A Besselian year is named after the German mathematician and astronomer Friedrich Bessel (1784–1846)

The beginning of a Besselian year to be the moment at which the mean longitude of the Sun, including the effect of aberration and measured from the mean equinox of the date, is exactly 280 degrees.

This moment falls near the beginning of the corresponding Gregorian year. The definition depended on a particular theory of the orbit of the Earth around the Sun, that of Newcomb (1895), which is now obsolete; for that reason among others, the use of Besselian years has also become or is becoming obsolete.

”Besselian epoch” can be calculated from the Julian date according to the following formula:

$\mathrm{B} = 1900.0 + (\mathrm{J_{date}} − 2415020.31352) / 365.242198781$