105. _The Rotation of the Moon._--A careful examination of the spots on the disc of the moon reveals the fact that she always presents the same side to the earth. In order to do this, she must rotate on her axis while making a revolution around the earth, or in about twenty-seven days. 106. _Librations of the Moon._--The moon appears to rock slowly to and fro, so as to allow us to see alternately a little farther around to the right and the left, or above and below, than we otherwise could. This apparent rocking of the moon is called _libration_. The moon has three librations:-- (1) _Libration in Latitude._--This libration enables us to see alternately a little way around on the northern and southern limbs of the moon. This libration is due to the fact that the axis of the moon is not quite perpendicular to the plane of her orbit. The deviation from the perpendicular is six degrees and a half. As the axis of the moon, like that of the earth, maintains the same direction, the poles of the moon will be turned alternately six degrees and a half toward and from the earth. (2) _Libration in Longitude._--This libration enables us to see alternately a little farther around on the eastern and western limbs of the moon. [Illustration: Fig. 119.] It is due to the fact that the moon's axial motion is uniform, while her orbital motion is not. At perigee her orbital motion will be in advance of her axial motion, while at apogee the axial motion will be in advance of the orbital. In Fig. 119, _E_ represents the earth, _M_ the moon, the large arrow the direction of the moon's motion in her orbit, and the small arrow the direction of her motion of rotation. When the moon is at _M_, the line _AB_, drawn perpendicular to _EM_, represents the circle which divides the visible from the invisible portion of the moon. While the moon is passing from _M_ to _M'_, the moon performs less than a quarter of a rotation, so that _AB_ is no longer perpendicular to _EM'_. An observer on the earth can now see somewhat beyond _A_ on the western limb of the moon, and not quite up to _B_ on the eastern limb. While the moon is passing from _M'_ to _M''_, her axial motion again overtakes her orbital motion, so that the line _AB_ again becomes perpendicular to the line joining the centre of the moon to the centre of the earth. Exactly the same side is now turned towards the earth as when the moon was at _M_. While the moon passes from _M''_ to _M'''_, her axial motion gets in advance of her orbital motion, so that _AB_ is again inclined to the line joining the centres of the earth and moon. A portion of the eastern limb of the moon beyond _B_ is now brought into view to the earth, and a portion of the western limb at _A_ is carried out of view. While the moon is passing from _M'''_ to _M_, the orbital motion again overtakes the