
1) Celestial mechanic 2) Tidal force
Found on
https://www.crosswordclues.com/clue/tidal-locking

is a net result of continued tidal braking wherein the satellite orbits with the same face always pointed toward its primary. The Moon is tidally locked with the Earth.
Found on
http://en.wikipedia.org/wiki/Glossary_of_astronomy

Tidal locking (also called gravitational locking or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as i...
Found on
http://en.wikipedia.org/wiki/Tidal_locking
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