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 Since 1984, Besselian equinoxes and epochs have been superseded by Julian equinoxes and epochs.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:
“But real action is in silent moments. The epochs of our life are not in the visible facts of our choice of a calling, our marriage, our acquisition of an office, and the like, but in a silent thought by the way-side as we walk; in a thought which revises our entire manner of life, and says,—”Thus hast thou done, but it were better thus.””
—Ralph Waldo Emerson (1803–1882)