A set of five quantities that are tabulated at daily intervals in The Astronomical Almanac. When combined with a set of star constants, they allow the apparent right ascension and declination of a star to be calculated from its mean place. The latter, together with the star constants, can be obtained from a star catalogue. In this way the effects of general precession, nutation, and annual aberration are included in an efficient manner, avoiding more detailed calculation. They are named after the German astronomer F. W. Bessel, who first introduced them. See also Day Number.
Subjects: Astronomy and Astrophysics.