Julian day

"Julian date" redirects here. For dates in the Julian calendar, see Julian calendar. For numbered day of year, see Ordinal date. For the comic book character Julian Gregory Day, see Calendar Man. For the artist and composer, see Julian Day (artist).
Not to be confused with Julian year (astronomy).

The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date).[1]

The Julian period is a chronological interval of 7980 years, derived from three multi-year cycles: the Indiction, Solar, and Lunar cycles. The last year that was simultaneously the beginning of all three cycles was 4713 BC (−4712),[2] so that is year 1 of the current Julian period, making AD 2024 year 6737 of that Period. The next Julian Period begins in the year AD 3268. Historians used the period to identify Julian calendar years within which an event occurred when no such year was given in the historical record, or when the year given by previous historians was incorrect.[3]

The Julian day number (JDN) shares the epoch of the Julian period, but counts days instead of years. Specifically, Julian day number 0 is assigned to the day starting at noon Universal Time on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar),[4][5][6].[a] For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2451545.[7]

The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added.[8] For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2456293.520833.[9] This article was loaded at 2024-11-21 21:22:14 (UTC) – expressed as a Julian date this is 2460636.3904398.

  1. ^ "Julian date" n.d.
  2. ^ Astronomical Almanac for the year 2017 p. B4, which states 2017 is year 6730 of the Julian Period.
  3. ^ Grafton 1975
  4. ^ Dershowitz & Reingold 2008, 15.
  5. ^ Seidelman 2013, 15.
  6. ^ "Astronomical Almanac Online" 2016, Glossary, s.v. Julian date. Various timescales may be used with Julian date, such as Terrestrial Time (TT) or Universal Time (UT); in precise work the timescale should be specified.
  7. ^ McCarthy & Guinot 2013, 91–92
  8. ^ "Resolution B1" 1997.
  9. ^ US Naval Observatory 2005


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