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 expiration date)."Julian date" n.d.
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 ,Astronomical Almanac for the year 2017 p. B4, which states 2017 is year 6730 of the Julian period. so that is year 1 of the current Julian period, making AD year 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.Grafton 1975
The Julian day number (JDN) has the same epoch as the Julian period, but counts the number of days since the epoch rather than the number of years since then. 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).Dershowitz & Reingold 2008, 15.Seidelman 2013, 15."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. For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was .McCarthy & Guinot 2013, 91–92
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."Resolution B1" 1997. For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is .US Naval Observatory 2005 This article was loaded at (UTC) – expressed as a Julian date this is .
Terminology The term Julian date may also refer, outside of astronomy, to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry,USDA . or it may refer to dates in the Julian calendar. For example, if a given "Julian date" is "October 5, 1582", this means that date is in the Julian calendar (which was October 15, 1582, in the Gregorian calendarthe date it was first established). Without an astronomical or historical context, a "Julian date" given as "36" most likely means the 36th day of a given Gregorian year, namely February 5. Other possible meanings of a "Julian date" of "36" include an astronomical Julian Day Number, or the year AD 36 in the Julian calendar, or a duration of 36 astronomical Julian years. This is why the terms "ordinal date" or "day-of-year" are preferred. In contexts where a "Julian date" means simply an ordinal date, calendars of a Gregorian year with formatting for ordinal dates are often called "Julian calendars", but this could also mean that the calendars are of years in the Julian calendar system.
Historically, Julian dates were recorded relative to Greenwich Mean Time (GMT) (later, Ephemeris Time), but since 1997 the International Astronomical Union has recommended that Julian dates be specified in Terrestrial Time.Resolution B1 on the use of Julian Datesof the XXIIIrd International Astronomical Union General Assembly, Kyoto, Japan, 1997 Seidelmann indicates that Julian dates may be used with International Atomic Time (TAI), Terrestrial Time (TT), Barycentric Coordinate Time (TCB), or Coordinated Universal Time (UTC) and that the scale should be indicated when the difference is significant.Seidelmann 2013, p. 15. The fraction of the day is found by converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction. Time intervals calculated from differences of Julian Dates specified in non-uniform time scales, such as UTC, may need to be corrected for changes in time scales (e.g. leap seconds).
Variants
Because the starting point or reference epoch is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. In the following table, times are given in 24-hour notation.
In the table below, Epoch refers to the point in time used to set the origin (usually zero, but (1) where explicitly indicated) of the alternative convention being discussed in that row. The date given is a Gregorian calendar date unless otherwise specified. JD stands for Julian Date. 0h is 00:00 midnight, 12h is 12:00 noon, UT unless otherwise specified. Current value is at and may be cached. []
Name Epoch Calculation Current value Notes Julian date JD Reduced JD 12:00 November 16, 1858 JD − Hopkins 2013, p. 257.Pallé, Esteban 2014. Modified JD 0:00 November 17, 1858 JD − Introduced by SAO in 1957 Truncated JD 0:00 May 24, 1968 floor(JD − ) Introduced by NASA in 1979 Dublin JD 12:00 December 31, 1899 JD − Introduced by the IAU in 1955 CNES JD 0:00 January 1, 1950 JD − Introduced by the CNESTheveny 2001. CCSDS JD 0:00 January 1, 1958 JD − Introduced by the CCSDS Modified JD2000 (MJD2000) 0:00 January 1, 2000 JD - Introduced by ESAESA Earth Observation Mission CFI Software Conventions document Lilian date day 1 = October 15, 1582 floor (JD − ) Count of days of the Gregorian calendar Rata Die day 1 = January 1, 1 floor (JD − ) Count of days of the Common Era Mars Sol Date 12:00 December 29, 1873 (JD − )/ Count of Martian days Unix time 0:00 January 1, 1970 (JD − ) × Count of seconds,Astronomical almanac for the year 2001, 2000, p. K2 excluding leap seconds JavaScript Date 0:00 January 1, 1970 (JD − ) × Count of milliseconds, excluding leap seconds EXT4 File Timestamps 0:00 January 1, 1970 (JD − ) × Count of nanoseconds, excluding leap seconds .NET DateTime 0:00 January 1, 1 (JD − ) × Count of 100-nanosecond ticks, excluding ticks attributable to leap seconds
The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 (36-bit machine) and using only 18 bits until August 7, 2576. MJD is the epoch of VAX/VMS and its successor OpenVMS, using 63-bit date/time, which allows times to be stored up to July 31, 31086, 02:48:05.47. The MJD has a starting point of midnight on November 17, 1858, and is computed by MJD = JD − 2400000.5 Winkler n.d. The Truncated Julian Day (TJD) was introduced by NASA/Goddard in 1979 as part of a parallel grouped binary time code (PB-5) "designed specifically, although not exclusively, for spacecraft applications". TJD was a 4-digit day count from MJD 40000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 50000, or October 10, 1995, "which gives a long ambiguity period of 27.4 years". (NASA codes PB-1–PB-4 used a 3-digit day-of-year count.) Only whole days are represented. Time of day is expressed by a count of seconds of a day, plus optional milliseconds, microseconds and nanoseconds in separate fields. Later PB-5J was introduced which increased the TJD field to 16 bits, allowing values up to 65535, which will occur in the year 2147. There are five digits recorded after TJD 9999.Chi 1979.SPD Toolkit Time Notes 2014. The Dublin Julian Date (DJD) is the number of days that has elapsed since the epoch of the solar and lunar ephemerides used from 1900 through 1983, Newcomb's Tables of the Sun and Ernest W. Brown's Tables of the Motion of the Moon (1919). This epoch was noon UT on :January 0, 1900, which is the same as noon UT on December 31, 1899. The DJD was defined by the International Astronomical Union at their meeting in Dublin, Ireland, in 1955.Ransom The Lilian day number is a count of days of the Gregorian calendar and not defined relative to the Julian Date. It is an integer applied to a whole day; day 1 was October 15, 1582, which was the day the Gregorian calendar went into effect. The original paper defining it makes no mention of the time zone, and no mention of time-of-day.Ohms 1986 It was named for Aloysius Lilius, the principal author of the Gregorian calendar.IBM 2004. Rata Die is a system used in Rexx, Go and Python."datetime – Basic date and time types – date Objects" (December 5, 2021). The Python Standard Library. Some implementations or options use Universal Time, others use local time. Day 1 is January 1, 1, that is, the first day of the Christian or Common Era in the proleptic Gregorian calendar.Dershowitz & Reingold 2008, 10, 351, 353, Appendix B. In Rexx, January 1 is Day 0."Chapter 3. Functions – DATE – Base" (September 29, 2022). z/VM: 7.1 REXX/VM Reference The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes (498 seconds), that being the time it takes light to reach Earth from the Sun.
History Julian period The Julian day number is based on the Julian period proposed by Joseph Scaliger, a classical scholar, in 1583 (one year after the Gregorian calendar reform) as it is the product of three calendar cycles used with the Julian calendar:
Its epoch occurs when all three cycles (if they are continued backward far enough) were in their first year together. Years of the Julian period are counted from this year, , as , which was chosen to be before any historical record.Richards 2013, pp. 591–592.
Scaliger corrected chronology by assigning each year a tricyclic "character", three numbers indicating that year's position in the 28-year solar cycle, the 19-year lunar cycle, and the 15-year indiction cycle. One or more of these numbers often appeared in the historical record alongside other pertinent facts without any mention of the Julian calendar year. The character of every year in the historical record was unique – it could only belong to one year in the 7980-year Julian period. Scaliger determined that 1 BC or year 0 was Julian period . He knew that 1 BC or year 0 had the character 9 of the solar cycle, 1 of the lunar cycle, and 3 of the indiction cycle. By inspecting a 532-year Paschal cycle with 19 solar cycles (each of 28 years, each year numbered 1–28) and 28 lunar cycles (each of 19 years, each year numbered 1–19), he determined that the first two numbers, 9 and 1, occurred at its year 457. He then calculated via remainder division that he needed to add eight 532-year Paschal cycles totaling 4256 years before the cycle containing 1 BC or year 0 in order for its year 457 to be indiction 3. The sum was thus JP 4713.Grafton 1975, p. 184
A formula for determining the year of the Julian period given its character involving three four-digit numbers was published by Jacques de Billy in 1665 in the Philosophical Transactions of the Royal Society (its first year).de Billy 1665 John F. W. Herschel gave the same formula using slightly different wording in his 1849 Outlines of Astronomy.Herschel 1849
Carl Friedrich Gauss introduced the modulo operation in 1801, restating de Billy's formula as:
where a is the year of the indiction cycle, b of the lunar cycle, and c of the solar cycle.Gauss 1966Gauss 1801
John Collins described the details of how these three numbers were calculated in 1666, using many trials.Collins 1666 A summary of Collin's description is in a footnote.+ Calculation of 4845, 4200, 6916 by Collins Try 2+ until = 19×15 = 285 = 285×17 = 19×15×17 = 4845 = 28×15 = 420 = 420×10 = 28×15×10 = 4200 = 28×19 = 532 = 532×13 = 28×19×13 = 6916