Julian day

   

The Julian day or Julian day number (JDN) is the number of days that have elapsed since noon Monday, January 1, 4713 BC (according to the proleptic Julian calendar; or November 24, 4714 BC, according to the proleptic Gregorian calendar). The Julian day system was intended to provide astronomers with a single system of dates that could be used when working with different calendars and to unify different historical chronologies.

The Julian Date (JD) is the Julian day number combined with the fraction of the day since noon at Greenwich, England (any Greenwich time such as Universal Time or Terrestrial Time may be used). The fraction of the day is found by converting the number of hours, minutes, and seconds since noon into a decimal fraction.

In other contexts, Julian date is also used to refer to:

Alternatives

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, that being the time it takes light to reach Earth from the Sun. The Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD.

Another version of the Julian day, introduced by Peter Meyer, is the chronological julian day (CJD), in which the starting point is set at midnight January 1, 4713 BC (proleptic Julian calendar) local time rather than noon UT. Chronographers found the Julian day concept useful, but they didn't like noon as the starting time. So CJD = JD + 0.5. Note that JD may use Universal Time (UT) or Terrestrial Time (TT), and so it is the same for all time zones and is independent of Summer Time or Daylight-Saving Time (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs. Users of CJD sometimes refer to the Julian day as astronomical Julian Day (AJD) to distinguish it from CJD.

Because the starting point 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.

The Modified Julian Day (MJD), introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik, is defined in terms of the Julian day as follows:

MJD = JD - 2400000.5

The offset of 0.5 means that MJD started at midnight at the beginning of November 17, 1858, and that every Modified Julian Day begins and ends at midnight UT or TT.

The Reduced Julian Day (RJD) is also used by astronomers and counts days from the same day as MJD, but from noon UT or TT, and thus is defined as:

RJD = JD - 2400000

The Truncated Julian Day (TJD) was introduced by NASA for the space program. TJD began at 24 May 1968. Since TJD exceeded four digits on 10 October 1995, some now count TJD from this date in order to maintain a four-digit number. It can be defined as:

TJD = JD - 2440000.5

or

TJD = (JD - 0.5) mod 10000

The Dublin Julian Day (DJD) is used by computer programmers, and is a count of days from midnight of January 1, 1900.

Other epochs, etc.

  • Jan. 1, 1970 = MJD 40,587
  • Jan. 1, 2000 = MJD 51,544
  • (Microsoft Excel day number) + 15,018 = MJD
    • Note: This does not hold for dates in January and February 1900, due to a bug. Lotus 1-2-3, the dominant spreadsheet program at the time of Excel's first release, treated 1900 as a leap year for ease of calculation, so Microsoft intentionally copied the bug for compatibility reasons.[1] (http://support.microsoft.com/default.aspx?scid=kb;en-us;214326)

Other day-count systems, or integer dates, include the Lilian Day Number with day one at October 15, 1582 (the first day of the Gregorian calendar); Rata Die assigns day one to January 1, 1 in the proleptic Gregorian calendar; and the ANSI Date uses January 1, 1601.

The day of the week can be determined from the Julian day number by calculating it modulo 7, where 0 means Monday.

JD mod 70 12 34 56
Day of the weekMon TueWed ThuFri SatSun

History

The Julian day number is based on the Julian Period proposed by Joseph Scaliger in 1583, at the time of the Gregorian calendar reform, but it is the multiple of three calendar cycles used with the Julian calendar:

15 (Indiction cycle) × 19 (Metonic cycle) × 28 (Solar cycle) = 7980 years

Its epoch falls at the last time when all three cycles were in their first year together, and Scaliger chose this because it pre-dated all historical dates.

Note: although many references say that the "Julian" in "Julian day" refers to Scaliger's father, Julius Scaliger, in the introduction to Book V of his Opus de Emendatione Tempore (Work on the Emendation of Time) he states: "Iulianum vocauimus: quia ad annum Iulianum dumtaxat accomodata est" which translates more or less as "We call this Julian merely because it is accommodated to the Julian year". This "Julian" refers to Julius Caesar, who introduced the Julian calendar in 46 BC.

In his book Outlines of Astronomy, published in 1849, the astronomer John Herschel recommended that the "day of the Julian Period" should be used as the standard system of time for astronomy. This has now become the standard system of Julian days. Julian days are typically used by astronomers to date astronomical events, thus eliminating the complications resulting from using standard calendar periods like eras, years, months, or weeks.

Julian days begin at noon because when Herschel recommended them, the astronomical day began at noon (it did so until 1925). The astronomical day had begun at noon ever since Ptolemy chose to begin the days in his astronomical periods at noon. He chose noon because the transit of the Sun across the observer's meridian occurs at the same apparent time every day of the year, unlike sunrise or sunset, which vary by several hours. Midnight was not even considered because it could not be accurately determined using water clocks. Nevertheless, he double dated most nighttime observations with both Egyptian days beginning at sunrise and Babylonian days beginning at sunset. Thus the astronomical day did not begin at noon to allow all observations of a single night to be in a single day.

Calculation

The Julian day number can be calculated using the following formulas:

All divisions are integer divisions, meaning the remainder in the division is discarded.

<math>\begin{matrix}a & = & {14 - month \over 12} \\ \\y & = & year + 4800 - a \\ \\m & = & month + 12a - 3 \\\end{matrix}<math>

For a date in the Gregorian calendar:

<math>\begin{matrix}JD & = & day + {153m + 2\over 5} + 365y + {y \over 4} - {y \over 100} + {y \over 400} - 32045\end{matrix}<math>

For a date in the Julian calendar:

<math>\begin{matrix}JD & = & day + {153m + 2\over 5} + 365y + {y \over 4} - 32083\end{matrix}<math>

See also

References

  • Gordon Moyer, "The Origin of the Julian Day System," Sky and Telescope 61 (April 1981) 311-313.
  • Explanatory Supplement to the Astronomical Almanac, edited by P. Kenneth Seidelmann. University Science Books, 1992. ISBN 0935702687

External links

da:Julianske dato de:Julianisches_Datum fr:Jour julien it:Giorno giuliano ja:ユリウス通日


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