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 motionProper motion is a component of the motion of stars. At first sight the stars seem to be in fixed positions with respect to each other, meaning they always form the same figures, and (for example) Ursa Major looks the same now as forty years ago. More car.
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.
In a calendarCalendars A calendar is a system for assigning calendar dates to days. The dates may be based on the perceived motion of astronomical objects. A calendar is also a physical device (often paper) that illustrates the system (for example, a desktop calendar, an epoch is the year, day, or instant from which its later (and earlier) years are counted.
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