Equinox (celestial coordinates)
The now used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" shows that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" showing that it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.