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 Since 1984, Besselian equinoxes and epochs have been superseded by Julian equinoxes and epochs.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.
Read more about this topic: Equinox (celestial Coordinates)
Famous quotes containing the word epochs:
“There are lone figures armed only with ideas, sometimes with just one idea, who blast away whole epochs in which we are enwrapped like mummies. Some are powerful enough to resurrect the dead. Some steal on us unawares and put a spell over us which it takes centuries to throw off. Some put a curse on us, for our stupidity and inertia, and then it seems as if God himself were unable to lift it.”
—Henry Miller (1891–1980)