Tidal lock

Tidal locking is the process by which two bodies in an orbital relationship will, over time, come to show the same face to one another. In other words, the period of rotation about the axis of the body will come to match the body's orbital period.

Mechanism

Considering two bodies, one larger (A) than the other (B): Body A will exert a gravitational force on B, causing B to be bulged towards body A.

As body B continues to rotate, a different part of the surface will be pulled towards A. In effect, the bulge will "move". However, the material of body B will resist being warped in this way, resulting in a torque that will affect the rotation of B.

This torque acts in such a way that eventually, the rotation of B is such that the same face always points towards A.

This process will also alter the rotation of body A, but if A is much larger, then the process will take a greater amount of time.

Example

One example of tidal locking is the Earth/Moon system. The moon has become tidally locked, and so continually shows the same face to inhabitants of planet Earth.

See also