Editing Axial precession (section)

===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.