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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 (12h 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.

Calculate Julian Day and day of Week

Day Month Year
Julian Day Day of Week

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

Constructed: November 1998, by: Eran Ofek