Epoch - Definition and Overview

The word epoch can mean either an interval of time, or a particular point in time used as a reference point.

• In common usage, the term is often used to apply to a period of time when significant related events took place. (see also: period, era)
• In geology, the recent (to the geologist) past is divided into a series of epochs of a few million years each. See geologic timescale.
• In computing, an epoch is a specific date and time used as the reference for all other times. The Unix epoch is an example.
• In telecommunication, an epoch date is a date in history, chosen as the reference date from which time is measured.
• In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. The current standard epoch is J2000.0
  • 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 epoch of a calendar era is the year, day, or instant from which the later (and earlier) years of a calendar are counted.