Astrophysics (Index)About

epoch J2000.0

(J2000.0, J2000)
(noon 1/1/2000 GMT as used as a time reference)

Epoch J2000.0 is a standard point in time used as a reference in astronomy. It is precisely defined, an exact point that is very close to noon, January 1, 2000 GMT, i.e., precisely Julian date 2451545.0 TT. It was adopted to be used as a basis for cited positions of objects, to replace epoch B1950.0, used previously. It can be cited with the position of an astronomical object to signify an object that was in that position at that precise time. This allows identification of an object by position, despite movement such as a star's proper motion. Knowing the movement, a position cited as "epoch J2000.0" can be extrapolated to its current position, and the reverse, and recorded positions at different such epochs allow proper motion to be determined. Citing equatorial coordinates with a J prefix (e.g., "J162702.56+432833.9", which for the purposes of this website, I've termed a J designator) indicates both epoch J2000.0 and J2000.0 equinox, the latter defining the positioning of the coordinate scales.

Further reading:

Referenced by pages:
epoch B1950.0
celestial reference frame
Catalogues of Fundamental Stars
galactic north
International Celestial Reference System (ICRS)
J2000.0 equinox
J designator
Luyten Half-second Catalog (LHS)
Principal Galaxies Catalog (PGC)
terrestrial time (TT)