Editing Precession (section)

== Astronomy ==

In astronomy, precession refers to any of several gravity-induced, slow and continuous changes in an astronomical body's rotational axis or orbital path. Precession of the equinoxes, perihelion precession, changes in the [[Axial tilt#Earth|tilt of Earth's axis]] to its orbit, and the [[Orbital eccentricity|eccentricity]] of its orbit over tens of thousands of years are all important parts of the astronomical theory of [[ice age]]s. ''(See [[Milankovitch cycles]].)''

=== Axial precession (precession of the equinoxes) ===
{{Main|Axial precession}}

Axial precession is the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone. In the case of Earth, this type of precession is also known as the ''precession of the equinoxes'', ''lunisolar precession'', or ''precession of the equator''. Earth goes through one such complete precessional cycle in a period of approximately 26,000 years or 1° every 72 years, during which the positions of stars will slowly change in both [[equatorial coordinates]] and [[ecliptic longitude]]. Over this cycle, Earth's north axial pole moves from where it is now, within 1° of [[Polaris]], in a circle around the [[ecliptic pole]], with an angular radius of about 23.5°.

The [[Greek astronomy|ancient Greek astronomer]] [[Hipparchus]] (c. 190–120 BC) is generally accepted to be the earliest known astronomer to recognize and assess the precession of the equinoxes at about 1° per century (which is not far from the actual value for antiquity, 1.38°),<ref>{{cite book |last=Barbieri |first=Cesare |title=Fundamentals of Astronomy |year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=71 }}</ref> although there is some minor dispute about whether he was.<ref>{{cite book |last = Swerdlow |first = Noel |title = On the cosmical mysteries of Mithras |publisher = Classical Philology, 86, (1991), 48–63 |date = 1991 |page = 59}}</ref> In [[ancient China]], the [[Jin dynasty (265–420)|Jin-dynasty]] scholar-official [[Yu Xi]] ({{fl.}} 307–345&nbsp;AD) made a similar discovery centuries later, noting that the position of the Sun during the [[winter solstice]] had drifted roughly one degree over the course of fifty years relative to the position of the stars.<ref>Sun, Kwok. (2017). ''Our Place in the Universe: Understanding Fundamental Astronomy from Ancient Discoveries'', second edition. Cham, Switzerland: Springer. {{ISBN|978-3-319-54171-6}}, p. 120; see also Needham, Joseph; Wang, Ling. (1995) [1959]. ''Science and Civilization in China: Mathematics and the Sciences of the Heavens and the Earth'', vol. 3, reprint edition. Cambridge: Cambridge University Press. {{ISBN|0-521-05801-5}}, p. 220.</ref> The precession of Earth's axis was later explained by [[classical mechanics|Newtonian physics]]. Being an [[oblate spheroid]], Earth has a non-spherical shape, bulging outward at the equator. The gravitational [[tidal force]]s of the [[Moon]] and [[Sun]] apply torque to the equator, attempting to pull the [[equatorial bulge]] into the plane of the [[ecliptic]], but instead causing it to precess. The torque exerted by the planets, particularly [[Jupiter]], also plays a role.<ref name="Bradt">{{cite book |last = Bradt |first = Hale |title = Astronomy Methods |publisher = [[Cambridge University Press]] |date = 2007 |pages = 66 |isbn = 978-0-521-53551-9}}</ref>

{{multiple image
 |direction = horizontal
 |align= center
 |width1= 158
 |width2= 308
 |width3= 180
 |alt3=Small white disks representing the northern stars on a black background, overlaid by a circle showing the position of the north pole over time 
 |image1=Earth precession.svg
 |image2=Equinox path.png
 |image3=Precession N.gif
 |footer=Precessional movement of the axis (left), precession of the equinox in relation to the distant stars (middle), and the path of the north celestial pole among the stars due to the precession. Vega is the bright star near the bottom (right).
}}

===Apsidal precession===
[[File:Precessing Kepler orbit 280frames e0.6 smaller.gif|thumb|upright=1.25|[[Apsidal precession]]—the orbit rotates gradually over time.]]
{{main|Apsidal precession}}{{See also|Anomalous perihelion precession}}
The [[orbit]]s of planets around the [[Sun]] do not really follow an identical ellipse each time, but actually trace out a flower-petal shape because the major axis of each planet's elliptical orbit also precesses within its orbital plane, partly in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called perihelion precession or [[apsidal precession]].

In the adjunct image, Earth's apsidal precession is illustrated. As the Earth travels around the Sun, its elliptical orbit rotates gradually over time. The eccentricity of its ellipse and the precession rate of its orbit are exaggerated for visualization. Most orbits in the Solar System have a much smaller eccentricity and precess at a much slower rate, making them nearly circular and nearly stationary.

Discrepancies between the observed perihelion precession rate of the planet [[Mercury (planet)|Mercury]] and that predicted by [[classical mechanics]] were prominent among the forms of experimental evidence leading to the acceptance of [[Albert Einstein|Einstein]]'s [[Theory of Relativity]] (in particular, his [[General relativity|General Theory of Relativity]]), which accurately predicted the anomalies.<ref>[[Max Born]] (1924), ''Einstein's Theory of Relativity'' (The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)</ref><ref>{{cite web| url = http://www.dailygalaxy.com/my_weblog/2008/03/18-billion-suns.html| title = An even larger value for a precession has been found, for a black hole in orbit around a much more massive black hole, amounting to 39 degrees each orbit.| date = 18 March 2008| access-date = 2023-11-15| archive-date = 2018-08-07| archive-url = https://web.archive.org/web/20180807131603/http://www.dailygalaxy.com/my_weblog/2008/03/18-billion-suns.html| url-status = bot: unknown}}</ref> Deviating from Newton's law, Einstein's theory of gravitation predicts an extra term of {{math|{{sfrac|''A''|''r''<sup>4</sup>}}}}, which accurately gives the observed excess turning rate of 43 [[arcsecond]]s every 100 years.

===Nodal precession===
{{main|Nodal precession}}
[[Orbital node]]s also [[nodal precession|precess]] over time.

{{for|the precession of the Moon's orbit|lunar precession}}