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The date and time at which an astronomical observation was made, or the date for which positions of celestial objects and orbital elements are calculated. Because of precession and nutation, celestial coordinates change with time, so that positions of celestial objects must be referred to a given date. The standard epoch now commonly used in ephemerides and star catalogues is 2000 January 1, 12 h (written as 2000.0).

Subjects: Astronomy and Astrophysics.

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