Julian Date

Julian Date (JD) A calendar and timekeeping system introduced to give unambiguous dates and times of celestial events, unaffected by changes in the civil calendar. The system was begun in 1582 by the French scholar Joseph Justus Scaliger (1540–1609), who named it to honour his father, the Italian-born French scholar Julius Caesar Scaliger (1484–1558). The starting date is chosen as January1 of the year 4713bc, sufficiently far in the past that it predates all known recorded astronomical observations. Time is measured from mean noon (12 h UT) on that date by the number of days and fractions of a day elapsed. For example, an observation made on 1962June24 at 18 h UT was made at JD 2 437 840.25. The integral part of the Julian date is called the Julian day number, and the fractional part is the universal time expressed as a fraction of a day. See also Modified Julian Date.