J2000

J2000, formally J2000.0, is a standard astronomical epoch used as a reference for celestial coordinates. It specifies the orientation of the celestial equator and the origin of right ascension at 12:00 Terrestrial Time on January 1, 2000. The epoch corresponds to the Julian Date 2451545.0 TT. It was adopted by the International Astronomical Union in 1984 as part of a system relating coordinates to a fixed inertial frame, replacing the older B1950.0 epoch.

Positions of optical objects are usually quoted for a given epoch. Because the Earth's rotation and the precession of the equator change with time, coordinates must be transformed (precessed and nutated) to a common epoch when comparing observations taken at different dates.

J2000.0 is the equinox and equator reference used to define many celestial coordinates. In practice, coordinates labeled J2000.0 refer to the equator and equinox of J2000.0, tied to the IAU-defined reference frame; modern inertial studies commonly use the International Celestial Reference System (ICRS), which is closely aligned with J2000.0 but not identical, requiring small corrections.

Because of its wide use, many star catalogs and orbital data present positions in J2000.0. It remains a common standard for historical data, even as newer epochs and reference frames are used for higher precision.