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Epoch B1950.0 is a standard point in time (epoch) used as a reference in astronomy, e.g., to specify the date and time regarding a cited position of a star that shows proper motion. It was commonly used before the currently preferred standard epoch, epoch J2000.0. Epoch B1950.0 is precisely defined, the exact point falling (according to our generally-used, everyday time standard) somewhat before midnight, December 31, 1949 GMT, specifically, Julian date 2433282.4235. The B indicates that it is a Besselian epoch, based upon a precise definition of the solar year, the Besselian year, which is now considered imperfect. Earlier Besselian epochs used in astronomy include B1900.0, B1925.0, and B1875.0.
This point in time, epoch B1950.0, is used to define the B1950.0 equinox, a basis for celestial coordinate systems commonly used during the mid-20th century. (This is analogous to the more recent use of epoch J2000.0 to define the J2000.0 equinox.)