Synchronous rotation

Synchronous rotation occurs when a body's spin period equals its orbital period around another body. It keeps the same hemisphere of the rotating body facing its orbital partner.

Tidal locking: The process by which tidal forces bring a body's rotation into a stable relationship with its orbit. It is the usual route by which a body's spin becomes synchronous.
Moon: Earth's natural satellite, whose rotation period is nearly equal to its orbital period. Its synchronous rotation keeps its near side generally facing Earth.
Rotation period: The time a body takes to complete one rotation about its axis. It is the spin period compared with the body's orbital period.
Spin-orbit coupling: An interaction linking the rotational and orbital motions of a system. Synchronous rotation is one possible outcome of this broader interaction.
Tidal force: A difference in gravitational pull across an extended body, caused by another body's gravity. Differential gravity raises the distortions that exert torques on a rotating body.
Charon: Pluto's largest moon, which rotates synchronously with its orbit around Pluto. Charon and Pluto are mutually synchronous, each keeping the same hemisphere turned toward the other.
Orbital period: The time a body takes to complete one orbit around another body or shared center of mass. Synchronous rotation is defined by equality with this period.
Mercury's 3:2 spin-orbit resonance: Mercury's rotational state, in which it spins three times for every two orbits around the Sun. It demonstrates that tidal evolution can settle into a resonance other than 1:1 synchrony.
Tidal torque: A torque produced by tidal deformation, which can change a body's rotation or orbit. It slows or speeds rotation until a stable spin-orbit state is reached.
Phobos: The innermost and larger of Mars's two small moons. Its rotation is synchronous with its orbit around Mars.

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