Equinox (celestial coordinates): Difference between revisions

contrast the other meanins of equinox in the lead paragraph even though there is a hatnote (since they are related, not just the same words), discuss the date of an equinox and epoch more clearly
(simplify the hatnote and lead a little)
(contrast the other meanins of equinox in the lead paragraph even though there is a hatnote (since they are related, not just the same words), discuss the date of an equinox and epoch more clearly)
{{Use mdy dates|date=April 2014}}
 
In [[astronomy]], '''equinox''' is either of two places on the [[celestial sphere]] at which the [[ecliptic]] intersects the [[celestial equator]].<ref name='aa2019'>{{cite book |last= |first= |authorlink= |title=Astronomical Almanac for the Year 2019 |url= |accessdate= |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 |authorlink= |title=Fundamentals of Astronomy |url= |accessdate= |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 |last1= |first1= |last2= |first2= |date=2007 |website=Paris Observatory |publisher= |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|spring]] and [[autumn equinox|fall]], or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.
 
InThe contrastequinox tomoves thebecause commonof usageperturbing offorces, [[springtherefore equinox|spring]]in andorder [[autumnto equinox|fall]],define ora vernalcoordinate andsystem autumnal,it equinoxes,is necessary to specify the celestialdate coordinatefor systemwhich the equinox is achosen. direction inThis spacedate rathershould thannot abe momentconfused inwith timethe [[epoch (astronomy)|epoch]]. However,Astronomical theobjects equinoxshow movesreal becausemovements ofsuch perturbingas forces[[orbit|orbital]] and [[proper motion]]s, thereforeand inthe orderepoch todefines definethe adate coordinatefor systemwhich itthe isposition necessaryof toan specifyobject applies. Therefore a complete specification of the timecoordinates for whichan astronomical objects requires both the date of the equinox, calledand anof [[epochthe (astronomy)|epoch]].<ref>{{cite book |editor-last=Seidelmann |editor-first=P. Kenneh |authorlink= |title=Explanatory Supplement to the Astronomical Almanac |url= |accessdate= |year=1998 |publisher=University Science Books |location=Mill Valley, CA |isbn=0-935702-68-7 |page=12 }}</ref>
 
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