Editing Tidal locking (section)

==Occurrence==

===Moons===
[[File:Synchronous rotation.svg|thumb|Due to tidal locking, the inhabitants of the central body will never be able to see the satellite's green area.]]
All twenty known moons in the [[Solar System]] that are [[List of gravitationally rounded objects of the Solar System|large enough to be round]] are tidally locked with their primaries, because they orbit very closely and tidal force increases rapidly (as a [[cubic function]]) with decreasing distance.<ref>{{cite book|last1=Schutz|first1=Bernard|title=Gravity from the Ground Up|publisher=Cambridge University Press|isbn=9780521455060|page=43|url=https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|access-date=24 April 2017|date=2003-12-04|archive-date=2023-08-06|archive-url=https://web.archive.org/web/20230806164032/https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|url-status=live}}</ref> On the other hand, most of the [[irregular satellite|irregular outer satellites]] of the [[giant planet]]s (e.g. [[Phoebe (moon)|Phoebe]]), which orbit much farther away than the large well-known moons, are not tidally locked.{{cn|date=May 2024}}

[[Pluto]] and [[Charon (moon)|Charon]] are an extreme example of a tidal lock. Charon is a relatively large moon in comparison to its primary and also has a very close [[orbit]]. This results in Pluto and Charon being mutually tidally locked. Pluto's other moons are not tidally locked; [[Styx (moon)|Styx]], [[Nix (moon)|Nix]], [[Kerberos (moon)|Kerberos]], and [[Hydra (moon)|Hydra]] all rotate [[chaos theory|chaotically]] due to the influence of Charon.<ref>{{cite journal | title=Resonant interactions and chaotic rotation of Pluto's small moons | last1=Showalter | first1=M. R. | last2=Hamilton | first2=D. P. | journal=Nature | date=June 2015 | volume=522 | issue=7554 | pages=45–49 | doi=10.1038/nature14469 | pmid=26040889 | bibcode=2015Natur.522...45S | s2cid=205243819 | url=https://esahubble.org/static/archives/releases/science_papers/heic1512a.pdf | access-date=2022-03-25 | archive-date=2022-06-08 | archive-url=https://web.archive.org/web/20220608040417/https://esahubble.org/static/archives/releases/science_papers/heic1512a.pdf | url-status=live }}</ref> Similarly, {{dp|Eris}} and [[Dysnomia (moon)|Dysnomia]] are mutually tidally locked.<ref name=Szakats2022/> {{dp|Orcus}} and [[Vanth (moon)|Vanth]] might also be mutually tidally locked, but the data is not conclusive.<ref name="Ortiz2011">{{Cite journal | last1 = Ortiz | first1 = J. L. | last2 = Cikota | first2 = A. | last3 = Cikota | first3 = S. | last4 = Hestroffer | first4 = D. | last5 = Thirouin | first5 = A. | last6 = Morales | first6 = N. | last7 = Duffard | first7 = R. | last8 = Gil-Hutton | first8 = R. | last9 = Santos-Sanz | first9 = P. | last10 = De La Cueva | first10 = I. | title = A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus | doi = 10.1051/0004-6361/201015309 | journal = Astronomy & Astrophysics | volume = 525 | pages = A31 | date = 2010 |bibcode = 2011A&A...525A..31O |arxiv = 1010.6187 | s2cid = 56051949 }}</ref>

The tidal locking situation for [[asteroid moon]]s is largely unknown, but closely orbiting binaries are expected to be tidally locked,{{cn|date=March 2022|reason=Not necessarily true given the YORP effect}} as well as [[Contact binary (asteroid)|contact binaries]].

====Earth's Moon====
[[File:Lunation animation April 2007.gif|thumb|This simulation shows the variability in the portion of the Moon visible from Earth due to libration over the course of an orbit. Lighting phases from the Sun are not included.]]
Earth's Moon's rotation and orbital periods are tidally locked with each other, so no matter when the Moon is observed from Earth, the same hemisphere of the Moon is always seen. Most of the [[Far side (Moon)|far side of the Moon]] was not seen until 1959, when photographs of most of the far side were transmitted from the [[Soviet Union|Soviet]] spacecraft ''[[Luna 3]]''.<ref>{{cite web|title=Oct. 7, 1959 – Our First Look at the Far Side of the Moon|url=http://www.universetoday.com/105326/oct-7-1959-our-first-look-at-the-far-side-of-the-moon/|website=Universe Today|date=2013-10-07|access-date=2015-02-15|archive-date=2022-08-12|archive-url=https://web.archive.org/web/20220812035122/https://www.universetoday.com/105326/oct-7-1959-our-first-look-at-the-far-side-of-the-moon/|url-status=live}}</ref>

When Earth is observed from the Moon, Earth does not appear to move across the sky. It remains in the same place while showing nearly all its surface as it rotates on its axis.<ref>{{Cite web|last=Cain|first=Fraser|date=2016-04-11|title=When Will Earth Lock to the Moon?|url=https://www.universetoday.com/128350/will-earth-lock-moon/|access-date=2020-08-03|website=Universe Today|language=en-US|archive-date=2022-05-28|archive-url=https://web.archive.org/web/20220528015905/https://www.universetoday.com/128350/will-earth-lock-moon/|url-status=live}}</ref>

Despite the Moon's rotational and orbital periods being exactly locked, about 59 percent of the Moon's total surface may be seen with repeated observations from Earth, due to the phenomena of [[libration]] and [[parallax]]. Librations are primarily caused by the Moon's varying orbital speed due to the [[eccentricity (orbit)|eccentricity]] of its orbit: this allows up to about 6° more along its perimeter to be seen from Earth. Parallax is a geometric effect: at the surface of Earth observers are offset from the line through the centers of Earth and Moon; this accounts for about a 1° difference in the Moon's surface which can be seen around the sides of the Moon when comparing observations made during moonrise and moonset.<ref>{{cite book
 | title=The Moon and How to Observe It
 | first=Peter
 | last=Grego
 | year=2006
 | pages=47–50
 | isbn=9781846282430
 | publisher=Springer London
 | url=https://books.google.com/books?id=0wh5HABxVksC&pg=PA48
 | access-date=2023-03-19
 | archive-date=2023-10-21
 | archive-url=https://web.archive.org/web/20231021104054/https://books.google.com/books?id=0wh5HABxVksC&pg=PA48
 | url-status=live
 }}</ref>

===Planets===
It was thought for some time that [[Mercury (planet)|Mercury]] was in synchronous rotation with the Sun. This was because whenever Mercury was best placed for observation, the same side faced inward. Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. Modeling has demonstrated that Mercury was captured into the 3:2 spin–orbit state very early in its history, probably within 10–20 million years after its formation.<ref name=Noyelles2012>{{Cite journal | bibcode=2014Icar..241...26N | last1=Noyelles | first1=Benoit | last2=Frouard | first2=Julien | last3=Makarov | first3=Valeri V. | last4=Efroimsky | first4=Michael | name-list-style=amp | title=Spin–orbit evolution of Mercury revisited | journal=Icarus | date=2014 | volume=241 | doi=10.1016/j.icarus.2014.05.045 | pages=26–44 | arxiv = 1307.0136 | s2cid=53690707 }}</ref>

The 583.92-day interval between successive close approaches of [[Venus]] to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.<ref>{{cite journal |last1=Gold |first1=T. |last2=Soter |first2=S. |date=1969 |title=Atmospheric tides and the resonant rotation of Venus |journal=Icarus |volume=11 |issue=3 |pages=356–366|bibcode=1969Icar...11..356G |doi=10.1016/0019-1035(69)90068-2 }}</ref>

The [[exoplanet]] [[Proxima Centauri b]] discovered in 2016 which orbits around [[Proxima Centauri]], is almost certainly tidally locked, expressing either synchronized rotation or a 3:2 spin–orbit resonance like that of Mercury.<ref>{{cite journal|title=Tidal locking of habitable exoplanets|url=https://link.springer.com/article/10.1007/s10569-017-9783-7|publisher=Springer|year=2017|doi=10.1007/s10569-017-9783-7|last1=Barnes|first1=Rory|journal=Celestial Mechanics and Dynamical Astronomy|volume=129|issue=4|pages=509–536|s2cid=119384474|arxiv=1708.02981|bibcode=2017CeMDA.129..509B|access-date=2021-03-29|archive-date=2021-02-26|archive-url=https://web.archive.org/web/20210226135913/https://link.springer.com/article/10.1007/s10569-017-9783-7|url-status=live}}</ref>

One form of hypothetical tidally locked [[exoplanet]]s are [[eyeball planet]]s, which in turn are divided into "hot" and "cold" eyeball planets.<ref>{{cite web|url=http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|title=Forget "Earth-Like"—We'll First Find Aliens on Eyeball Planets|publisher=Nautilus|language=en|author=Sean Raymond|date=20 February 2015|access-date=5 June 2017|archive-date=23 June 2017|archive-url=https://web.archive.org/web/20170623082602/http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|url-status=dead}}</ref><ref name="SA-20200105">{{cite news |last=Starr |first=Michelle |title=Eyeball Planets Might Exist, And They're as Creepy as They Sound |url=https://www.sciencealert.com/eyeball-planets-might-exist-yep-they-re-exactly-as-creepy-as-they-sound |date=5 January 2020 |work=ScienceAlert.com |access-date=6 January 2020 |archive-date=6 January 2020 |archive-url=https://web.archive.org/web/20200106014046/https://www.sciencealert.com/eyeball-planets-might-exist-yep-they-re-exactly-as-creepy-as-they-sound |url-status=live }}</ref>

===Stars===
Close [[binary star]]s throughout the universe are expected to be tidally locked with each other, and [[extrasolar planet]]s that have been found to orbit their primaries extremely closely are also thought to be tidally locked to them. An unusual example, confirmed by [[MOST (satellite)|MOST]], may be [[Tau Boötis]], a star that is probably tidally locked by its planet [[Tau Boötis b]].<ref name="space.com">{{cite web |url=http://www.space.com/scienceastronomy/050523_star_tide.html |title=Role Reversal: Planet Controls a Star |date=2005-05-23 |first=Michael |last=Schirber |publisher=space.com |access-date=2018-04-21 |archive-date=2008-08-04 |archive-url=https://web.archive.org/web/20080804180104/http://www.space.com/scienceastronomy/050523_star_tide.html |url-status=live }}</ref> If so, the tidal locking is almost certainly mutual.<ref>{{cite journal | title=Life on a tidally-locked planet | last=Singal | first=Ashok K. | journal=Planex Newsletter | volume=4 | issue=2 | page=8 | date=May 2014 | bibcode=2014arXiv1405.1025S | arxiv=1405.1025 }}</ref><ref>{{cite journal | title=MOST detects variability on tau Bootis possibly induced by its planetary companion | url=http://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | last1=Walker | first1=G. A. H. | last2=Croll | first2=B. | last3=Matthews | first3=J. M. | last4=Kuschnig | first4=R. | last5=Huber | first5=D. | last6=Weiss | first6=W. W. | last7=Shkolnik | first7=E. | last8=Rucinski | first8=S. M. | last9=Guenther | first9=D. B. | display-authors=1 | year=2008 | journal=Astronomy and Astrophysics | volume=482 | issue=2 | pages=691–697 | doi=10.1051/0004-6361:20078952 | arxiv=0802.2732 | bibcode=2008A&A...482..691W | s2cid=56317105 | access-date=2019-05-16 | archive-date=2021-02-25 | archive-url=https://web.archive.org/web/20210225212508/https://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | url-status=live }}</ref>