Tidal locking

A rotating astronomical body is tidally locked to a companion when tidal interactions have brought its spin into a stable relationship with its orbit. The common synchronous, or 1:1, state means that the body completes one rotation on its axis during each orbit around the companion. It therefore keeps roughly the same hemisphere turned toward that object. The body has not stopped rotating: its rotation matches its orbital motion.

This distinction explains why the Moon, despite always showing Earth nearly the same side, rotates relative to distant stars. The hemisphere facing away from Earth is commonly called the far side.

Gravity is slightly stronger on the side of a body nearer its companion than on the far side. This difference can stretch the body along the line joining them, creating a tidal deformation. Real bodies do not respond instantly: their material flexes and internal friction dissipates some mechanical energy as heat. The displaced deformation also exerts a torque, gradually changing the body's spin and the system's orbit.

When a body's spin is faster than its orbital motion, these effects can slow its rotation toward synchronous motion. The path and timescale depend on factors including the bodies' masses, separation, internal structure and orbital shape; tidal evolution is not an instantaneous switch. In some systems, the final spin-orbit state can be more complicated than the simple 1:1 example.

The Moon is in synchronous rotation with Earth: one rotation takes the same time as one orbit, so observers on Earth see nearly the same lunar hemisphere over time. NASA describes this as synchronous tidal locking. The Moon's spin was slowed by Earth's tidal forces during the system's evolution.

Tidal evolution also affects Earth's rotation and the Moon's orbit. Energy dissipated by tides raised on Earth slows Earth's spin, while the Moon's orbit gradually expands and the Moon recedes from Earth. These changes are measurable, although very small over a human lifetime.

Tidal locking is common among natural satellites. NASA reports that all of the Solar System's large moons are tidally locked to their planets. It is also relevant to close binary stars and to studies of planets orbiting other stars, though whether a particular object is locked depends on its system and history. The concept helps astronomers interpret rotation, orbital evolution and the conditions on worlds that keep one region oriented toward a nearby companion.