Explanation: This slow-loading time-lapse sequence shows what
our Moon looks like during a lunation, a complete lunar cycle. As the Moon orbits the
Earth, the half illuminated by the Sun first becomes increasingly visible, then
decreasingly visible. The Moon always keeps the same face toward the Earth. The Moon's
apparent size changes slightly, though, and a slight wobble called a libration is
discernable as it progresses along its elliptical orbit. During the cycle, sunlight
reflects from the Moon at different angles, and so illuminates different features
differently. A full lunation takes about 29.5 days, just under a month (moon-th).
Libration of the moon, is defined as any one of those small periodical changes in the position of the moon's surface relatively to the earth, in consequence of which narrow portions at opposite limbs become visible or invisible alternately. It receives different names according to the manner in which it takes place; as: (a) Libration in longitude, that which, depending on the place of the moon in its elliptic orbit, causes small portions near the eastern and western borders alternately to appear and disappear each month. (b) Libration in latitude, that which depends on the varying position of the moon's axis in respect to the spectator, causing the alternate appearance and disappearance of either pole. (c) Diurnal or parallactic libration, that which brings into view on the upper limb, at rising and setting, some parts not in the average visible hemisphere.
Credit: António Cidadão, and Astronomy Picture of the Day at: http://antwrp.gsfc.nasa.gov/apod/