Equinox (celestial coordinates)
When the mean equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be represented J2000 coordinates or simply J2000. This is different from the International Celestial Reference System (ICRS): the mean equator and equinox at J2000.0 are seperate from and of lower (high) quality than ICRS, but agree with ICRS to the limited (high) quality of the first thing mentioned. Use of the "mean" locations means that nutation is averaged out or left out. This means that the Earth's rotational North pole does not point quite at the J2000 celestial pole at the epoch J2000.0; the true pole of epoch nutates away from the mean one. The same differences relate to the equinox.