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In [[astronomy]], '''equinox''' is a moment when the [[vernal equinox|vernal]] point, [[celestial equator]], and other such elements are taken to be used in the definition of a [[celestial coordinate system]]. The position at other [[equinox]]es can be computed by taking into account [[precession]], [[nutation]], and [[aberration of light|aberration]], which directly affect e.g. [[right ascension]] and [[declination]].
Contrastingly, ''[[epoch (astronomy)|epoch]]'' is a moment for when a given position of an [[astronomical object]] is valid. The position at other epochs can be computed by taking into account [[proper motion]], [[parallax]], and
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.
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
* The [[Bonner Durchmusterung]] started by [[Friedrich Wilhelm August Argelander]] uses B1855.0
* The [[Henry Draper Catalog]] uses [[#B1900.0|B1900.0]]
* [[Constellation]] boundaries were defined in 1930 along lines of [[right ascension]] and [[declination]] for the B1875.0 epoch.
* Occasionally, non-standard equinoxes have been used, such as B1925.0 and B1970.0
* The [[Hipparcos Catalog]] uses the [[International Celestial Reference System
Epochs and equinoxes for orbital elements are usually given in [[Terrestrial Time]], in several different formats, including:
* [[NASA]]/[[North American Aerospace Defense Command|NORAD]]'s [[Two-line elements]] format with fractional day: 00001.50000000
A Besselian epoch, named after
Besselian epochs are calculated according to:
Since the [[right ascension]] and [[declination]] of stars are constantly changing due to [[precession]], astronomers always specify these with reference to a particular equinox. Historically used Besselian equinoxes include B1875.0, B1900.0, B1925.0 and B1950.0. The official constellation boundaries were defined in 1930 using B1875.0.
A Julian epoch is an epoch that is based on [[Julian year (astronomy)|Julian years]] of exactly 365.25 days. Since 1984, Julian epochs are used in preference to the earlier Besselian epochs.
Since the [[right ascension]] and [[declination]] of stars are constantly changing due to [[precession]], (and, for relatively nearby stars due to [[proper motion]]), astronomers always specify these with reference to a particular epoch. The earlier epoch that was in standard use was the [[#B1950.0|B1950.0]] epoch.
When the ''mean'' equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be denoted J2000 coordinates or simply J2000.
The "J" in the prefix indicates that it is a Julian equinox