In astronomy, an equinox is either of two places on the celestial sphere at which the ecliptic divides the celestial equator. 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.
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.
Movement of the equinox
The equinox moves, in the sense that as time goes forward it is in a different location with respect to the distant stars. As a result, star catalogs over the years, even over the course of a few decades, will list different ephemerides. This is due to precession and nutation, both of which can be modeled, as well as other minor perturbing (disturbing) forces which can only be decided by observation and are this way tabulated in astronomical almanacs.
Precession
Precession of the equinox was first noted by Hipparchus in 129 BC, when noting the location of Spica with respect to the equinox and comparing it to the location followed by Timocharis in 273 BC. It is a long term movement with a period of 25,800 years.
Nutation
Nutation is the oscillation of the ecliptic plane. It was first followed by James Bradley as a difference in the declination of stars. Bradley published this discovery in 1748. Because he did not have a correct enough clock, Bradley was unaware of the effect of nutation on the movement of the equinox along the celestial equator, although that is in the present day the more significant aspect of nutation. The period of oscillation of the nutation is 18.6 years.
Equinoxes and epochs
Besselian equinoxes and epochs
A Besselian epoch, named after 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 and epochs have been superseded by Julian equinoxes and epochs. The current standard equinox and epoch is J2000.0, which is a Julian epoch.
Besselian epochs are calculated according to:
B = 1900.0 + (Julian date − 2415020.31352) / 365.242198781
The previous standard equinox and epoch were B1950.0, a Besselian 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 borders were defined in 1930 using B1875.0.
Julian equinoxes and epochs
A Julian epoch is an epoch that is based on Julian years of exactly 365.25 days. Since 1984, Julian epochs are used in preference to the earlier Besselian epochs.
Julian epochs are calculated according to:
J = 2000.0 + (Julian date − 2451545.0)/365.25
The standard equinox and epoch now in use are J2000.0, which is equal to January 1, 2000 12:00 Terrestrial Time (TT).
J2000.0
The J2000.0 epoch is exactly Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT. This is equal to January 1, 2000, 11:59:27.816 TAI or January 1, 2000, 11:58:55.816 UTC.
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.