 The complete cycle of the moon’s orbit is called a lunation. Interestingly, several curious things are happening as the moon orbits the earth. It is rotating on its axis at a constant speed (it takes 29.5 days to rotate once; its “day” is 29.5 days). Its orbit is not circular, but rather slightly elliptical and so the moon’s speed as it orbits varies (it orbits more slowly at its farthest point from earth - apogee - and fastest at its nearest approach - perigee). As a result, the moon appears slightly larger during one part of it’s orbit than another. But more interesting, considering the moon’s constant rotation and varying orbital speed, is the result that at a certain point in its orbit the moon is moving too fast for it’s rotation, so some of the moon’s far side comes into view along a limb, and at another point, the moon is moving too slow for its rotation, so some of the moon’s far side comes into view at the moon’s other limb. This is libration in longitude. Another fascinating occurrence through a lunation is that the moon appears to wobble This is libration in latitude, a result of the moon’s axis of rotation being not quite perpendicular to its plane of orbit around the earth.
The above simulation shows the obvious affects of libration in longitude and libration in latitude through a lunation. Antonio Cidadao created a wonderful animated gif showing a lunation where the librations are apparent.
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