Epoch (astronomy) - Besselian Years

Besselian Years

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

Lieske says that a "Besselian epoch" can be calculated from the Julian date according to

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

This relationship is included in the SOFA software library.

Lieske's definition is not exactly consistent with the earlier definition in terms of the mean longitude of the Sun. When using Besselian years, specify which definition is being used.

To distinguish between calendar years and Besselian years, it became customary to add ".0" to the Besselian years. Since the switch to Julian years in the mid-1980s, it has become customary to prefix "B" to Besselian years. So, "1950" is the calendar year 1950, and "1950.0" = "B1950.0" is the beginning of Besselian year 1950.

• The IAU constellation boundaries are defined in the equatorial coordinate system relative to the equinox of B1875.0.
• The Henry Draper Catalog uses the equinox B1900.0.
• The classical star atlas Tabulae Caelestes used B1925.0 as its equinox.

According to Meeus, and also according to the formula given above,

• B1900.0 = JDE 2415020.3135 = 1900 January 0.8135 TT
• B1950.0 = JDE 2433282.4235 = 1950 January 0.9235 TT