the beginning (the start of day 1 of year 1) of a calendar. This
need not be the first day on which the calendar is used. For example,
the epoch of the Gregorian calendar lies over 1500 years before the
first day on which it was used.
the time for which a set of orbital elements is valid.
Predictions of positions based on those orbital elements will become
less accurate the further from the epoch the time is for which the
prediction is made.