Editing First point of Aries

{{Short description|Point on the celestial sphere}}
[[File:Constellations, equirectangular plot, Menzel families.svg|right|300px|thumb|The first point of Aries defines the [[ecliptic coordinate system|ecliptic coordinate]] of (0°, 0°). It is on the [[celestial equator]] at both the left and right extremes of the [[star chart|sky chart]], with the [[ecliptic]] (the orange dotted [[sine wave|sine curve]]) passing through it.]]

The '''first point of Aries''', also known as the '''cusp of Aries''', is the location of the [[March equinox|March]] [[equinox (celestial coordinates)|equinox]] (the [[spring (season)|vernal]] equinox in the northern hemisphere, and the [[autumn]]al equinox in the southern), used as a reference point in [[celestial coordinate system]]s. In diagrams using such coordinate systems, it is often indicated with the symbol ♈︎. Named for the [[constellation]] of [[Aries (constellation)|Aries]], it is one of the two points on the [[celestial sphere]] at which the [[celestial equator]] crosses the [[ecliptic]], the other being the [[first point of Libra]], located exactly 180° from it. Due to [[axial precession|precession of the equinoxes]] since the positions were originally named in antiquity, the [[position of the Sun]] when at the March equinox is now in [[Pisces (constellation)|Pisces]]; when it is at the [[September equinox]], it is in [[Virgo (constellation)|Virgo]] (as of [[epoch (astronomy)|J2000]]).

Along its yearly path through the [[zodiac]], the [[Sun]] meets the celestial equator as it travels from south to north at the first point of Aries, and from north to south at the first point of Libra. The first point of Aries is considered to be the celestial "[[prime meridian]]" from which [[right ascension]] is calculated.

==History==
[[File:Equinozio da Pizzo Vento,tramonto fondachelli fantina, sicilia.JPG|thumb|equinox seen from the site of Pizzo Vento at [[Fondachelli-Fantina]], [[Sicily]]]]

The choice of starting position from which to measure the Sun's motion across the celestial sphere is arbitrary. The equinoxes are preferred as an equinox marks the point in time when the Sun has neither northern nor southern declination but is crossing the celestial equator. Of the two possible equinoxes the ancient Greeks chose the March equinox as the starting point.  This coincided with the festival of [[Hilaria]], a time of optimism and beginnings where farmers began to sow or observed the first growth and blossoming of trees and summer crops.<ref>R Turcan. 1996. The Cults of the Roman Empire. pp.  44-47</ref>  The naming of [[Aries (constellation)|Aries]] is late in the [[Babylonian zodiac]] where the equinox was in its earliest tradition marked as in the early [[Middle Bronze Age]] by actual coincidence with the [[Pleiades]].{{sfn|Rogers, ''Mesopotamian Traditions''|1998}} The time also corresponds to the time of [[castration]] of male calves, mules and donkeys, ''Sanguia'' on the vernal equinox and marked the start of spring proper.{{sfn|Ridpath, ''Star Tales'' Aries: The Ram}}{{sfn|Winterburn|2008|pp=230–231}}

The first point of Aries is so called because, when [[Hipparchus]] defined it in 130 BCE, it was located in the western extreme of the constellation of Aries, near its border with [[Pisces (constellation)|Pisces]] and the star [[Gamma Arietis|γ Arietis]]. Due to the [[Sun]]'s eastward movement across the sky throughout the year, this western end of Aries was the point at which the Sun entered the constellation, hence the name ''first point'' of Aries.{{citation needed|date=January 2025}}

==Definition==
Due to Earth's [[axial precession]], this point gradually moves westwards at a rate of about one degree every 72 years. This means that, since the time of Hipparchus, it has shifted across the sky by about 30°, and is currently located within Pisces, near its border with [[Aquarius (constellation)|Aquarius]]. The Sun now appears in Aries from late April until mid-May, though the constellation is still associated with the beginning of the northern spring.{{sfn|Ridpath, ''Star Tales'' Aries: The Ram}}{{sfn|Winterburn|2008|pp=230–231}}{{sfn|Ridpath|2001|pp=84–85}}

The first point of Aries is important to the fields of astronomy, nautical navigation and astrology. [[Nautical almanac|Navigational ephemeris tables]] record the geographic position of the first point of Aries as the reference for position of [[List of stars for navigation|navigational stars]]. Due to the slow [[Precession (astronomy)|precession of the equinoxes]], the [[zenith]] view (above a location) of constellations at a time of year from a given location have slowly moved west (by using solar [[epoch (astronomy)|epoch]]s the drift is known). The [[tropical Zodiac]] is similarly affected and no longer corresponds with the [[constellation]]s (the Cusp of Libra today is located within [[Virgo (constellation)|Virgo]]). In [[sidereal astrology]], by contrast, the first point of Aries remains aligned with the Aries constellation.{{citation needed|date=January 2025}}

== See also ==
* [[Ras Hammel]] "the head of the ram"

== References ==
{{reflist}}

* {{cite web
  | last = Ridpath
  | first = Ian
  | date = 1988
  | title = Aries: The Ram
  | work = Star Tales
  | url = http://www.ianridpath.com/startales/aries.htm
  | access-date = 24 May 2012
  | ref = {{sfnRef|Ridpath, ''Star Tales'' Aries: The Ram}}
  }}
* {{cite book |last=Ridpath |first=Ian |year=2001 |title=Stars and Planets Guide |publisher=Princeton University Press |isbn=978-0-691-08913-3}}
* {{cite journal
  | last = Rogers
  | first = John H.
  | year = 1998
  | title = Origins of the Ancient Constellations: I. The Mesopotamian Traditions
  | journal = Journal of the British Astronomical Association
  | volume = 108
  | issue = 1
  | pages = 9–28
  | bibcode = 1998JBAA..108....9R
  | ref = {{sfnRef|Rogers, ''Mesopotamian Traditions''|1998}}
  }}
* {{cite book
  | last = Winterburn
  | first = Emily
  | date = 2008
  | title = The Stargazer's Guide: How to Read Our Night Sky
  | publisher = [[HarperCollins]]
  | isbn = 978-0-06-178969-4
  }}

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[[Category:Astronomy]]
[[Category:Astronomical coordinate systems]]
[[Category:Dynamics of the Solar System]]
[[Category:Time in astronomy]]
[[Category:Aries (constellation)]]
[[Category:Technical factors of Western astrology]]
[[Category:Pisces (constellation)]]
[[Category:Spring equinox]]