Equinox (celestial coordinates): Difference between revisions

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(this paragraph is unclear, and in order to check its meaning a citation is needed)
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 equinox, it is a place in space and not a moment in time.<ref>{{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>
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}}
Equinox is often confused with [[epoch (astronomy)|epoch]], with the difference between the two being that the equinox addresses changes in the coordinate system, while the epoch addresses changes in the position of the celestial body itself. The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.<ref name="Equinoccicc">{{cite book |url=https://books.google.com/books?id=WDjJIww337EC&pg=PA20&lpg=PA20&dq=julian+epoch+equinox&source=bl&ots=p8s-ilXgiV&sig=Y7PYY-JtJ0537ELO8BLJ7nNKjHk&hl=ca&ei=Cv1aSt3LH4_QjAex4bQb&sa=X&oi=book_result&ct=result&resnum=3 |title=Astronomy on the Personal Computer, p. 20 |accessdate=July 13, 2007 |publisher=[[Google books]]}}</ref>