Page:The Moon (Pickering).djvu/30

10 Earth's attraction on this projecting surface will retard its rotation by just the proper amount to bring about a coincidence with exactly the same result that a tide would have had.

Thus we never have seen and never shall see what is on the other side of the Moon. But we may perhaps console ourselves with the belief that it is undoubtedly very like the side that we do see. Hansen formerly suggested that the Moon's centre of gravity was thirty miles farther from us than its centre of figure. If this were true, we should be looking down upon a great tableland, while all of the Moon's atmosphere and oceans would have flowed around to the other side, rendering that side fertile while this side remained an absolute desert. This would be interesting if true, but we now know that such is not the case.

As we all know, the Moon apparently revolves about the Earth in an elliptical orbit once a month. If we disregard its motion about the Sun, this statement is literally true. This fact is determined by noting the difference in its apparent size in different portions of its orbit. Thus, owing to its varying distance, it sometimes appears about twelve per cent, larger than it does at others. While this difference is not noticeable to the unassisted eye, we can detect it on some of the photographs. Thus if we examine Plate 5A we shall see a large dark plain near the centre of it, known as the Mare Serenitatis. If now we turn to Plate 5E we shall see that the mare appears decidedly smaller. This is because when the second picture was taken the Moon was at a greater distance from the Earth. The greatest distance from us that the Moon ever gets is 253,000 miles; the nearest it ever comes to us is 222,000 miles. Its mean distance is 239,000 miles, or about one four-htmdredth part as far as the Sun. Its maximum apparent diameter is 33′ 33″, its minimum apparent diameter 29′ 24″, and its mean diameter 31′ 08″.

The Moon is constantly moving rapidly eastward among the stars, although carried still more rapidly with them toward the west, owing to the Earth's rotation. Since its orbit is considerably inclined to the plane of the Earth's equator, it also sometimes moves northward and sometimes southward in the sky. When it is moving rapidly northward it rises at pretty nearly the same time night after night for several nights in succession. In the latitude of 40° the difference in the time of rising sometimes amounts to less than twenty-five minutes, instead of fifty—the average amount. As we go north the difference becomes less and less, until finally when far enough north we may at times find the Moon for a few evenings rising at an earlier and earlier hour. When the