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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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Term and definition status: This term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".