One of the most remarkable features of the Moon's motions is that our satellite rotates on its axis in exactly the same period of time occupied by its revolution around the Earth, from which it results that the Moon always presents to us the same face. To explain this peculiarity, astronomers have supposed that the figure of our satellite is not perfectly spherical, but elongated, so that the attraction of the Earth, acting more powerfully upon its nearest portions, always keeps them turned toward us, as if the Moon were united to our globe by a string. It is not exactly true, however, that the Moon always presents its same side to us, although its period of rotation exactly equals that of its revolution; since in consequence of the inclination of its axis of rotation to its orbit, combined with the irregularities of its orbital motion about us, apparent oscillations in latitude and in longitude, called librations, are created, from which it results that nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at one time or another. The Moon is a familiar object, and every one is aware that our satellite, especially when it is fully illuminated, presents a variety of bright and dark markings, which, from their distant resemblance to a human face, are popularly known as "the man in the moon." A day or two after New Moon, when the thin crescent of our satellite is visible above the western horizon after sunset, the dark portion of its disk is plainly visible, and appears of a pale, ashy gray color, although not directly illuminated by the Sun. This phenomenon is due to the Earth-shine, or to that portion of solar light which the illuminated surface of our globe reflects to the dark side of the Moon, exactly in the same manner that the Moon-shine, on our Earth, is due to the solar light reflected to our globe by the illuminated Moon. Seen with a telescope of moderate power, or even with a good opera-glass, the Moon presents a peculiar mottled appearance, and has a strong resemblance to a globe made of plaster of Paris, on the surface of which numerous roundish, saucer-shaped cavities of various sizes are scattered at random. This mottled structure is better seen along the boundary line called the _terminator_, which divides the illuminated from the dark side of the Moon. The line of the terminator always appears jagged, and it is very easy to recognize that this irregularity is due to the uneven and rugged structure of the surface of our satellite.