The term Julian day has different meanings. It is sometimes confused with Julian date, which also has more than one meaning. Just as the Gregorian date is a date in the Gregorian calendar, a Julian date is a date in the Julian calendar. Some people use the term Julian date as synonymous with Julian Day or Julian Day Number. Such use makes it ambiguous, for which reason is better to reserve the term Julian date to refer to a date in the Julian calendar.
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 system was intended to provide a single system of dates that could be used when working with different calendars and to unify different historical chronologies.
Given that the Julian Day Number (and modifications of it) has been widely used by astronomers, it is also called "Astronomical Julian Day (AJD)".
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 Daylight-Saving Times (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs.
Because the starting point is so long ago, numbers in the Julian day can be quite large and cumbersome. To make numbers more convenient, a more recent starting point is sometimes used, for instance by dropping the leading digits.
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 - 2,299,159.5The Modified Julian Day, introduced by space scientists in the 1950s, is defined in terms of the Julian Day as follows:
MJD = AJD - 2400000.5The 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 subtracting 2,440,000.5 from the AJD.
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15 (Indiction cycle) * 19 (Metonic cycle) * 28 (Solar cycle) = 7980 yearsIts epoch falls at the last time when all three cycles were in their first year together, and Josephus Scaliger chose this because it pre-dated all known 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" in "proleptic Julian calendar" and "Julian year" 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 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 days. Julian days 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.