The Julian Day (JD) or Julian Day Number is the time that has elapsed since noon January 1, 4713 BC[?] (according to the proleptic Julian calendar; or November 24, 4714 BC according to the proleptic Gregorian calendar), expressed in days and fractions of a day. The Julian day is based on the Julian period proposed by Joseph Scaliger in 1583. 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 accomodated to the Julian year". This "Julian" in "proleptic Julian calendar" and "Julian year" refers to Julius Caesar, who introduced the Julian calendar in 46 BC.
Given that the Julian Day Number (and modifications of it) has been widely used by astronomers, it is also called "Astronomical Julian Day (AJD)". Because the starting point is so long ago, numbers in the Julian day can be quite large and cumbersome. For this reason, adaptations have been made by substracting a number of days from the Julian day to obtain the Modified Julian Day (MJD) or Truncated Julian Day (TJD) (See below.)
The most well known version of the Julian Day is perhaps the Chronological Julian Day (CJD), a modification of the Astronomical Julian Day, in which the starting point is set at midnight January 1, 4713 BC (Julian calendar) rather than noon. Chronographers found the Julian Day concept useful, but they didn't like noon as starting time. So CJD = AJD + 0.5. Note that AJD uses Coordinated Universal Time (UTC), and so it is the same for all time zones and is independent of DaylightSaving Times (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs.
To make numbers more convenient, a more recent starting point is sometimes used. For example, the Lilian Day number (LD) counts from October 14, 1582 C.E. in the Gregorian Calendar, which is the date before the day on which the Gregorian calendar was adopted. Where CJD is the Chronological Julian day number: LD = CJD  2,299,160 = AJD  2,299,159.5
The Julian date system was intended to provide a single system of dates that could be used to unify different historical chronologies. Its epoch was fixed by Scaliger to a time that he believed predated all known historical dates.
In his book Outlines of Astronomy, published in 1849, the astronomer John Herschel recommended that a version of Scaliger's scheme should be used to make a standard system of time for astronomy. This has now become the standard system of Julian dates. Julian dates are typically used by astronomers to calculate astronomical events, and eliminate the complications resulting from using standard calendar periods. There are two particular advantages: first, starting so far back in time allows historical observations to be handled easily (when studying ancient records of, eg, eclipses); second, because Julian days begin at noon a single night of astronomical observation will fall within the same Julian day.
The Modified Julian Date, invented by space scientists in the 1950s, is defined in terms of the Julian Date as follows:
MJD = AJD  2400000.5
The offset of 0.5 means that MJDs start midnight of November 17th, 1858 CE.
Modified Julian Days are always based on the Universal Time system, not local time.
The Truncated Julian Day (TJD) is obtained by substracting 2,440,000.5 to the AJD.
See also: time, time scales, era, epoch, epoch (astronomy)
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