Standard time
In Astronomy, the standard time corresponds to the time (i.e. the moment) used to determine the orientation of the axis of the poles Ground stress compared to the celestial Sphère in order to determine the origin of the equatorial Frame of reference, composed of the Right ascension and the variation. Since 1984, the standard time is J2000.0. Previously it was B1950.0.
The need for choosing one standard time in addition to the coordinates of a star comes owing to the fact that the point of reference, which is the vernal Point, changes (certainly very slowly) during time because of the Précession of the Ground around its axis of rotation.
See too
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