Editing Celestial equator

{{Short description|Projection of Earth's equator out into space}}

[[File:AxialTiltObliquity.png|right|thumb|300px|The celestial equator is currently inclined by about 23.44° to the [[ecliptic]] plane. The image shows the relations between Earth's [[axial tilt]] (or ''obliquity''), [[rotation|rotation axis]], and [[orbital plane]].]]

The '''celestial equator''' is the [[great circle]] of the imaginary [[celestial sphere]] on the [[coplanarity|same plane]] as the [[equator]] of [[Earth]]. By extension, it is also a [[plane of reference]] in the [[equatorial coordinate system]]. In other words, the celestial equator is an abstract [[graphical projection|projection]] of the terrestrial equator into [[outer space]].<ref>{{Cite web
  | title = Celestial Equator
  | url=http://scienceworld.wolfram.com/astronomy/CelestialEquator.html
  | access-date = 5 August 2011 }}</ref> Due to Earth's [[axial tilt]], the celestial equator is currently inclined by about 23.44° with respect to the [[ecliptic]] (the plane of [[Earth's orbit]]), but has varied from about 22.0° to 24.5° over the past 5&nbsp;million years<ref>
{{cite journal
 |last=Berger |first=A.L.
 |date=1976
 |title=Obliquity and Precession for the Last 5000000 Years
 |journal=[[Astronomy and Astrophysics]]
 |volume=51 |issue= 1|pages=127–135
 |bibcode=1976A&A....51..127B
}}</ref> due to [[Milankovitch cycles]] and [[perturbation (astronomy)|perturbation]] from other planets.

An observer standing on Earth's [[equator]] visualizes the celestial equator as a [[semicircle]] passing through the [[zenith]], the point directly overhead. As the observer moves north (or south), the celestial equator tilts towards the opposite [[horizon]]. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on Earth. At the [[geographical pole|poles]], the celestial equator coincides with the astronomical horizon. At all [[latitude]]s, the celestial equator is a uniform arc or circle because the observer is only finitely far from the plane of the celestial equator, but infinitely far from the celestial equator itself.<ref>{{Cite book
  | last = Millar
  | first = William
  | title = The Amateur Astronomer's Introduction to the Celestial Sphere
  | publisher = [[Cambridge University Press]]
  | date = 2006
  | isbn = 978-0-521-67123-1}}</ref>

[[Astronomical object]]s near the celestial equator appear above the horizon from most places on earth, but they [[culmination|culminate]] (reach the meridian) highest near the equator. The celestial equator currently passes through these [[constellation]]s:<ref>{{cite web|url=https://in-the-sky.org/data/constellations_map.php|title=Map of the Constellations|last=Ford|first=Dominic|website=in-the-sky.org|access-date=1 Feb 2021}}</ref>

{| width="60%"
|-valign="top"
| width="20%" |
*[[Pisces (constellation)|Pisces]] (contains the [[first point of Aries]] above its southern border)
*[[Cetus]]
*[[Taurus (constellation)|Taurus]]
*[[Eridanus (constellation)|Eridanus]]
*[[Orion (constellation)|Orion]] (near [[Orion's belt]])
| width="20%" |
*[[Monoceros (constellation)|Monoceros]]
*[[Canis Minor]]
*[[Hydra (constellation)|Hydra]]
*[[Sextans]]
*[[Leo (constellation)|Leo]]
| width="20%" |
*[[Virgo (constellation)|Virgo]] (contains the first point of Libra)
*[[Serpens]] (caput)
*[[Ophiuchus]]
*[[Serpens]] (cauda)
*[[Aquila (constellation)|Aquila]]
*[[Aquarius (constellation)|Aquarius]]
|}

These are the most globally visible constellations.

Over thousands of years, the orientation of Earth's equator and thus the constellations the celestial equator passes through will change due to [[axial precession]].

Celestial bodies other than Earth also have similarly defined celestial equators.<ref>{{cite journal|vauthors=Tarasashvili MV, ((Sabashvili ShA)), Tsereteli SL, Aleksidze NG|date=26 Mar 2013|title=New model of Mars surface irradiation for the climate simulation chamber 'Artificial Mars'|url=https://www.researchgate.net/publication/258789199|journal=International Journal of Astrobiology|volume=12|issue=2|pages=161–170|doi=10.1017/S1473550413000062|bibcode=2013IJAsB..12..161T|s2cid=120041831 }}</ref><ref>{{cite web|url=https://www.dlr.de/media/en/desktopdefault.aspx/tabid-4986/8423_page-7//8423_read-12880|title=Equal length of day and night on Saturn: the start of spring in the northern hemisphere|publisher=[[German Aerospace Center]]|access-date=1 Feb 2021|archive-date=28 June 2021|archive-url=https://web.archive.org/web/20210628090055/https://www.dlr.de/media/en/desktopdefault.aspx/tabid-4986/8423_page-7//8423_read-12880|url-status=dead}}</ref>


==See also==
* [[Axial precession]]
* [[Celestial pole]]
* [[Declination]]
* [[Rotation around a fixed axis]] (pole)

==References==
{{reflist}}

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{{DEFAULTSORT:Celestial Equator}}

[[Category:Astronomical coordinate systems|Equator]]
[[Category:Dynamics of the Solar System]]
[[Category:Technical factors of astrology]]
[[Category:Circles]]
[[Category:Planes (geometry)]]