Equinox (celestial Coordinates) - Besselian Equinoxes/epochs

Besselian Equinoxes/epochs

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 Since 1984, Besselian equinoxes and epochs have been superseded by Julian equinoxes and epochs. The current standard equinox and 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 previous standard equinox and epoch were B1950.0, a Besselian epoch.

Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular equinox. Historically used Besselian equinoxes include B1875.0, B1900.0, B1925.0 and B1950.0. The official constellation boundaries were defined in 1930 using B1875.0.