Besselian year(be-see -lee-ăn, -sel -ee-) The length of the tropical year in 1900. Before 1976 it was defined as the period of one complete revolution in right ascension of the mean Sun. The beginning of the Besselian year was used as a standard epoch until 1984.
(named for F. Bessel) the tropical year, whose beginning is taken to be the moment of time at which the mean longitude of the sun, decreased by a constant aberration coefficient (20.496”), is exactly equal to 280°. The beginning of the Besselian year occurs at the same moment of time for any point on earth. The duration of the Besselian year is equal to the duration of the tropical year and can be expressed in days by the formula T0 = 365.24219879–0.00000614 T, where T is the number of centuries past the year 1900.