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Julian Date

(JD, Julian Day)
(time in days since Noon UT November 24 4714 BC Gregorian)

The Julian Date (Julian Day, JD) is the number of days since a chosen fixed time, specifically Noon, UT, November 24, 4714 BC Gregorian Calendar, (Noon January 1, 4713 BC Julian Calendar). Variants:

  • Modified Julian Day (MJD) is JD minus 2400000.5, for convenience in writing a smaller number.
  • Heliocentric Julian Day (HJD) is JD corrected to the position of the Sun. In astronomy, when placing the time of an observation, it is the time that the observation would reach the Sun.
  • Barycentric Julian Day (BJD) is similar but for the center of mass of the Solar System. It varies from HJD by up to 4 seconds. It has become standard for astronomical observations.

MHJD can mean "Modified HJD" as above, or can be used for a version adjusted by some other constant. Analogously for MBJD.

A Julian Year is somewhat distinct from Julian dates and the Julian calendar: it is exactly 365.25 days if 86400 SI seconds. It is used in astronomy as a measure of time.


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