Tidal lock

Tidal locking (also called gravitational locking or captured rotation) occurs when the long-term interaction between a pair of co-orbiting astronomical bodies drives the rotation rates into a harmonic ratio with the orbital period. This effect arises from the gravitational gradient (tidal force) between the co-orbiting bodies, acting over a sufficiently long period of time. Once tidal locking is achieved for one of the bodies, there is no more net transfer of angular momentum between the two objects, although there can be some back and forth transfer over the course of an orbit. In the special case where the orbital eccentricity is nearly zero, tidal locking results in one hemisphere of the revolving object constantly facing its partner, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth, although there is some libration because the Moon's orbit is not perfectly circular. A tidally locked body in synchronous rotation takes just as long to rotate around its own axis as it does to revolve around its partner.

Usually, only the satellite is tidally locked to the larger body. However, if both the mass difference between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon.

This effect is employed to stabilize some artificial satellites.

The possibility of lifeforms existing on tidally-locked planets has been debated.

One form of hypothetical tidal locked planets are eyeball planets, that in turn are divided into "hot" and "cold" eyeball planets.

The change in rotation rate necessary to tidally lock a body B to a larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces.

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