Editing Tidal locking (section)

===Eccentric orbits===
{{Quote
|text=A widely spread misapprehension is that a tidally locked body
permanently turns one side to its host.
|author=Heller et al. (2011)<ref name=Heller_Leconte_Barnes_2011/>
}}

For orbits that do not have an eccentricity close to zero, the [[rotation]] rate tends to become locked with the [[orbital speed]] when the body is at [[periapsis]], which is the point of strongest tidal interaction between the two objects. If the orbiting object has a companion, this third body can cause the rotation rate of the parent object to vary in an oscillatory manner. This interaction can also drive an increase in orbital eccentricity of the orbiting object around the primary &ndash; an effect known as eccentricity pumping.<ref name=Correia2012>{{citation
 | title=Pumping the Eccentricity of Exoplanets by Tidal Effect
 | last1=Correia | first1=Alexandre C. M. | last2=Boué | first2=Gwenaël
 | last3=Laskar | first3=Jacques | postscript=.
 | journal=The Astrophysical Journal Letters
 | volume=744 | issue=2 | id=L23 | pages=5 | date=January 2012
 | doi=10.1088/2041-8205/744/2/L23 | bibcode=2012ApJ...744L..23C | arxiv=1111.5486| s2cid=118695308 }}</ref>

In some cases where the orbit is [[eccentricity (orbit)|eccentric]] and the tidal effect is relatively weak, the smaller body may end up in a so-called '''spin–orbit resonance''', rather than being tidally locked. Here, the ratio of the rotation period of a body to its own orbital period is some simple fraction different from 1:1. A well known case is the rotation of [[Mercury (planet)|Mercury]], which is locked to its own orbit around the Sun in a 3:2 resonance.<ref name=Clouse_et_al_2022>{{citation |title=Spin-orbit gravitational locking-an effective potential approach |display-authors=1 |last1=Clouse |first1=Christopher |last2=Ferroglia |first2=Andrea |last3=Fiolhais |first3=Miguel C. N. |journal=European Journal of Physics |postscript= |volume=43 |issue=3 |id=035602 |pages=13 |date=May 2022 |doi=10.1088/1361-6404/ac5638 |arxiv=2203.09297 |bibcode=2022EJPh...43c5602C
|s2cid=246962304 }}</ref> This results in the rotation speed roughly matching the orbital speed around perihelion.<ref>{{citation |title=Rotational Period of the Planet Mercury |last=Colombo |first=G. |journal=Nature |volume=208 |issue=5010 |page=575 |date=November 1965 |doi=10.1038/208575a0 |bibcode=1965Natur.208..575C
|s2cid=4213296 |doi-access=free }}</ref>

Many [[exoplanet]]s (especially the close-in ones) are expected to be in spin–orbit resonances higher than 1:1. A Mercury-like terrestrial planet can, for example, become captured in a 3:2, 2:1, or 5:2 spin–orbit resonance, with the probability of each being dependent on the orbital eccentricity.<ref name=Makarov2012>{{citation |title=Conditions of Passage and Entrapment of Terrestrial Planets in Spin–orbit Resonances |last1=Makarov |first1=Valeri V. |journal=The Astrophysical Journal |volume=752 |issue=1 |id=73 |pages=8 |date=June 2012 |doi=10.1088/0004-637X/752/1/73 |bibcode=2012ApJ...752...73M  |arxiv=1110.2658 |s2cid=119227632 |postscript= }}</ref>