A timekeeping system which does not have months and years. It is
used primarily by astronomers to avoid confusion
due to the use of different calendars at different times and
places. Julian date is the interval of time in days and fractions
of a day since noon 1 January 4713 B.C. (12h Universal Time).
For example, January 1, 1970 is JD 2440588. Decimal fractions
correspond to fractions of a day so that, for example, an observation
made at 15h on June 24, 1962 is given as JD 2437840.13.
→ modified Julian date (MJD).
Note that the “Julius” involved is not Julius Caesar, and this system is
unrelated to the Julian calendar, as explained below.
See also: The system was proposed by the French scholar Joseph Justus Scaliger (1540-1609)
in 1583 and named after his father, Julius Caesar Scaliger.
His choice of starting year was based on the convergence in 4713 B.C. of
three calendrical cycles (indication cycle, Metonic cycle, and solar cycle).
→ date.
