Glossary term: Tidal locking

Description: Tidal locking is a process where tidal forces alter the rotation of one body orbiting another, changing its orbital period. A good example of this is the Moon. The Moon is subtly stretched by tidal forces from Earth's gravity, changing its shape so it is subtly elongated with the long axis pointing towards the Earth. As it orbits the Earth, the Earth exerts a subtle force on this tidal bulge. If the Moon were to rotate so that its elongated axis no longer faced the Earth, the tidal force would drag the Moon back so that its tidal bulge would again point towards the Earth. This means that the Moon always presents the same face toward the Earth as tidal locking has made its rotation period equal to its orbital period. This one to one relationship between period and orbit is not always true for tidally locked bodies. Mercury's relatively elliptical orbit around the Sun, combined with tidal locking, means that Mercury rotates three times every time it completes two orbits of the Sun.

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