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The J2000.0 epoch is used in astronomy.

It is precisely Julian date 2451545.0 TT, or January 1 2000, 12h TT. This is equivalent to January 1 2000, 11:59:27.816 TAI or January 1 2000, 11:58:55.816 UTC.

Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch. The earlier epoch that was in standard use was the B1950.0 epoch.

The "J" in the prefix indicates that it is a Julian epoch, as opposed to a Besselian epoch.

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