### 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).
It is of note that Julian dates are noon-to-noon rather than
midnight-to-midnight, i.e., date 2451545.0 represents
what we would call noon UT of some day
and 2451544.9583 (i.e., 2451545.0 minus 1/24)
is what we would call 11AM UT of the same day.

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 of 86400 SI seconds. It is
used in astronomy as a measure of time.

(*time,measure*)
http://en.wikipedia.org/wiki/Julian_day

**Referenced by:**

epoch B1950.0

epoch

epoch J2000.0

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