Editing Precession (section)

=== 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
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 |width1= 158
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 |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).
}}