A Besselian epoch, named after the German mathematician and astronomer Friedrich Bessel (1784 – 1846), is an epoch that is based on a Besselian year of 365.242198781 days, which is a tropical year measured at the point where the Sun's longitude is exactly 280°.

Since 1984, Besselian epochs have been superseded by Julian epochs. The current standard epoch is J2000.0, which is a Julian epoch.

Besselian epochs are calculated according to:

B = 1900.0 + (Julian date − 2415020.31352) / 365.242198781

The standard epoch that was in use before the current standard epoch (J2000.0) was B1950.0, a Besselian epoch.

Astronomical epochs

Bessel-Epoche