In astronomy, an epoch is a moment in time used as a reference point for data that specify the measure, often over an extended period of time, of some time-varying astronomical quantity, for example celestial coordinates, or elliptical orbital elements of a celestial body, where these are (as usual) subject to perturbations and vary with time. The time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, or of the node of its orbit relative to a reference-plane, or of the directon of the apogee or aphelion of its orbit, or the size of the major axis of its orbit. The main uses of astronomical quantities specified in this way include their use to calculate other parameters of relevant motions, e.g. in order to predict future positions and velocities.
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