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The Julian Day (JD) or Julian Day number 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 a single system of dates that could be used when working with different calendars and to unify different historical chronologies.
The Julian Date is the Julian Day combined with the fraction of the day, starting from noon Universal Time (formerly called Greenwich Mean Time). The fraction of the day is found by dividing the time of the day, in hours, by twenty-four (00:00 hours being noon and 12:00 hours midnight).
In other contexts, Julian date is also used to refer to:
Astronomers often use a Julian year of exactly 365.25 days for ephemeris purposes, because of the direct and simple conversion to Julian Days.
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 sixteen minutes, that being the time it takes light to cross the orbit of the earth. 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 (Julian calendar) local time rather than noon UT. Chronographers found the Julian Day concept useful, but they didn't like noon as starting time. So CJD = JD + 0.5. Note that JD uses Universal Time (UT), 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 Astronomical Observatory in 1958 to record the orbit of Sputnik, is defined in terms of the Julian day as follows:
The offset of 0.5 means that MJD started at midnight of November 17th, A.D. 1858, and that every modified Julian Day begins and ends at midnight Universal Time.
The Reduced Julian Day (RJD) is also used by astronomers and counts days from the same day as MJD, but from noon UT, and thus is defined as:
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:
or
The Dublin Julian Day (DJD) is used by computer programmers, and is a count of days from midnight of January 1, A.D. 1900.
Other day-count systems, or integer dates, include the Lilian Date with an epoch of October 14, A.D. 1582 (the day before the Gregorian calendar was adopted); Rata Die counts from the beginning of the Christian or Common Era, January 1, A.D. 1 (Julian calendar) and the ANSI Date, which uses January 1, A.D. 1601.
You can determine the day of the week from the Julian Day modulo 7.
| JD mod 7 | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
| Day of the week | Mon | Tue | Wed | Thu | Fri | Sat | Sun |
The Julian Day is based on the Julian period proposed by Joseph Scaliger in 1583, at the time of the Gregorian calendar reform. It is a multiple of three calendar cycles:
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 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 by counting the "day of the Julian period." 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 use astronomical time, which begins at noon, a single night of astronomical observation will fall within the same Julian Day. Astronomical time was used for Greenwich Mean Time until 1925. It is now regulated by the International Astronomical Union.
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>
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