Use of the "mean" locations means that nutation is averaged out or omitted. 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 pertain to the equinox. The "J" in the prefix indicates that it is a Julian equinox or epoch rather than a Besselian equinox or epoch. Equinox of Date There is a special meaning of the expression "equinox (and ecliptic/equator) of date". This reference frame is defined by the positions of the ecliptic and the celestial equator as of the date/epoch on which the position of something else (typically a solar system object) is being specified. Other equinoxes and their corresponding epochs Other equinoxes and epochs that have been used include: The Bonner Durchmusterung started by Friedrich Wilhelm August Argelander uses B1855.0 The Henry Draper Catalog uses B1900.0 Constellation boundaries were defined in 1930 along lines of right ascension and declination for the B1875.0 epoch. Occasionally, non-standard equinoxes have been used, such as B1925.0 and B1970.0 The Hipparcos Catalog uses the International Celestial Reference System (ICRS) coordinate system (which is essentially equinox J2000.0) but uses an epoch of J1991.25. For objects with a significant proper motion, assuming that the epoch is J2000.0 leads to a large position error. Assuming that the equinox is J1991.25 leads to a large error for nearly all objects. Epochs and equinoxes for orbital elements are usually given in Terrestrial Time, in several different formats, including: Gregorian date with 24-hour time: 2000 January 1, 12:00 TT Gregorian date with fractional day: 2000 January 1.5 TT Julian day with fractional day: JDT 2451545.0 NASA/NORAD's Two-line elements format with fractional day: 00001.50000000 Sidereal time and the equation of the equinoxes Sidereal time is the hour angle of the equinox. However, there are two types: if the mean equinox is used (that which only includes precession), it is called mean sidereal time; if the true equinox is used (the actual location of the equinox at a given instant), it is called apparent sidereal time. The difference between these two is known as the equation of the equinoxes, and is tabulated in the Astronomical Almanac. A related concept is known as the equation of the origins, which is the arc length between the Celestial Intermediate Origin and the equinox. Alternatively, the equation of