J2000.0

In Astronomy, the expression J2000.0 (or J2000 ), indicates the date 2.451 545.0 TT of the Calendrier Julien, that is to say on January 1st 2000 in the Gregorian Calendrier, towards 12:00 UTC. This date is defined like the standard time international since 1984 for the variable data in time. It is in particular the case of the equatorial celestial coordinated whose J2000.0 coordinates are used as reference. When the coordinates of a star are given, it is theoretically necessary to specify the time because of the Précession and the Nutation of the Ground. These two phenomena, which one can calculate perfectly today, change the position of the vernal Point, which is used as point of reference.

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