Page:The Moon (Pickering).djvu/32

12 first from the Sun, thence reflected from the Earth, thence from the Moon, and so back to the Earth. It is, in fact, earthlight shining on the Moon.

The first photographs which were probably ever taken of this faint illumination were made at the Harvard Observatory in 1888. From them it was possible to show that the Earth was photographically 23.6 times as bright as the Moon, or about 1.7 times as bright per unit area.

The light given out to us by the Moon varies very rapidly with the phase. Thus the light of the Moon when it is full is about eight times as bright as when it is on the quarter. The full moon is about 60,000 times as bright as Vega, the brightest star in the northern heavens. The Sun is about 400,000 times as bright as the full moon, and is therefore about 24,000,000,000 times as bright as Vega. Astronomically, these facts are expressed by saying that Vega is of the 0.0 magnitude, the full moon is of the -12.0 magnitude, and the Sun is of the -26.0 magnitude.

As the Moon moves at different speeds in different portions of its orbit, according to its distance from the Earth, while its rotation on its axis is uniform, the result is that we sometimes see a little farther around one edge of the Moon and sometimes around the other. This is called the Moon's libration—i. e., balancing—in longitude. As the axis of the Moon is somewhat inclined to the plane of its orbit, it turns sometimes one pole toward the Earth and sometimes the other, for the same reason that the Earth turns its poles alternately toward the Sun. This is called the Moon's libration in latitude. These librations each amount to about seven degrees, and as a result we do see a little of the other side of the Moon—in fact, about nine per cent.; there therefore remains only forty-one per cent, of which we never see anything at all. The librations of the Moon are quite readily detected with the naked eye if we are used to the configurations of its surface. They may be seen on the photographs if we compare Plate 16D with 16E and Plate 10B with 10C.

Although as compared with most of the celestial motions the speed of the Moon in its orbit about the Earth is extremely slow, yet as compared with our terrestrial standards it is quite rapid. It moves with a mean velocity of 3,350 feet per second, or a little faster than the highest speed yet given to a cannon-ball. This velocity is most readily observed at the time of a total solar eclipse, when the observer, if located upon a hill or mountain, may watch the shadow of the Moon as it sweeps over him across the