Editing Axial precession (section)

==History==
[[File:Precession table Metius.jpg|thumb|right|"Table indicating the longitude of three stars observed at different times." Compiled by [[Adriaan Metius]], 1624.]]
===Hellenistic world===
====Hipparchus====
The discovery of precession usually is attributed to [[Hipparchus]] (190–120 BC) of [[Rhodes]] or [[İznik|Nicaea]], a [[Greek astronomy|Greek astronomer]]. According to [[Ptolemy]]'s ''[[Almagest]]'', Hipparchus measured the longitude of [[Spica]] and other bright stars. Comparing his measurements with data from his predecessors, [[Timocharis]] (320–260 BC) and [[Aristillus]] (~280 BC), he concluded that Spica had moved 2° relative to the [[September equinox|autumnal equinox]]. He also compared the lengths of the [[tropical year]] (the time it takes the Sun to return to an equinox) and the [[sidereal year]] (the time it takes the Sun to return to a fixed star), and found a slight discrepancy. Hipparchus concluded that the equinoxes were moving ("precessing") through the zodiac, and that the rate of precession was not less than 1° in a century, in other words, completing a full cycle in no more than 36,000 years.<ref name=Ptolemy>{{citation |author=Ptolemy |author-link=Ptolemy |title=Ptolemy's Almagest |translator-last=Toomer |translator-first=G. J. |translator-link=Gerald J. Toomer |year=1998 |orig-year=1984 {{circa|150}} |publisher=Princeton University Press |pages=131–141, 321–340 |isbn=0-691-00260-6}}</ref>

Virtually all of the writings of Hipparchus are lost, including his work on precession. They are mentioned by Ptolemy, who explains precession as the rotation of the [[celestial sphere]] around a motionless Earth. It is reasonable to presume that Hipparchus, similarly to Ptolemy, thought of precession in [[geocentric]] terms as a motion of the heavens, rather than of the Earth.

====Ptolemy====
The first astronomer known to have continued Hipparchus's work on precession is Ptolemy in the second century AD. Ptolemy measured the longitudes of [[Regulus]], [[Spica]], and other bright stars with a variation of Hipparchus's lunar method that did not require eclipses. Before sunset, he measured the longitudinal arc separating the Moon from the Sun. Then, after sunset, he measured the arc from the Moon to the star. He used Hipparchus's model to calculate the Sun's longitude, and made corrections for the Moon's motion and its [[parallax]].<ref>Evans 1998, pp.&nbsp;251–255</ref> Ptolemy compared his own observations with those made by Hipparchus, [[Menelaus of Alexandria]], [[Timocharis]], and [[Agrippa (astronomer)|Agrippa]]. He found that between Hipparchus's time and his own (about 265 years), the stars had moved 2°40', or 1° in 100 years (36" per year; the rate accepted today is about 50" per year or 1° in 72 years). It is possible, however, that Ptolemy simply trusted Hipparchus' figure instead of making his own measurements. He also confirmed that precession affected all fixed stars, not just those near the ecliptic, and his cycle had the same period of 36,000 years as that of Hipparchus.<ref name=Ptolemy/>

====Other authors====
Most ancient authors did not mention precession and, perhaps, did not know of it. For instance, [[Proclus]] rejected precession, while [[Theon of Alexandria]], a commentator on Ptolemy in the fourth century, accepted Ptolemy's explanation. Theon also reports an alternate theory:

:"According to certain opinions ancient astrologers believe that from a certain epoch the solstitial signs have a motion of 8° in the order of the signs, after which they go back the same amount. ..." (Dreyer 1958, p. 204)

Instead of proceeding through the entire sequence of the zodiac, the equinoxes "trepidated" back and forth over an arc of 8°. The theory of [[trepidation]] is presented by Theon as an alternative to precession.

===Alternative discovery theories===
====Babylonians====
Various assertions have been made that other cultures discovered precession independently of Hipparchus. According to [[Al-Battani]], the [[Babylonian astronomy|Chaldean astronomers]] had distinguished the [[tropical year|tropical]] and [[sidereal year]] so that by approximately 330 BC, they would have been in a position to describe precession, if inaccurately, but such claims generally are regarded as unsupported.<ref>{{Cite journal |jstor = 595428|title = The Alleged Babylonian Discovery of the Precession of the Equinoxes|journal = Journal of the American Oriental Society|volume = 70|issue = 1|pages = 1–8|last1 = Neugebauer|first1 = O.|year = 1950|doi = 10.2307/595428}}</ref>

====Maya====
Archaeologist Susan Milbrath has speculated that the [[Mesoamerican Long Count calendar]] of "30,000 years involving the [[Pleiades]]...may have been an effort to calculate the precession of the equinox."<ref>Susan Milbrath, [https://web.archive.org/web/20110726181418/http://www.instituteofmayastudies.org/Milbrath2012.pdf "Just How Precise is Maya Astronomy?"], Institute of Maya Studies newsletter, December 2007.</ref> This view is held by few other professional [[Mayanist|scholars of Maya civilization]].{{citation needed|date=January 2017}}

====Ancient Egyptians====
Similarly, it is claimed the precession of the equinoxes was known in [[Ancient Egypt]], prior to the time of Hipparchus (the [[Ptolemaic Kingdom|Ptolemaic]] period). These claims remain controversial. Ancient Egyptians kept accurate calendars and recorded dates on temple walls, so it would be a simple matter for them to plot the "rough" precession rate. 

The [[Dendera Zodiac]], a star-map inside [[Dendera Temple complex#Hathor temple|the Hathor temple at Dendera]], allegedly records the precession of the equinoxes.<ref>Tompkins, 1971</ref> In any case, if the ancient Egyptians knew of precession, their knowledge is not recorded as such in any of their surviving astronomical texts.

Michael Rice, a popular writer on Ancient Egypt, has written that Ancient Egyptians must have observed the precession,<ref>Rice, Michael. ''Egypt's Legacy'', p.&nbsp;128). "Whether or not the ancients knew of the mechanics of the Precession before its definition by Hipparchos the Bithynian, in the second century BC is uncertain, but as dedicated watchers of the night sky they could not fail to be aware of its effects."</ref> and suggested that this awareness had profound affects on their culture.<ref>Rice, p. 10 "...the Precession is fundamental to an understanding of what powered the development of Egypt"; p. 56 "...in a sense Egypt as a nation-state and the king of Egypt as a living god are the products of the realisation by the Egyptians of the astronomical changes effected by the immense apparent movement of the heavenly bodies which the Precession implies."</ref> Rice noted that Egyptians re-oriented temples in response to precession of associated stars.<ref>Rice, p.&nbsp;170
 "to alter the orientation of a temple when the star on whose position it had originally been set moved its position as a consequence of the Precession, something which seems to have happened several times during the New Kingdom."</ref>

===India===
Before 1200, India had two theories of [[trepidation]], one with a rate and another without a rate, and several related models of precession. Each had minor changes or corrections by various commentators. The dominant of the three was the trepidation described by the most respected Indian astronomical treatise, the ''[[Surya Siddhanta]]'' (3:9–12), composed {{circa|400}} but revised during the next few centuries. It used a sidereal epoch, or [[ayanamsa]], that is still used by all [[Indian national calendar|Indian calendar]]s, varying over the [[ecliptic longitude]] of 19°11′ to 23°51′, depending on the group consulted.<ref name=Reform>{{citation |author=Government of India |title=Report of the Calendar Reform Committee |publisher=Council of Scientific and Industrial Research |year=1955 |page=262 |url=https://dspace.gipe.ac.in/xmlui/bitstream/handle/10973/39692/GIPE-043972.pdf |quote=The longitudes of the first point of Aries, according to the two schools therefore differ by 23°[51]′ (–) 19°11′ ... [Upper limit was increased by 42′ of accumulated precession 1950–2000.]}}</ref> This epoch causes the roughly 30 Indian calendar years to begin 23–28 days after the modern [[March equinox]]. The March equinox of the ''Surya Siddhanta'' librated 27° in both directions from the sidereal epoch. Thus the equinox moved 54° in one direction and then back 54° in the other direction. This cycle took 7200 years to complete at a rate of 54″/year. The equinox coincided with the epoch at the beginning of the ''[[Kali Yuga]]'' in −3101 and again 3,600 years later in 499. The direction changed from prograde to retrograde midway between these years at −1301 when it reached its maximum deviation of 27°, and would have remained retrograde, the same direction as modern precession, for 3600 years until 2299.<ref name=Surya>{{citation |author=Surya |author-link=Surya |title=Translation of Surya Siddhanta: A Textbook of Hindu Astronomy |publisher=University of Calcutta |year=1935 |orig-year=1860 |translator-last=Burgess |translator-first=Ebenezzer |editor-last=Gangooly |editor-first=Phanindralal |url=https://archive.org/details/TranslationOfTheSuryaSiddhanta/page/n169 |pages=114}}</ref><ref name=Pingree>{{citation |last=Pingree |first=David |title=Precession and trepidation in Indian astronomy before A.D. 1200 |journal=Journal for the History of Astronomy |volume=3 |pages=27–35 |year=1972|bibcode=1972JHA.....3...27P |doi=10.1177/002182867200300104 |s2cid=115947431 }}</ref>{{rp|29–30}}

Another trepidation was described by [[Varāhamihira]] ({{circa|550}}). His trepidation consisted of an arc of 46°40′ in one direction and a return to the starting point. Half of this arc, 23°20′, was identified with the Sun's maximum [[declination]] on either side of the equator at the solstices. But no period was specified, thus no annual rate can be ascertained.<ref name=Pingree/>{{rp|27–28}}

Several authors have described precession to be near 200,000{{spaces}}revolutions in a [[Kalpa (aeon)|Kalpa]] of 4,320,000,000{{spaces}}years, which would be a rate of {{sfrac|200,000×360×3600|4,320,000,000}}{{spaces}}= 60″/year. They probably deviated from an even 200,000{{spaces}}revolutions to make the accumulated precession zero near 500. Visnucandra ({{circa|550–600}}) mentions 189,411{{spaces}}revolutions in a Kalpa or 56.8″/year. [[Bhaskara I]] ({{circa|600–680}}) mentions [1]94,110{{spaces}}revolutions in a Kalpa or 58.2″/year. [[Bhāskara II]] ({{circa|1150}}) mentions 199,699{{spaces}}revolutions in a Kalpa or 59.9″/year.<ref name=Pingree/>{{rp|32–33}}

===Chinese astronomy===
[[Yu Xi]] (fourth century AD) was the first [[Chinese astronomy|Chinese astronomer]] to mention precession. He estimated the rate of precession as 1° in 50 years.<ref>Pannekoek 1961, p.&nbsp;92</ref>

===Middle Ages and Renaissance===
In [[Astronomy in medieval Islam|medieval Islamic astronomy]], precession was known based on Ptolemy's ''Almagest'', and by observations that refined the value.

[[Al-Battani]], in his work ''Zij Al-Sabi'', mentions Hipparchus's calculation of precession, and Ptolemy's value of 1 degree per 100 solar years, says that he measured precession and found it to be one degree per 66 solar years.<ref>{{Cite web
 |title=Zij Al-Sabi'
 |author=Al-Battani
 |url=http://shamela.ws/browse.php/book-452#page-132
 |access-date=30 September 2017
 |archive-url=https://web.archive.org/web/20170105192525/http://shamela.ws/browse.php/book-452#page-132
 |archive-date=5 January 2017
 |url-status=dead
 }}</ref>

Subsequently, [[Al-Sufi]], in his ''[[Book of Fixed Stars]]'', mentions the same values  that Ptolemy's value for precession is 1 degree per 100 solar years. He then quotes a different value from ''Zij Al Mumtahan'', which was done during [[Al-Ma'mun]]'s reign, of 1 degree for every 66 solar years. He also quotes the aforementioned ''Zij Al-Sabi'' of Al-Battani as adjusting coordinates for stars by 11 degrees and 10 minutes of arc to account for the difference between Al-Battani's time and Ptolemy's.<ref>
{{Cite web
 |title=Book of Fixed Stars
 |author=Al-Sufi
 |url=https://www.wdl.org/ar/item/18412/view/1/20/
}}</ref>

Later, the ''[[Zij-i Ilkhani]]'', compiled at the [[Maragheh observatory]], sets the precession of the equinoxes at 51 arc seconds per annum, which is very close to the modern value of 50.2 arc seconds.<ref>{{Cite journal |title=The Influence of Islamic Astronomy in Europe and the Far East |last=Rufus |first=W. C. |journal=Popular Astronomy |volume=47 |issue=5 |date=May 1939 |pages=233–238 [236] 
|bibcode = 1939PA.....47..233R}}.</ref>

In the Middle Ages, Islamic and Latin Christian astronomers treated "trepidation" as a motion of the fixed stars to be ''added to'' precession. This theory is commonly attributed to the [[Arab]] astronomer [[Thabit ibn Qurra]], but the attribution has been contested in modern times. [[Nicolaus Copernicus]] published a different account of trepidation in ''[[De revolutionibus orbium coelestium]]'' (1543). This work makes the first definite reference to precession as the result of a motion of the Earth's axis. Copernicus characterized precession as the third motion of the Earth.<ref>{{cite book |last=Gillispie |first=Charles Coulston |author-link=Charles Coulston Gillispie|title=The Edge of Objectivity: An Essay in the History of Scientific Ideas |year=1960 |publisher=Princeton University Press |isbn=0-691-02350-6 |url=https://archive.org/details/edgeofobjectivit00char |page=24}}</ref>

===Modern period===
Over a century later, [[Isaac Newton]] in ''[[Philosophiae Naturalis Principia Mathematica]]'' (1687) explained precession as a consequence of [[gravitation]].<ref>Evans 1998, p.&nbsp;246</ref> However, Newton's original precession equations did not work, and were revised considerably by [[Jean le Rond d'Alembert]] and subsequent scientists.