Editing Equatorial coordinate system (section)

{{short description|Celestial coordinate system used to specify the positions of celestial objects}}
{{Use British English|date=March 2021}}
[[File:Celestial Sphere - Equatorial Coordinate System.png|thumb|330x330px|Model of the equatorial coordinate system. [[Declination]] (vertical arcs, degrees) and [[hour angle]] (horizontal arcs, hours) is shown. For hour angle, [[right ascension]] (horizontal arcs, degrees) can be used as an alternative.]]
The '''equatorial coordinate system''' is a [[celestial coordinate system]] widely used to specify the positions of [[astronomical object|celestial objects]]. It may be implemented in [[spherical coordinate system|spherical]] or [[Cartesian coordinate system|rectangular]] coordinates, both defined by an [[origin (mathematics)|origin]] at the centre of [[Earth]], a [[fundamental plane (spherical coordinates)|fundamental plane]] consisting of the [[projective geometry|projection]] of Earth's [[equator]] onto the [[celestial sphere]] (forming the [[celestial equator]]), a primary direction towards the [[March equinox|March]] [[equinox (celestial coordinates)|equinox]], and a [[right-hand rule|right-handed]] convention.<ref>
{{cite book
 | url = https://archive.org/details/astronomicalalmanac1961
 | author = Nautical Almanac Office, U.S. Naval Observatory
 | author2 = H.M. Nautical Almanac Office
 | author3 = Royal Greenwich Observatory
 | title = Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
 | publisher = H.M. Stationery Office, London (reprint 1974)
 | date = 1961
 | pages = [https://archive.org/details/astronomicalalmanac1961/page/n34 24], 26
}}</ref><ref name="Vallado">
{{cite book
 | title=Fundamentals of Astrodynamics and Applications
 | first=David A.
 | last=Vallado
 | publisher=Microcosm Press, El Segundo, CA
 | date=2001
 | isbn=1-881883-12-4
 | page=157
}}</ref>

The origin at the centre of Earth means the coordinates are ''[[geocentric model|geocentric]]'', that is, as seen from the centre of Earth as if it were [[transparency and translucency|transparent]].<ref>
{{cite book
 | author = U.S. Naval Observatory Nautical Almanac Office
 | author2 = U.K. Hydrographic Office
 | author3 = H.M. Nautical Almanac Office
 | title = The Astronomical Almanac for the Year 2010
 | publisher = U.S. Govt. Printing Office
 | date = 2008
 | page = M2, "apparent place"
 | isbn = 978-0-7077-4082-9
}}</ref> The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's [[equator]] and [[geographic pole|pole]], does not rotate with the Earth, but remains relatively fixed against the background [[fixed stars|stars]]. A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.