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Tidal locking (or captured rotation) is when one side of an astronomical body always face another. It is also called synchronous rotation. The classic example is the Moon: the same side of the Moon always faces the Earth.
A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This makes one hemisphere constantly face the partner body. Usually, at any given time only the satellite is tidally locked around the larger body, but if the difference in mass between the two bodies and their physical separation is small, each may be tidally locked to the other, as is the case between Pluto and Charon.
If the Moon were not spinning at all, it would alternately show its near and far sides to Earth, while moving around Earth in orbit.
It is possible to work out how long it takes for a particular case of tidal locking to occur.
List of known tidally locked bodies
Locked to the Earth
Locked to Mars
Locked to Jupiter
Locked to Saturn
Locked to Uranus
Locked to Neptune
Locked to Pluto
- Charon (Pluto is itself locked to Charon)
Libration is an oscillating motion of orbiting bodies relative to each other. Examples include the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.
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 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.