Julian day
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.
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, 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.
The Julian day number 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. For example, the Julian day number for the day starting at 12:00 UT on January 1, 2000, was.
The Julian date 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. For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is. This article was loaded at – expressed as a Julian date this is.
Terminology
The term Julian date may also refer, outside of astronomy, to the day-of-year number in the Gregorian calendar, especially in computer programming, the military and the food industry, 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. 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 , but since 1997 the International Astronomical Union has recommended that Julian dates be specified in Terrestrial Time. Seidelmann indicates that Julian dates may be used with International Atomic Time, Terrestrial Time, Barycentric Coordinate Time, or Coordinated Universal Time and that the scale should be indicated when the difference is significant. 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.
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 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 − | ||
| Modified JD | 0:00 November 17, 1858 | JD − | Introduced by SAO in 1957 | |
| Truncated JD | 0:00 May 24, 1968 | floor | 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 CNES | |
| CCSDS JD | 0:00 January 1, 1958 | JD − | Introduced by the CCSDS | |
| Modified JD2000 | 0:00 January 1, 2000 | JD - | Introduced by ESA | |
| Lilian date | day 1 = October 15, 1582 | floor | Count of days of the Gregorian calendar | |
| Rata Die | day 1 = January 1, 1 | floor | Count of days of the Common Era | |
| Mars Sol Date | 12:00 December 29, 1873 | / | Count of Martian days | |
| Unix time | 0:00 January 1, 1970 | × | Count of seconds, excluding leap seconds | |
| JavaScript Date | 0:00 January 1, 1970 | × | Count of milliseconds, excluding leap seconds | |
| EXT4 File Timestamps | 0:00 January 1, 1970 | × | Count of nanoseconds, excluding leap seconds | |
| .NET DateTime | 0:00 January 1, 1 | × | Count of 100-nanosecond ticks, excluding ticks attributable to leap seconds |
- The Modified Julian Date was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 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
- The Truncated Julian Day was introduced by NASA/Goddard in 1979 as part of a parallel grouped binary time code "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". 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.
- The Dublin Julian Date 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. 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.
- 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. It was named for Aloysius Lilius, the principal author of the Gregorian calendar.
- Rata Die is a system used in Rexx, Go and Python. 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. In Rexx, January 1 is Day 0.
- The Heliocentric Julian Day 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, 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 as it is the product of three calendar cycles used with the Julian calendar:Its epoch occurs when all three cycles 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.
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 and 28 lunar cycles, 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.
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. John F. W. Herschel gave the same formula using slightly different wording in his 1849 Outlines of Astronomy.
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.
John Collins described the details of how these three numbers were calculated in 1666, using many trials. A summary of Collin's description is in a footnote.
Reese, Everett and Craun reduced the dividends in the Try column from 285, 420, 532 to 5, 2, 7 and changed remainder to modulo, but apparently still required many trials.
The specific cycles used by Scaliger to form his tricyclic Julian Period were, first, the indiction cycle with a first year of 313. Then he chose the dominant 19-year Alexandrian lunar cycle with a first year of 285, the Era of Martyrs and the Diocletian Era epoch, or a first year of 532 according to Dionysius Exiguus.Dionysius Exiguus 2003/525 Finally, Scaliger chose the post-Bedan solar cycle with a first year of 776, when its first quadrennium of concurrents,, began in sequence. Although not their intended use, the equations of de Billy or Gauss can be used to determined the first year of any 15-, 19-, and 28-year tricyclic period given any first years of their cycles. For those of the Julian Period, the result is AD 3268, because both remainder and modulo usually return the lowest positive result. Thus 7980 years must be subtracted from it to yield the first year of the present Julian Period, −4712 or 4713 BC, when all three of its sub-cycles are in their first years.
Scaliger got the idea of using a tricyclic period from "the Greeks of Constantinople" as Herschel stated in his quotation below in Julian day numbers. Specifically, the monk and priest Georgios wrote in 638/39 that the Byzantine year 6149 AM had indiction 14, lunar cycle 12, and solar cycle 17, which places the first year of the Byzantine Era in 5509/08 BC, the Byzantine Creation. Dionysius Exiguus called the Byzantine lunar cycle his "lunar cycle" in argumentum 6, in contrast with the Alexandrian lunar cycle which he called his "nineteen-year cycle" in argumentum 5.
Although many references say that the Julian in "Julian Period" refers to Scaliger's father, Julius Scaliger, at the beginning of Book V of his Opus de Emendatione Temporum he states, "Iulianam vocauimus: quia ad annum Iulianum accomodata", which Reese, Everett and Craun translate as "We have termed it Julian because it fits the Julian year". Thus Julian refers to the Julian calendar.