epochal moment

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 computed 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 on our solar system.Epoch is often confused with equinox. While the former specifies a selected time for the position (or the orbital elements) of the body itself, the latter specifies a selected time for a changing coordinate system (such as precession causing changes in right ascension and declination). When specifying the position or the orbital elements of some celestial body, one can select the same moment of time for both epoch and equinox, but quite frequently different times are selected. For comets or asteroids it is common to select an epoch some 200 days within the present time, but select a standard equinox such as J2000.0.
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