A quantity introduced by F. W. Bessel in the nineteenth century that has been used into the twentieth century. Bessel introduced a system whereby it would be convenient to identify any instant of time by giving the year and the decimal fraction of the year to a few places, but the starting time of the year was not convenient for dynamical studies that utilize Julian dates (see definition for Julian date), differing by 0. 5 day, and the Besselian year varies slowly. The recent change to Julian year usage in dynamical astronomy (and the J2000. 0 equinox) took effect in solar-system ephemerides of the Minor Planet Center and Central Bureau for Astronomical Telegrams on Jan. 1, 1992. (See Julian year. )