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Julian day
The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UTC) on 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.
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Julian Date
The Julian Date (JD) is the Julian day number added to the decimal fraction of the day elapsed since noon. Historical Julian Dates were recorded in respect to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian Dates be specified in Terrestrial Time, and that when necessary to specify Julian Dates using a different time scale, that the time scale used be indicated when required, such as JD(UT1). The fraction of the day is found by dividing the number of hours, including any fraction thereof, by 24, or by using the formula: JD = JDN+(hours+(minutes+(seconds/60))/60)/24.
The term Julian date is also used to refer to:
 Julian calendar dates
 ordinal dates (dayofyear)
The use of Julian date to refer to the dayofyear (ordinal date) is usually considered to be incorrect.
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 DaylightSaving 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 fourdigit number. It can be defined as:

 TJD = JD  2440000.5
 or

 TJD = (JD  0.5) mod 10000
 The Dublin Julian Day (DJD) is a count of days from midnight of January 1, 1900. The source of the name is unknown. It is used in computer programs, such as Lotus 123 and Microsoft Excel. In these particular programs, this date is counted as day 1, instead of day 0, because the year 1900 was erroneously http://support.microsoft.com/default.aspx?scid=kb;enus;214326 treated as a leap year.
 The Lilian day number defines day 1 as October 15, 1582, which was the first day of the Gregorian Calendar. It was named for Aloysius Lilius, who is credited as the principal author of the Gregorian Calendar.
 The ANSI Date defines January 1, 1601 as day 1, and is used as the origin of COBOL integer date s. This epoch is the beginning of the previous 400year cycle of leap years in the Gregorian Calendar, which ended with the year 2000.
 Rata Die is the epoch used in Calendrical Calculations by Edward M. Reingold and Nachum Dershowitz, where day 1 is January 1, 1, that is, the first day, in the proleptic Gregorian Calendar.
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 predated 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 wrote:
 The last year of the current Julian period, or that of which the number in each of the three subordinate cycles is 1, was the year 4713 B.C., and the noon of the 1st of January of that year, for the meridian of Alexandria, is the chronological epoch, to which all historical eras are most readily and intelligibly referred, by computing the number of integer days intervening between that epoch and the noon (for Alexandria) of the day, which is reckoned to be the first of the particular era in question. The meridian of Alexandria is chosen as that to which Ptolemy refers the commencement of the era of Nabonassar, the basis of all his calculations.
Astronomers adopted Herschel's Julian Days in the late 19th century, but using the meridian of Greenwich instead of Alexandria, after the former was made the Prime Meridian by international conference in 1884. This has now become the standard system of Julian days. Julian days are typically used by astronomers to date astronomical observations, 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.
For a date in the Gregorian calendar:
For a date in the Julian calendar:
The day of the week can be determined from the Julian day number by calculating it modulo 7, where 0 means Monday.
JD mod 7  0  1  2  3  4  5  6 
Day of the week  Mon  Tue  Wed  Thu  Fri  Sat  Sun 
See also
References
 Gordon Moyer, "The Origin of the Julian Day System," Sky and Telescope 61 (April 1981) 311313.
 Explanatory Supplement to the Astronomical Almanac, edited by P. Kenneth Seidelmann. University Science Books, 1992. ISBN 0935702687
External links
 Article 'Julian Day Numbers' http://hermetic.nofadz.com/cal_stud/jdn.htm by Peter Meyer
 U.S. Naval Observatory Julian Date Converter http://aa.usno.navy.mil/data/docs/JulianDate.html
 Julian Day and Civil Date calculator http://www.nr.com/julian.html
 U.S. Naval Observatory Time Service article http://tycho.usno.navy.mil/mjd.html
 Outlines of Astronomy by John Herschel http://visualiseur.bnf.fr/Visualiseur?Destination=Gallica&O=NUMM94926
 International Astronomical Union Resolution 1B: On the Use of Julian Dates http://www.iers.org/iers/earth/resolutions/UAI_b1.html
 Calendrica http://emr.cs.iit.edu/home/reingold/calendarbook/Calendrica.html
 Another Julian Day calculator with conversions to many other calendars http://isotropic.org/uw/date/ valid from 1 January 100 proleptic Gregorian calendar