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## Julian Day

Julian Day (**JD**) is continuous count of
days and fractions,
begined in January 1st, 4713 B.C. (-4712) at
Greenwich mean noon
(12^{h} UT).
The Julian day method was invented by
**Josefh Justus Scaliger** and he
named it "Julian" in honor of his father Julius Scaliger.

This method is commomly used by astronomer in order to avoid the
complexity of the other
calendars.

**Modified Julian Day (MJD)** is JD - 2400000.5

**Julian Ephemeris Day (JDE or JED)** is the JD coresponds to an
instant of time measured in the
Terrestrial Dynamical Time scale.

**Sidereal Julian Day** is
Julian Day in which its days are
sidereal days instead of
mean solar days.

The **Day of Week** could be calculated from the JD
by taking the JD (of midnight) modulus 7 and adding 4 to
the result.

Read more (in Hebrew) about Julian day in the Hebrew Astropedia of the TAU AstroClub.

Constructed: November 1998, by: Eran Ofek