In astronomy, equinox is a moment in time at which the 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 equinoxes can be computed by taking into account precession, nutation and aberration, which directly affect e.g. right ascension and declination.
Equinox is often confused with 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 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/epochs were used. Since that time Julian equinoxes/epochs have been used.
Other equinoxes/epochs that have been used include:
Epochs and equinoxes for orbital elements are usually given in Terrestrial Time, in several different formats, including:
A Besselian epoch, named after the German mathematician and astronomer Friedrich Bessel (1784 – 1846), is an epoch that is based on a Besselian year of 365.242198781 days, which is a tropical year measured at the point where the Sun's longitude is exactly 280°. Since 1984, Besselian equinoxes/epochs have been superseded by Julian equinoxes/epochs. The current standard equinox/epoch is J2000.0, which is a Julian equinox/epoch.
Besselian equinoxes/epochs are calculated according to:
The previous standard equinox/epoch was B1950.0, a Besselian equinox/epoch.
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.
Julian equinoxes/epochs are calculated according to:
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 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. Technically, this is different from, but similar to, the International Celestial Reference System (ICRS): the mean equator and equinox at J2000.0 are distinct from and of lower precision than ICRS, but agree with ICRS to the limited precision of the former. Use of the "mean" locations means that nutation is averaged out or omitted. Novices are sometimes confused by finding that the Earth's rotational North pole does not point quite at the J2000 celestial pole at the epoch J2000.0; the reason is that the true pole of epoch nutates away from the mean one. The same differences pertain to the equinox.
The "J" in the prefix indicates that it is a Julian equinox/epoch rather than a Besselian equinox/epoch.