Editing Libration (section)

==Lunar libration==
[[File:Libration longitude 1080p30 1.gif|thumb|Animation showing the changing position of the Moon due to libration, in relation to a fictitious red position on perfectly circular orbit.]]
The [[Moon]] keeps one [[wikt:hemisphere|hemisphere]] of itself facing the [[Earth]] because of [[tidal locking]]. Therefore, the first view of the [[far side of the Moon]] was not possible until the Soviet probe [[Luna 3]] reached the Moon on October 7, 1959, and further [[lunar exploration]] by the [[United States]] and the [[Soviet Union]]. This simple picture is only approximately true since over time, slightly ''more'' than half (about 59% in total) of the Moon's surface is seen from Earth because of libration.<ref>{{cite encyclopedia |encyclopedia=World Book at NASA |first=Paul D. |last=Spudis |url=http://www.nasa.gov/worldbook/moon_worldbook.html |title=Moon |date=2004 |access-date=May 27, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20130703162844/http://www.nasa.gov/worldbook/moon_worldbook.html |archive-date=July 3, 2013 }}</ref>

Lunar libration arises from three changes in perspective because of the non-circular and inclined orbit, the finite size of the Earth, and the orientation of the Moon in space. The first of these is called '''optical libration''', the second '''parallax''', and the third '''physical libration'''. Each of these can be divided into two contributions.

The following are the three types of lunar libration:
* '''Optical libration''', the combined libration of longitudinal and latitudinal libration produces a movement of the sub-Earth point and a wobbling view between the temporarily visible parts of the Moon, during a lunar orbit. This is not to be confused with the change of the Moon's [[apparent size]] because of the changing [[Lunar distance (astronomy)|distance between the Moon and the Earth]] during the [[Orbit of the Moon|Moon's elliptic orbit]], or with the change of [[positional angle]] because of the change in the position of the Moon's tilted axis, or with the observed swinging motion of the Moon because of the relative position of the Earth's tilted axis during an orbit of the Moon.<ref name="SVS 2021">{{cite web | last=SVS | first=NASA's | title=SVS: Moon Phase and Libration, 2020 | website=Home - NASA Scientific Visualization Studio | date=2021-08-18 | url=https://svs.gsfc.nasa.gov/4768 | access-date=2022-06-01}}</ref>
** '''Libration in [[Ecliptic coordinate system#Spherical coordinates|longitude]]''' results from the [[orbital eccentricity|eccentricity]] of the [[orbit of the Moon]] around the Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position. The lunar libration in longitude was discovered by [[Johannes Hevelius]] in 1648.<ref name="Bergeron">{{cite book |editor=Jacqueline Bergeron |title=Highlights of Astronomy: As Presented at the XXIst General Assembly of the IAU, 1991 |publisher=Springer Science & Business Media |year=2013 |page=521 |isbn=978-9401128285}}</ref> It can reach 7°54′ in [[amplitude]].<ref name="Ratkowski">{{cite web |first1=Rob |last1=Ratkowski |first2=Jim |last2=Foster |url=https://epod.usra.edu/blog/2014/05/libration-of-the-moon.html |work=Earth Science Picture of the Day |date=May 31, 2014 |title=Libration of the Moon}}</ref> Longitudinal libration allows an observer on Earth to view at times further into the Moon's west and east respectively at different phases of the Moon's orbit.<ref name="SVS 2021"/>
** '''Libration in [[Ecliptic coordinate system#Spherical coordinates|latitude]]''' results from the Moon's [[axial tilt]] (about 6.7°) between its rotation axis and orbital axis around Earth. This is analogous to how Earth's seasons [[Season#Axial tilt|arise]] from [[Ecliptic#Obliquity of the ecliptic|its axial tilt]] (about 23.4°) between its rotation axis and orbital axis about the Sun. [[Galileo Galilei]] is sometimes credited with the discovery of the lunar libration in latitude in 1632<ref name="Bergeron" /> although [[Thomas Harriot]] or [[William Gilbert (astronomer)|William Gilbert]] might have done so before.<ref>Stephen Pumfrey: ''Harriot's Maps of the Moon: New Interpretations.'' Notes Rec. R. Soc. 63, 2009, [[doi:10.1098/rsnr.2008.0062]].</ref> Note [[Cassini's laws]]. It can reach 6°50′ in amplitude.<ref name="Ratkowski" />  The 6.7° depends on the orbit inclination of 5.15° and the negative equatorial tilt of 1.54°. Latitudinal libration allows an observer on Earth to view beyond the Moon's north pole and south pole at different phases of the Moon's orbit.<ref name="SVS 2021"/>
* '''Parallax libration''' depends on both the longitude and latitude of the location on Earth from which the Moon is observed.
** '''[[Diurnal motion|Diurnal]] libration''' is the small daily libration and oscillation from [[Earth's rotation]], which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—since the observer is on Earth's surface, not at its center. It reaches less than 1° in amplitude.<ref name="Ratkowski" />
*'''Physical libration''' is the oscillation of orientation in space about uniform rotation and precession. There are physical librations about all three axes. The sizes are roughly 100 seconds of arc. As seen from the Earth, this amounts to less than 1 second of arc. '''Forced physical librations''' can be predicted given the orbit and shape of the Moon. The periods of '''free physical librations''' can also be predicted, but their amplitudes and phases cannot be predicted.