What is "tidal locking"

n. (context astronomy English) The locking of the rotation of a satellite (moon or planet) to its orbit, such that one side always faces the body around which it is orbiting.

Tidal locking (also called gravitational locking or captured rotation) occurs when, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. This state can result from the gravitational gradient (tidal force) between two co-orbiting bodies, acting over a sufficiently long period of time. In the case where the orbital eccentricity is exactly 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.