A method of specifying the date as a year with a decimal part, as for example in 1995.5. The unit is the Julian year of 365.25 days and the time-scale used is Barycentric Dynamical Time (TDB). It is often convenient to use the standard epoch, J2000.0, for example, in star catalogues. The prefix ‘J’ is added to distinguish this epoch from the more complicated Besselian epoch used prior to 1984. The standard epoch is defined as
J2000.0 = 2000 January 1.5 = 2000 January 1 12 h TDB
The date for other epochs can be calculated from this. For example, J1999.0 is exactly 365.25 days earlier, that is, 1999 January 1.25.
Subjects: Astronomy and Astrophysics.