Equinox (celestial coordinates): Difference between revisions

→‎J2000.0: clean up using AWB
m (→‎J2000.0: Journal cites: fix journal name, using AWB (11952))
m (→‎J2000.0: clean up using AWB)
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|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.<ref name="Equinoj2000">{{cite journal |title=Rotation matrix from the mean dynamical equator and equinox at J2000.0 to the ICRS|doi=10.1051/0004-6361:20031552|authorauthor1=Hilton, J. L. and |author2=Hohenkerk, C. Y.| journal=Astronomy & Astrophysics | volume=413 | pages=765–770 |year=2004 | bibcode=2004A&A...413..765H}}</ref>
The "J" in the prefix indicates that it is a Julian equinox/epoch rather than a Besselian equinox/epoch.