Equinox (celestial coordinates): Difference between revisions

Gen fixes (page/s, endash, &nbsp, et al., unicodify, concising wikilinks, ref cleanup, etc.) using AWB
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{{Use mdy dates|date=April 2014}}
{{About|the celestial coordinate system|the moment when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment|Equinox}}
{{Use mdy dates|date=April 2014}}
In [[astronomy]], '''equinox''' is a moment in time at which the vernal point, [[celestial equator]], and other such elements are taken to be used in the definition of a [[celestial coordinate system]]. The position at other equinoxes can be computed by taking into account [[precession]], [[nutation]] and [[aberration of light|aberration]], which directly affect e.g. [[right ascension]] and [[declination]].
* [[Constellation]] boundaries were defined in 1930 along lines of [[right ascension]] and [[declination]] for the B1875.0 epoch.
* Occasionally, non-standard equinoxes have been used, such as B1925.0 and B1970.0
* The [[Hipparcos Catalog]] uses the [[International Celestial Reference System|ICRS]] coordinate system (which is essentially equinox J2000.0) but uses an epoch of J1991.25. For objects with a significant [[proper motion]] assuming that the epoch is J2000.0 leads to a large position error. Assuming that the equinox is J1991.25 leads to a large error for nearly all objects.<ref>{{cite journal | url=http://adsabs.harvard.edu/cgi-bin/bib_query?1997A&A...323L..49P | author=Perryman, M.A.C. et al. | title=The Hipparcos Catalogue | journal= Astronomy &amp; Astrophysics | volume=323| pages=L49-L52|year=1997}}</ref>
Epochs and equinoxes for orbital elements are usually given in [[Terrestrial Time]], in several different formats, including: