Tidal locking

Tidal locking is a state in which an orbiting body's rotation period matches its orbital period, keeping the same hemisphere facing its companion.

Tidal force: A gravitational force difference across an extended body caused by another body's varying attraction. The companion's uneven pull raises the deformations that exert locking torques.
Moon: Earth's natural satellite, which completes one rotation for each orbit around Earth. Its synchronous rotation is the familiar case of one lunar hemisphere facing Earth.
Pierre-Simon Laplace: A French mathematician and astronomer whose work helped establish celestial mechanics. His analyses of planetary motion helped develop explanations of lunar rotation.
Lunar far side: The hemisphere of the Moon that generally faces away from Earth. Synchronous rotation keeps this hemisphere hidden from direct Earth-based view.
Synchronous rotation: Rotation in which a body's spin period equals its orbital period around another body. This is the precise rotational condition described by tidal locking.
Tidal torque: A twisting effect produced when gravitational forces act on a body's tidal deformation. It changes the body's spin until rotation and orbital motion synchronize.
Charon: Pluto's largest moon, in a mutual synchronous rotation with Pluto. Both bodies keep the same hemispheres facing one another.
George Darwin: A British astronomer who developed a mathematical theory of tides and the evolution of the Earth–Moon system. His tidal-evolution model connected friction, lunar recession, and rotational change.
Terminator: The boundary between the illuminated and dark hemispheres of a celestial body. On a synchronously rotating planet, its day–night boundary can remain nearly fixed relative to the star.
Double synchronous rotation: A state in which two mutually orbiting bodies each rotate once per mutual orbit. Pluto and Charon illustrate locking in both directions, not just one.

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