Equinox (celestial coordinates): Difference between revisions

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In [[astronomy]], an '''equinox''' is either of two places on the [[celestial sphere]] at which the [[ecliptic]] intersects the [[celestial equator]].<ref name='aa2019'>{{cite book|title=Astronomical Almanac for the Year 2019|year=2018 |publisher=United States Naval Observatory |location=Washington, DC |isbn=978-0-7077-41925 |page=M6 }}</ref><ref>{{cite book |last=Barbieri |first=Cesare|title=Fundamentals of Astronomy|year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=31 }}</ref><ref>{{cite web |url=https://syrte.obspm.fr/iauWGnfa/NFA_Glossary.html |title=IAU Nomenclature for Fundamental Astronomy|date=2007 |website=Paris Observatory|accessdate=December 23, 2018 }}</ref> Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's [[ascending node]] is used as the origin of celestial coordinate systems and referred to simply as ''the'' equinox. In contrast to the common usage of [[spring equinox (disambiguation)|spring]] and [[fall equinox (disambiguation)|fall]], or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.
The equinox moves because of perturbing forces, and therefore in order to define a coordinate system it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the [[epoch (astronomy)|epoch]]. Astronomical objects show real movements such as [[orbit]]al and [[proper motion]]s, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical objects requires both the date of the equinox and of the epoch.<ref>{{cite book |editor-last=Seidelmann |editor-first=P. Kenneh|title=Explanatory Supplement to the Astronomical Almanac|year=1998 |publisher=University Science Books |location=Mill Valley, CA |isbn=978-0-935702-68-2 |page=12 }}</ref>
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