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Epoch (astronomy)

moment in time used as a reference point for some time-varying astronomical quantity

In astronomy, an epoch (or sometimes epochal moment) is a moment in time for which the position or the orbital elements of a celestial body are specified.

The position at other times can be measured by taking into account proper motion or orbital motion. In the case of orbital elements for a solar system body, these change due to perturbation by other bodies in our solar system.