modified Julian day

modified Julian day
(MJD)
   a count of days used by astronomers, space agencies, and others. Astronomers have long used the Julian day, a count of days beginning at noon Universal Time January 1, 4713 BC, as a means of specifying a date independent of all calendars. One problem with this is that the numbers are large, more than 2.4 million, for current dates. Also, the old astronomical custom of beginning a day at noon is awkward for converting Julian dates to the ordinary calendar. To ease these problems, space engineers introduced the modified Julian date, equal to the Julian date minus 2 400 000.5. The result is a count of days beginning at 0 hours (midnight) Universal Time on 17 November 1858. Thus (for example) 0 hours 1 January 2005 is MJD 53371.0.

Dictionary of units of measurement. 2015.

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