, an epoch
is a moment in time for which celestial coordinates or orbital elements
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 epoch is J2000.0, which corresponds to the situation at (Universal Time) 12:00, January 1, 2000, corresponding to the Julian day 2451545. Moving to a different epoch one must use a year length of exactly 365.25 days.