Editing Zodiac (section)

== Precession of the equinoxes ==
{{Further|Axial precession|Epoch (astronomy)|Sidereal and tropical astrology|Astrological age|Ayanamsa}}
[[File:Equinox path.png|thumb|upright=1.36|Path taken by the point of the [[March equinox]] along the ecliptic over the past 6,000 years]]

The zodiac system was developed in [[Neo-Babylonian|Babylonia]], some 2,500 years ago, during the "[[Age of Aries]]".<ref name=Sachs>{{cite journal | last=Sachs | first=A. | author-link=Abraham Sachs | title=A Classification of the Babylonian Astronomical Tablets of the Seleucid Period | journal=Journal of Cuneiform Studies | publisher=University of Chicago Press | volume=2 | issue=4 | year=1948 | jstor=3515929| doi=10.2307/3515929 | pages=271–290| s2cid=164038422 }}</ref> At the time, it is assumed, the precession of the equinoxes was unknown. Contemporary use of the coordinate system is presented with the choice of interpreting the system either as [[sidereal astrology|sidereal]], with the signs fixed to the [[Fixed stars|stellar background]], or as [[tropical astrology|tropical]], with the signs fixed to the point ([[Euclidean vector|vector]] of the Sun) at the [[March equinox]].<ref name="Rochberg"/>

[[Western astrology]] takes the tropical approach, whereas [[Hindu astrology]] takes the sidereal one. This results in the originally unified zodiacal coordinate system drifting apart gradually, with a clockwise (westward) precession of 1.4 degrees per century.

For the tropical zodiac used in Western astronomy and astrology, this means that the tropical ''sign'' of Aries currently lies somewhere within the ''constellation'' Pisces ("[[Age of Pisces]]").

The sidereal coordinate system takes into account the [[ayanamsa]], ''ayan'' meaning "transit" or "movement", and ''amsa'' meaning 'small part', i.e. movement of equinoxes in small parts. It is unclear when Indians became aware of the precession of the equinoxes, but [[Bhāskara II]]'s 12th-century treatise ''[[Siddhanta Shiromani]]'' gives equations for measurement of precession of equinoxes, and says his equations are based on some lost equations of ''[[Suryasiddhanta]]'' plus the equation of Munjaala.{{cn|date=February 2025}}

The discovery of precession is attributed to Hipparchus around 130 BC. Ptolemy quotes from Hipparchus' now-lost work entitled "On the Displacement of the Solstitial and Equinoctial Points" in the seventh book of his 2nd century astronomical text, ''Almagest'', where he describes the phenomenon of precession and estimates its value.<ref name="Springer"/> Ptolemy clarified that the convention of Greek mathematical astronomy was to commence the zodiac from the point of the vernal equinox and to always refer to this point as "the first degree" of Aries.<ref name=Geminos2>{{cite book|last1=Evans|first1=James|last2=Berggren|first2=J. Lennart|title=Geminos's Introduction to the Phenomena|date=2006|publisher=Princeton University Press|isbn=069112339X|page=115}}</ref> This is known as the "tropical zodiac" (from the Greek word trópos, turn)<ref>{{cite web|title=tropo-|url=http://dictionary.reference.com/browse/tropo-|website=Dictionary.com|publisher=Random House, Inc.|access-date=21 May 2015|archive-date=2 March 2015|archive-url=https://web.archive.org/web/20150302034306/http://dictionary.reference.com/browse/tropo-|url-status=live}}</ref> because its starting point revolves through the circle of background constellations over time.

The principle of the vernal point acting as the first degree of the zodiac for Greek astronomers is described in the 1st century BC astronomical text of [[Geminus]] of Rhodes. Geminus explains that Greek astronomers of his era associate the first degrees of the zodiac signs with the two solstices and the two equinoxes, in contrast to the older Chaldean (Babylonian) system, which placed these points within the zodiac signs.<ref name=Geminos2/> This illustrates that Ptolemy merely clarified the convention of Greek astronomers and did not originate the principle of the tropical zodiac, as is sometimes assumed.

Ptolemy demonstrates that the principle of the tropical zodiac was well known to his predecessors within his astrological text, the ''[[Tetrabiblos]]'', where he explains why it would be an error to associate the regularly spaced signs of the seasonally aligned zodiac with the irregular boundaries of the visible constellations:

{{blockquote|The beginnings of the signs, and likewise those of the terms, are to be taken from the equinoctial and tropical points. This rule is not only clearly stated by writers on the subject, but is especially evident by the demonstration constantly afforded, that their natures, influences and familiarities have no other origin than from the tropics and equinoxes, as has been already plainly shown. And, if other beginnings were allowed, it would either be necessary to exclude the natures of the signs from the theory of prognostication, or impossible to avoid error in then retaining and making use of them; as the regularity of their spaces and distances, upon which their influence depends, would then be invaded and broken in upon.<ref name="Ptolemy's Tetrabiblos"/>}}