Equinox (celestial coordinates): Difference between revisions

another citation on the basic meaning -- IAU
(equation of the equinoxes)
(another citation on the basic meaning -- IAU)
{{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> When 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}}