# Julian date (JD)

The Julian date (JD) is a timekeeping system used by astronomers to avoid the ambiguities and computational
complexities of the civilian calendar; the 'Julius' involved is not Julius
Caesar, and this system is unrelated to the Julian
calendar. The Julian date is the number of days that have elapsed since
noon (12h Universal Time) on January 1, 4713 BC. 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 2437840.63; the whole number
part is called the **Julian date number**.

The system was proposed by the French scholar Joseph Justus Scaliger (1540–1609)
in 1582 and named after his father, Julius Caesar Scaliger. His choice of
starting year was based on the convergence in 4713 BC of three calendrical cycles, one of which was the 15-year ancient Roman
tax cycle of Emperor Constantine, so is of no practical consequence. For
convenience, the **modified Julian date** (MJD) is sometimes
used. This is defined as starting at midnight on November 17.0, 1858, so
that MJD = JD - 2,400,000.5 day.