In astronomy, libration is a perceived oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of trojan asteroids relative to planets. Lunar libration is distinct from the slight changes in the Moon's apparent size viewed from Earth. Although this appearance can also be described as an oscillating motion, it is caused by actual changes in the physical distance of the Moon because of its elliptic orbit around Earth. Lunar libration is caused by three phenomena detailed below.
The Moon keeps one hemisphere of itself facing the Earth, due to tidal locking. Therefore, the first view of the far side of the Moon was not possible until the Soviet probe Luna 3 reached the Moon on October 7th, 1959 and further lunar exploration by the U.S. and the Soviet Union. However, this simple picture is only approximately true: over time, slightly more than half (about 59%) of the Moon's surface is seen from Earth due to libration.
Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.
There are three types of lunar libration:
In 1772 Lagrange's analyses determined that small bodies can stably share the same orbit as a planet if they remain near Lagrange points, which are 60° ahead of or behind the planet in its orbit. Such 'trojan asteroids' have been found co-orbiting with Earth, Jupiter, Mars, and Neptune. Trojan asteroids associated with Earth are difficult to observe in the visible spectrum, as their libration paths are such that they would be visible primarily in the daylight sky. In 2010, however, using infrared observation techniques, the asteroid was found to be a trojan companion of the Earth; it librates around the leading Lagrange point, L4, in a stable orbit.