Equinox (celestial coordinates)

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In astronomy, an equinox is either of two places on the celestial sphere at which the ecliptic divides the celestial equator.^[1][2][3] Although there are two such intersections, the equinox connected with the Sun's ascending node is used as the ordinary origin of celestial coordinate systems and referred to simply as "the equinox". In contrast to the common usage of spring/vernal and Autumn/autumnal equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.

In a cycle of about 25,800 years, the equinox moves westward with respect to the celestial sphere because of perturbing forces; 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. Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete detailed description of the coordinates for an astronomical object needs both the date of the equinox and of the epoch.^[4]

The now used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" shows that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" showing that it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.^[5]

1. ↑ Astronomical Almanac for the Year 2019. Washington, DC: United States Naval Observatory. 2018. p. M6. ISBN 978-0-7077-41925.
2. ↑ Barbieri, Cesare (2007). Fundamentals of Astronomy. New York: Taylor and Francis Group. p. 31. ISBN 978-0-7503-0886-1.
3. ↑ "IAU Nomenclature for Fundamental Astronomy". Paris Observatory. 2007. Retrieved December 23, 2018.
4. ↑ Seidelmann, P. Kenneh, ed. (1998). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 12. ISBN 978-0-935702-68-2.
5. ↑ Montenbruck, Oliver; Pfleger, Thomas (2005). Astronomy on the Personal Computer, p. 20 (corrected 3rd printing of 4th ed.). Springer. ISBN 9783540672210. Retrieved January 23, 2019.