Equinox (celestial coordinates): Difference between revisions

further clarification
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{{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]]. In contrast to the common usage of "spring and fall equinoxes", it is a place in space and not a moment in time.<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> TheWhen used as an origin for a coordinate system or star catalog, by convention the equinox associated with the sun's [[ascending node]] is used, and is typically simply referred to as ''the'' equinox. In contrast to the common usage of "spring and fall equinoxes", this sense of the term "equinox" is a place in space (on the celestial sphere) and not a moment in time. However the equinox moves because of perturbing forces, and hence is a function of time. In order to define a coordinate system using an equinox, it is necessary to define it for a specific time, called an [[epoch (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>
 
In the case of orbital elements for objects within the [[Solar System]], only a few of the classical orbital elements are affected by a switch of equinox: the [[longitude of the ascending node]] and (to a much lesser extent) the [[orbital inclination|inclination]]. If another set of orbital elements are used, such as the position and velocity vectors for a particular epoch, all components can be affected by a switch of equinox.{{Citation needed|reason=Your explanation here|date=December 2018}}