In astronomy, libration is an 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.
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The Moon generally has one hemisphere facing the Earth, due to tidal locking. Therefore, humans' first view of the far side of the Moon resulted from lunar exploration in the 1960s. 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.[1]
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 2011, however, using infrared observation techniques, the asteroid 2010 TK7 was found to be a Trojan companion of the Earth; it librates around the leading Lagrange point, L4, in a stable orbit.[2]