B1950.0

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 This standard was superseded by the succeeding epoch, which is currently in standard use, the J2000.0 epoch. Nevertheless, since many old star tables are still useful, B1950.0 cannot just be shelved. For comparison to modern J2000.0 positions, the old ones have to be corrected for precession and nutation.

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).
• 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).