Calendars and time
Epoch
Direct answer
Epoch: An Epoch is a specified reference instant to which coordinates, orbital elements, time counts, or other changing quantities are tied.
An Epoch is the reference instant for time-dependent astronomical data.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Besselian Epoch, Era, Calendar, Julian Date
An Epoch is the reference instant for time-dependent astronomical data.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Epoch
Catalog makers have long selected standard epochs to compare observations made at different dates.
Why Epoch matters
Positions and orbital elements change, so their numbers are incomplete without the instant at which they apply.
Epoch in practice: an example
A star catalog can give position at epoch J2016.0 plus proper motion for propagation to another date.
Where readings of Epoch go wrong
A common error is to confuse the epoch of observation with the equinox or reference frame of the coordinates.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory Circular 179: The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models - Technical reference for TAI, TT, TCG, TCB, TDB, UT1, sidereal time, epochs, and Earth rotation.
- IAU SOFA: Time Scale and Calendar Tools - IAU-standard algorithms and guidance for civil calendars, Julian dates, and seven recognized astronomical time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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More in Calendars & Timekeeping
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Common questions
What is an Epoch?
An Epoch is a specified reference instant to which coordinates, orbital elements, time counts, or other changing quantities are tied.
What do people get wrong about an Epoch?
A common error is to confuse the epoch of observation with the equinox or reference frame of the coordinates.
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