The B1950.0 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 This standard was superceded by the suceeding epoch, which is currently in standard use, the J2000.0 epoch.

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

Reference

• S. Aoki, et al., "Conversion matrix of epoch B 1950.0 FK 4-based positions of stars to epoch J 2000.0 positions in accordance with the new IAU resolutions", Astron. Astrophys. 128(2), 263–267, (1983). (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1983A%26A...128..263A)
• E. M. Standish, "Conversion of positions and proper motions from B1950.0 to the IAU system at J2000.0", Astron. Astrophys. 115(1), 20–22, (1982). (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1982A%26A...115...20S)