The J2000.0 epoch is used in astronomy.
It is precisely Julian date 2451545.0 TT, or January 1 2000, 12h TT. This is equivalent 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.
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, but more properly it is the International Celestial Reference Frame or ICRS as referenced below. 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 suffers nutation ("is nutated") away from the mean one. The same differences pertain to the equinox.
The "J" in the prefix indicates that it is a Julian epoch, as opposed to a Besselian epoch.
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