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Julian date(JD) The number of days that have elapsed since noon GMT on Jan. 1 4713 bc – the Julian day number – plus the decimal fraction of the day that has elapsed since the preceding noon. The consecutive numbering of days makes the system independent of the length of month or year and the JD is thus used to calculate the frequency of occurrence or the periodicity of phenomena over long periods. The system was devised in 1582 by the French scholar Joseph Scaliger. A specific Julian date can be determined from astronomical tables of Julian day numbers: thus since March 1 1980, noon GMT, is tabulated as day number 244 4300 then March 17 1980, 6 p.m. GMT, is JD 244 4316.25 (see table). The modified Julian date (MJD) is found by subtracting 240 0000.5 from the Julian date.
Julian date[′jül·yən ¦dāt]
The sum of the Julian day number and the fraction of a day elapsed since the previous noon.
Julian dateA calendar notation in which the date is represented by one number. For example, the Julian date for December 11, 1942 is 2430705; while December 12, 1942 is 2430706. The Julian date is widely used with computers because it requires less programming to compare dates that are single integers. Julian numbers are also used within a single year; for example, February 1 is Julian 32.
Developed in the 16th century by Joseph Scaliger, the numbers were based on a 7,980-year cycle that began on January 1, 4713 BC and ends January 22, 3268. Contrast with "Gregorian date," which is the common calendar notation of month, day and year.