In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. In the case of celestial coordinates, the position at other times can be computed by taking into account precession and proper motion. In the case of orbital elements, it is necessary to take account of perturbation by other bodies in order to calculate the orbital elements for a different time.
The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch.
Besselian epochs were used before 1984, however Julian epochs are now used.
Epochs for orbital elements are usually given in Terrestrial Time, in several different formats, including:
Gregorian date with 24-hour time: 2000 Jan. 1, 12:00 TT
- Gregorian date with fractional day: 2000 Jan. 1.5 TT
Julian Day with fractional day: JDT 2451545.0
NASA/NORAD's Two-Line Elements format with fractional day: 00001.50000000