portion of the usually invisible hemisphere into view, are of three kinds: First, the libration in latitude, arising from the combined effects of the inclination of the moon’s orbit to the plane of the earth’s orbit, and the inclination of her axis of rotation to the plane of her own orbit. When added together these two inclinations make the axis of the moon lean one way or the other with respect to the earth about 6½°. But, since the inclination of the moon’s orbit to that of the earth is continually varying to a small extent, the amount of this libration is also variable. Its effect is to cause now the North and now the South Pole of the moon to incline slightly toward the observer on the earth, so that he can see alternately a little way round the northern and the southern edges of the moon’s disk.