Julian Day

Julian Day

In 1583 C.E., French scientist Joseph Scaliger devised the Julian Period of 7980 years beginning 4713 B.C.E of Julian calendar. In 1849, English astronomer John F. W. Herschel suggested, for convenience of the astronomical and historical chronology, to use simple, continuous numeration of days beginning from January 1, 4713 B.C.E. of Julian calendar, counting the Greenwich midday as the beginning of the day. The numbers of days counted from this date were considered to be called Julian Days and marked as JD. For example, January 1, 2000 of Julian calendar corresponds to 2451558 JD, and of Gregorian calendar � 2451545 JD. Julian Days became very convenient for calculations and are widely used in astronomy.

In order not to operate with the unnecessary big numbers while calculating close to contemporary time events, astronomers introduced Modified Julian Day (MJD). The start of counting by Modified Julian Days began on 00:00 of Wednesday November 17, 1858 C.E. of Gregorian calendar. The Greenwich midnight is taken as the beginning of the new day.

Modified Julian Day = Julian day - 2400000.5
or
MJD = JD - 2400000.5

Try converting dates using the Calendar Converter.