An obsolete method of specifying the date of astronomical events. It was the standard before 1984, but has since been replaced by the simpler Julian epoch. The prefix ‘B’ is now added to Besselian epoch dates to avoid confusion with the Julian epoch. The Besselian epoch was expressed as the year plus a decimal part, e.g. 1975.2406. The time-scale used was Ephemeris Time, and the unit of time was the Besselian year. The standard epochs of 1900.0 and 1950.0 used in the twentieth century were Besselian epochs, differing by 50 tropical years.