Julian Epoch (J)
Julian Epoch (J) :
A measure of time used in astronomy. The word epoch comes from Greek, and means a fixed or standard instant of time. Other times are stated with reference to this fixed time using years and fractions of years. In 1984, astronomers agreed to fix the standard epoch at 12 hours Universal Time of 2000 January 1 (JD 2 451 545.0). This instant is designated J2000.0. Other times are specified with reference to this time using a year of length 365.25 days, the average length of the year in the Julian calendar. This means that J2001.0, for example, is 18 hours Universal Time of 2001 January 1 (exactly 365.25 days after J2000.0). See Julian year, below
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