Tidal locking

Tidal locking (also called gravitational locking or captured rotation) occurs when the gravitational gradient makes one hemisphere of a revolving astronomical body constantly face the partner body. This effect is known as synchronous rotation. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth. 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.