B1900.0

From Freepedia

The B1900.0 epoch is used in astronomy.

Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch. This standard was superseded by the succeeding epoch, B1950.0. The current one in standard use is the J2000.0 epoch.

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