Editing Tidal locking (section)

{{short description|Situation in which an astronomical object's orbital period matches its rotational period}}
[[File:Tidal locking of the Moon with the Earth.gif|thumb|300px|At left, the Moon rotates at the same rate it orbits the Earth, keeping the same face toward the planet. At right, if the Moon did not rotate then the face would change over the course of an orbit. Viewed from above; not to scale.]]
[[File:Pluto-Charon_system-new.gif|thumb|300px|A side view of the Pluto–Charon system. [[Pluto]] and [[Charon (moon)|Charon]] are tidally locked to each other.]]

'''Tidal locking''' between a pair of co-[[orbit]]ing [[astronomical body|astronomical bodies]] occurs when one of the objects reaches a state where there is no longer any net change in its [[rotation rate]] over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the [[Moon]] always faces [[Earth]], although there is some [[libration|variability]] because the Moon's orbit is not perfectly circular. Usually, only the [[natural satellite|satellite]] is tidally locked to the larger body.<ref>{{cite web|title=When Will Earth Lock to the Moon?|url=http://www.universetoday.com/128350/will-earth-lock-moon/|website=Universe Today|date=2016-04-12|access-date=2017-01-02|archive-date=2016-09-23|archive-url=https://web.archive.org/web/20160923082215/http://www.universetoday.com/128350/will-earth-lock-moon/|url-status=live}}</ref> However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for [[Pluto]] and [[Charon (moon)|Charon]], and for [[Eris (dwarf planet)|Eris]] and [[Dysnomia (moon)|Dysnomia]]. Alternative names for the tidal locking process are '''gravitational locking''',<ref name=Clouse_et_al_2022/> '''captured rotation''', and '''spin–orbit locking'''. 

The effect arises between two bodies when their [[gravitational interaction]] slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of [[energy transfer|energy exchange]] and heat [[dissipation]]. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked.<ref name=Barnes_2010>{{cite book |title=Formation and Evolution of Exoplanets |editor1-first=Rory |editor1-last=Barnes |publisher=John Wiley & Sons |year=2010 |isbn=978-3527408962 |page=248 |url=https://books.google.com/books?id=-7KimFtJnIAC&pg=PA248 |access-date=2016-08-16 |archive-date=2023-08-06 |archive-url=https://web.archive.org/web/20230806163538/https://books.google.com/books?id=-7KimFtJnIAC&pg=PA248 |url-status=live }}</ref> The object tends to stay in this state because leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object.

There is ambiguity in the use of the terms 'tidally locked' and 'tidal locking', in that some scientific sources use it to refer exclusively to 1:1 synchronous rotation (e.g. the Moon), while others include non-synchronous orbital resonances in which there is no further transfer of angular momentum over the course of one orbit (e.g. Mercury).<ref name=Heller_Leconte_Barnes_2011>{{cite journal |last1=Heller |first1=R. |last2=Leconte |first2=J. |last3=Barnes |first3=R. |title=Tidal obliquity evolution of potentially habitable planets |journal=Astronomy & Astrophysics |volume=528 |id=A27 |pages=16 |date=April 2011 |doi=10.1051/0004-6361/201015809 |bibcode=2011A&A...528A..27H |arxiv=1101.2156 |s2cid=118784209 }}</ref> In [[Mercury (planet)|Mercury's]] case, the planet completes three rotations for every two revolutions around the Sun, a 3:2 spin–orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.<ref name=Barnes_2010/><ref name=Heller_Leconte_Barnes_2011/><ref>{{cite book |title=Mercury |first1=T. J. |last1=Mahoney |publisher=Springer Science & Business Media |year=2013 |isbn=978-1461479512 |url=https://books.google.com/books?id=iC65BAAAQBAJ&pg=PA44 |access-date=2018-04-20 |archive-date=2023-08-06 |archive-url=https://web.archive.org/web/20230806163607/https://books.google.com/books?id=iC65BAAAQBAJ&pg=PA44 |url-status=live }}</ref>
Regardless of which definition of tidal locking is used, the hemisphere that is visible changes slightly due to [[Libration|variations]] in the locked body's [[Orbital speed|orbital velocity]] and the [[Axial tilt|inclination of its rotation axis]] over time.