Editing Astronomical coordinate systems (section)

== Coordinate systems ==
The following table lists the common coordinate systems in use by the astronomical community. The [[Fundamental plane (spherical coordinates)|fundamental plane]] divides the [[celestial sphere]] into two equal [[Celestial sphere|hemispheres]] and defines the baseline for the latitudinal coordinates, similar to the [[equator]] in the [[geographic coordinate system]]. The poles are located at ±90° from the fundamental plane. The primary direction is the starting point of the longitudinal coordinates. The origin is the zero distance point, the "center of the celestial sphere", although the definition of [[celestial sphere]] is ambiguous about the definition of its center point.

{| class="wikitable" style="text-align:center;"
|-
! rowspan="2" | Coordinate system<ref>{{cite web 
 |url=http://www.faculty.virginia.edu/ASTR5610/lectures/COORDS/coords.html 
 |title=Coordinate Systems 
 |last=Majewski 
 |first=Steve 
 |publisher=UVa Department of Astronomy 
 |access-date=19 March 2011 
 |archive-date=12 March 2016 
 |archive-url=https://web.archive.org/web/20160312195329/http://faculty.virginia.edu/ASTR5610/lectures/COORDS/coords.html 
 |url-status=dead 
 }}</ref>
! rowspan="2" | Center point<br/>(origin)
! rowspan="2" | Fundamental plane<br/>(0° latitude)
! rowspan="2" | Poles
! colspan="2" | Coordinates
! rowspan="2" | Primary direction<br/>(0° longitude)
|-
! Latitude
! Longitude
|-
| [[Horizontal coordinate system|Horizontal]] (also called {{abbr|alt|altitude}}-{{abbr|az|azimuth}} or {{abbr|el|elevation}}-{{abbr|az|azimuth}})
| Observer
| [[Horizon]]
| [[Zenith]], [[nadir (astronomy)|nadir]]
| Altitude ({{math|''a''}}) or elevation
| [[Azimuth]] ({{math|''A''}})
| [[North]] or [[south]] point of horizon
|-
| [[Equatorial coordinate system|Equatorial]]
| rowspan="2" | Center of the [[Earth]]{{nbsp}}(geocentric), or [[Sun]]{{nbsp}}(heliocentric)
| [[Celestial equator]]
| [[Celestial pole]]s
| [[Declination]] ({{math|''δ''}})
| [[Right ascension]] ({{math|''α''}})<br/>or [[hour angle]] ({{math|''h''}})
| rowspan="2" | [[Equinox (celestial coordinates)|March equinox]]
|-
| [[Ecliptic coordinate system|Ecliptic]]
| [[Ecliptic]]
| [[Ecliptic pole]]s
| [[Ecliptic latitude]] ({{math|''β''}})
| [[Ecliptic longitude]] ({{math|''λ''}})
|-
| [[Galactic coordinate system|Galactic]]
| Center of the [[Sun]]
| [[Galactic plane]]
| [[Galactic pole]]s
| Galactic latitude ({{math|''b''}})
| Galactic longitude ({{math|''l''}})
| [[Galactic Center]]
|-
| [[Supergalactic coordinate system|Supergalactic]]
|
| [[Supergalactic plane]]
| Supergalactic poles
| Supergalactic latitude ({{math|''SGB''}})
| Supergalactic longitude ({{math|''SGL''}})
| Intersection of supergalactic plane and galactic plane
|}

===Horizontal system===
{{Main|Horizontal coordinate system}}
[[File:Equatorial_and_horizontal_celestial_coordinates_E.svg|350px|thumb|Equatorial (red) and horizontal (blue) celestial coordinates]]
The ''horizontal'', or [[Horizontal coordinate system|altitude-azimuth]], system is based on the position of the observer on Earth, which revolves around its own axis once per [[sidereal day]] (23 hours, 56 minutes and 4.091 seconds) in relation to the star background. The positioning of a celestial object by the horizontal system varies with time, but is a useful coordinate system for locating and tracking objects for observers on Earth.  It is based on the position of stars relative to an observer's ideal horizon.

===Equatorial system===
{{Main|Equatorial coordinate system}}
The ''equatorial'' coordinate system is centered at Earth's center, but fixed relative to the celestial poles and the [[Equinox (celestial coordinates)|March equinox]]. The coordinates are based on the location of stars relative to Earth's equator if it were projected out to an infinite distance.  The equatorial describes the sky as seen from the [[Solar System]], and modern star maps almost exclusively use equatorial coordinates.

The ''equatorial'' system is the normal coordinate system for most professional and many amateur astronomers having an equatorial mount that follows the movement of the sky during the night. Celestial objects are found by adjusting the telescope's or other instrument's scales so that they match the equatorial coordinates of the selected object to observe.

Popular choices of pole and equator are the older [[B1950]] and the modern [[J2000]] systems, but a pole and equator "of date" can also be used, meaning one appropriate to the date under consideration, such as when a measurement of the position of a planet or spacecraft is made. There are also subdivisions into "mean of date" coordinates, which average out or ignore [[astronomical nutation|nutation]], and "true of date," which include nutation.

===Ecliptic system===
{{Main|Ecliptic coordinate system}}
The fundamental plane is the plane of the Earth's orbit, called the ecliptic plane. There are two principal variants of the ecliptic coordinate system: geocentric ecliptic coordinates centered on the Earth and heliocentric ecliptic coordinates centered on the center of mass of the Solar System.

The geocentric ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon, and planets.<ref>[[Asger Aaboe|Aaboe, Asger]]. 2001 ''Episodes from the Early History of Astronomy.'' New York: Springer-Verlag., pp. 17–19.</ref> It was used to define the twelve [[astrological sign]]s of the [[zodiac]], for instance.

The heliocentric ecliptic system describes the planets' orbital movement around the Sun, and centers on the [[Center of mass#Barycenter in astrophysics and astronomy|barycenter]] of the Solar System (i.e. very close to the center of the Sun). The system is primarily used for computing the positions of planets and other Solar System bodies, as well as defining their [[orbital elements]].

===Galactic system===
{{Main|Galactic coordinate system}}
The galactic coordinate system uses the approximate plane of the Milky Way Galaxy as its fundamental plane.  The Solar System is still the center of the coordinate system, and the zero point is defined as the direction towards the [[Galactic Center]]. Galactic latitude resembles the elevation above the galactic plane and galactic longitude determines direction relative to the center of the galaxy.

===Supergalactic system===
{{Main|Supergalactic coordinate system}}
The supergalactic coordinate system corresponds to a fundamental plane that contains a higher than average number of local galaxies in the sky as seen from Earth.