Julian Date Definition

Ask the average technical person to define the Julian Date (JD) and, probably, they will say the number of days from January 1 of the current year. This is the relatively modern, skewed definition of the Julian Date.

Actually, the JD, defined by Joseph Scaliger in 1582, is the number of continuous days elapsed since Noon, UTC, January 1, 4713 B.C. Analogous to the UNIX Epoch, Astronomers use Scaliger's discovery as a reference point for performing long interval date calculations.

Although there's no consensus, some contend Scaliger's discovery is the Julian DATE, and the number of days from January 1 of the current year is the Julian DAY. See:

http://newport.pmel.noaa.gov/nemo2000/qa.html

In this article, we refer to Scaliger's JD as the Astro-Julian Date. We hope this differentiates between Scaliger's Julian Date and the Julian Date from January 1 of the current year.

Of the many Web resources dealing with JD, check out this U.S. Naval Observatory (USNO) link:

http://aa.usno.navy.mil/data/docs/JulianDate.html

The USNO provides an extensive treatment of the Julian Date and Gregorian calendar. It also contains a JD calculator.

This USNO link:

http://aa.usno.navy.mil/faq/docs/JD_Formula.html

explains algorithms used to convert between Julian Dates and the Gregorian calendar. According to the USNO, the original algorithms were coded in Fortran taking advantage of that language's "truncation feature of integer arithmetic". The Korn and Bash Shells share that same feature.

Here are two important Julian Dates and their corresponding Gregorian dates:

JD 2440588 is Jan. 1, 1970

JD 2451545 is Jan. 1, 2000