A concept used in a convention for expressing time in dynamical calculations. The length of the Besselian year was originally defined as the period taken for the Sun’s right ascension to increase by 24 hours, and is almost equal to the length of a tropical year. In 1976, it was redefined as the length of the tropical year in 1900. The Besselian year is defined to commence at the instant when the Sun’s mean longitude is 280°. In practice, this falls close to the beginning of the calendar year. The simpler Julian year system is now usually preferred.