Editing Axial precession (section)

{{Short description|Change of rotational axis in an astronomical body}}
{{About|the astronomical concept|precession of the axes outside of astronomy|Precession|non-axial astronomical precession|Precession#Astronomy{{!}}Astronomical precession}}
{{Use dmy dates|date=December 2022}}

[[File:Earth precession.svg|thumb|right|Precessional movement of Earth. Earth rotates (white arrows) once a day around its rotational axis (red); this axis itself rotates slowly (white circle), completing a rotation in approximately 26,000 years<ref name=":crs-esaa-99"/>]]

In [[astronomy]], '''axial precession''' is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's [[Rotation around a fixed axis|rotational axis]]. In the absence of precession, the astronomical body's orbit would show [[axial parallelism]].<ref name="Lerner and Lerner 2003">{{cite book | last1=Lerner | first1=K. Lee | last2=Lerner | first2=Brenda Wilmoth | title=World of earth science | publisher=Thomson-Gale | publication-place=Farmington Hills, MI | date=2003 | isbn=0-7876-9332-4 | oclc=60695883 | page=105 and 454|quote=During revolution about the Sun, the earth's polar axis exhibits parallelism to Polaris (also known as the North Star). Although observing parallelism, the orientation of Earth's polar axis exhibits precession – a circular wobbling exhibited by gyroscopes – that results in a 28,000-year-long precessional cycle. Currently, Earth's polar axis points roughly in the direction of Polaris (the North Star). As a result of precession, over the next 11,000 years, Earth's axis will precess or wobble so that it assumes an orientation toward the star Vega.}}</ref> In particular, axial precession can refer to the gradual shift in the orientation of [[Earth]]'s axis of rotation in a cycle of approximately 26,000 years.<ref name=":crs-esaa-99">Hohenkerk, C.Y., Yallop, B.D., Smith, C.A., & Sinclair, A.T. "Celestial Reference Systems" in Seidelmann, P.K. (ed.) ''Explanatory Supplement to the Astronomical Almanac''. Sausalito: University Science Books. p. 99.</ref> This is similar to the [[precession]] of a spinning top, with the axis tracing out a pair of [[Cone (geometry)|cones]] joined at their [[Apex (geometry)|apices]]. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—[[astronomical nutation|nutation]] and [[polar motion]]—are much smaller in magnitude.

Earth's precession was historically called the '''precession of the equinoxes''', because the [[Equinox (celestial coordinates)|equinoxes]] moved westward along the [[ecliptic]] relative to the [[fixed star]]s, opposite to the yearly motion of the [[Sun]] along the ecliptic. Historically,<ref name="Astro 101">[http://www.wwu.edu/depts/skywise/a101_precession.html Astro 101 – Precession of the Equinox] {{Webarchive|url=https://web.archive.org/web/20090102124621/http://www.wwu.edu/depts/skywise/a101_precession.html |date=2 January 2009 }}, [[Western Washington University]] [[Planetarium]]. Retrieved 30 December 2008</ref> 
the discovery of the precession of the equinoxes is usually attributed in the West to the 2nd-century-BC astronomer [[Hipparchus]]. With improvements in the ability to calculate the gravitational force between planets during the first half of the nineteenth century, it was recognized that the ecliptic itself moved slightly, which was named '''planetary precession''', as early as 1863, while the dominant component was named '''lunisolar precession'''.<ref>Robert Main, [https://archive.org/details/practicalandsph00maingoog/page/n226 <!-- pg=204 --> Practical and Spherical Astronomy] (Cambridge: 1863) pp.203–4.</ref> Their combination was named '''general precession''', instead of precession of the equinoxes.

Lunisolar precession is caused by the gravitational forces of the [[Moon]] and Sun on Earth's [[equatorial bulge]], causing Earth's axis to move with respect to [[inertial space]]. Planetary precession (an advance) is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane (the ecliptic), causing the plane of the ecliptic to shift slightly relative to inertial space. Lunisolar precession is about 500 times greater than planetary precession.<ref name=Williams/> In addition to the Moon and Sun, the other planets also cause a small movement of Earth's axis in inertial space, making the contrast in the terms lunisolar versus planetary misleading, so in 2006 the [[International Astronomical Union]] recommended that the dominant component be renamed the '''precession of the equator''', and the minor component be renamed '''precession of the ecliptic''', but their combination is still named general precession.<ref>{{Cite web |url=http://www.iau.org/static/resolutions/IAU2006_Resol1.pdf |title=IAU 2006 Resolution B1: Adoption of the P03 Precession Theory and Definition of the Ecliptic |access-date=28 February 2009 |archive-url=https://web.archive.org/web/20111021185416/http://iau.org/static/resolutions/IAU2006_Resol1.pdf |archive-date=21 October 2011 |url-status=dead }}</ref> Many references to the old terms exist in publications predating the change.