A periodic wobble of a celestial body (see diagram). The most familiar librations are those of the Moon as seen from Earth. In libration in longitude, the Moon appears to swing slightly from side to side (east–west) by up to 8° 08′ in each direction. This occurs because the Moon’s speed along its elliptical orbit varies with its distance from Earth, while its axial rotation remains constant. In libration in latitude, the Moon appears to nod from north to south by up to 6° 53′ in each direction. This is because the Moon’s axis of rotation is not perpendicular to its orbital plane. As a result of these two librations, we can see up to 59% of the Moon’s surface. A third libration, diurnal libration, occurs because we view the Moon from different sides of the Earth at moonrise and moonset, so we can again see slightly round the eastern and western limbs, although by only about 1°. Libration occurs when two motions are locked in resonance (a synchronous orbit). For this reason, the planet Mercury also exhibits a libration, as do some planetary satellites.

libration:

(a) Libration in longitude is due to the elliptical shape of the Moon’s orbit. The Moon rotates on its axis at constant rate, but its orbital velocity about the Earth varies, so we can see slightly round its east and west edges during the course of an orbit. (b) Libration in latitude is due to the Moon’s equator being inclined to the plane of its orbit, so we can see slightly over its north and south poles.