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Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers.
The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar). ^{[1]} For example, the Julian day number for the day starting at 12:00 UT on January 1, 2000, was 2,451,545.^{[2]}
The Julian date (JD) of any instant is the Julian day number for the preceding noon in Greenwich Mean Time plus the fraction of the day since that instant. Julian dates are expressed as a Julian day number with a decimal fraction added.^{[3]} For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2,456,293.520833.^{[4]}
The term Julian date may also refer, outside of astronomy, to the dayofyear number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry,^{[5]}— or it may refer to dates in the Julian calendar. For example, if a given "Julian date" is "May 12, 1629", this means that date in the Julian calendar (which is May 22, 1629, in Gregorian calendar— the date of the Treaty of Lübeck). Outside of an astronomical or historical context, if a given "Julian date" is "40", this most likely means the fortieth day of a given Gregorian year, namely February 9. But the potential for mistaking a "Julian date" of "40" to mean an astronomical Julian Day Number (or even to mean the year 40 ad in the Julian calendar, or even to mean a duration of 40 astronomical Julian years) is justification for preferring the terms "ordinal date" or "dayofyear" instead. In contexts where a "Julian date" means simply an ordinal date, calendars of a Gregorian year with formatting for ordinal dates are often called "Julian calendars",^{[5]} in spite of the potential for misinterpreting this as meaning that the calendars are of years in the Julian calendar system.
The Julian Period is a chronological interval of 7980 years beginning 4713 BC. It has been used by historians since its introduction in 1583 to convert between different calendars. 2015 is year 6728 of the current Julian Period. The next Julian Period begins in the year 3268 AD.
Historical Julian dates were recorded relative 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 converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction. Time intervals calculated from differences of Julian Dates specified in nonuniform time scales, such as Coordinated Universal Time (UTC), may need to be corrected for changes in time scales (e.g. leap seconds).^{[3]}
Because the starting point or reference epoch 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. In the following table, times are given in 24hour notation.
In the table below, Epoch refers to the point in time used to set the origin (usually zero, but (1) where explicitly indicated) of the alternative convention being discussed in that row. The date given is a Gregorian calendar date if it is October 15, 1582, or later, but a Julian calendar date if it is earlier. JD stands for Julian Date. 0h is 00:00 midnight, 12h is 12:00 noon, UT unless specified else wise.
Name  Epoch  Calculation  Value for 16:45, 22 January 2015 (UTC)  Notes 

Julian Date  12h Jan 1, 4713 BC  2457045.19792  
Reduced JD  12h Nov 16, 1858  JD − 2400000  57045.19792  
Modified JD  0h Nov 17, 1858  JD − 2400000.5  57044.69792  Introduced by SAO in 1957 
Truncated JD  0h May 24, 1968  JD − 2440000.5  17044  Introduced by NASA in 1979 
Dublin JD  12h Dec 31, 1899  JD − 2415020  42025.19792  Introduced by the IAU in 1955 
Chronological JD  0h Jan 1, 4713 BC  JD + 0.5 + tz  2457045.69792(UT)  Specific to time zone 
Lilian date  Oct 15, 1582 (1)  floor (JD − 2299159.5)  157885  Count of days of the Gregorian calendar^{[6]} 
ANSI Date  Jan 1, 1601 (1)  floor (JD − 2305812.5)  151232  Origin of COBOL integer dates 
Rata Die  Jan 1, 1 (1)  floor (JD − 1721424.5)  735620  Count of days of the Common Era (Gregorian) 
Unix Time  0h Jan 1, 1970  (JD − 2440587.5) × 86400  1421945100  Count of seconds ^{[7]} 
Mars Sol Date  12h Dec 29, 1873  (JD − 2405522)/1.02749  50144.65857  Count of Martian 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 8.3 minutes (498 seconds), that being the time it takes the Sun's light to reach Earth.
To illustrate the ambiguity that could arise, consider the two separate astronomical measurements of an astronomic object from the earth: Assume that three objects — the Earth, the Sun, and the astronomical object targeted, that is whose distance is to be measured — happen to be in a straight line for both measure. However, for the first measurement, the Earth is between the Sun and the targeted object, and for the second, the Earth is on the opposite side of the Sun from that object. Then, the two measurements would differ by about 1000 lightseconds: For the first measurement, the Earth is roughly 500 light seconds closer to the target than the Sun, and roughly 500 light seconds further from the target astronomical object than the Sun for the second measure.
An error of about 1000 lightseconds is over 1% of a lightday, which can be a significant error when measuring temporal phenomena for short period astronomical objects over long time intervals. To clarify this issue, the ordinary Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD.
The Julian day number is based on the Julian Period proposed by Joseph Scaliger in 1583, at the time of the Gregorian calendar reform, as it is the multiple of three calendar cycles used with the Julian calendar:
Its epoch falls at the last time when all three cycles (if they are continued backward far enough) were in their first year together — Scaliger chose this because it preceded all historical dates. Years of the Julian Period are counted from this year, 4713 BC.
Although many references say that the Julian in "Julian Period" refers to Scaliger's father, Julius Scaliger, in the introduction to Book V of his Opus de Emendatione Temporum ("Work on the Emendation of Time") he states, "Iulianum vocavimus: quia ad annum Iulianum dumtaxat accomodata est", which translates more or less as "We have called it Julian merely because it is accommodated to the Julian year." Thus Julian refers to Julius Caesar, who introduced the Julian calendar in 46 BC.
Originally the Julian Period was used only to count years, and the Julian calendar was used to express historical dates within years. In his book Outlines of Astronomy, first published in 1849, the astronomer John Herschel added the counting of days elapsed from the beginning of the Julian Period:
The period thus arising of 7980 Julian years, is called the Julian period, and it has been found so useful, that the most competent authorities have not hesitated to declare that, through its employment, light and order were first introduced into chronology.^{[17]} We owe its invention or revival to Joseph Scaliger, who is said to have received it from the Greeks of Constantinople. The first 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 BC, 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.^{[18]}
Astronomers adopted Herschel's "days of the Julian period" in the late nineteenth century, but used the meridian of Greenwich instead of Alexandria, after the former was adopted as the Prime Meridian after the International Meridian Conference in Washington in 1884. This has now become the standard system of Julian days numbers.
The French mathematician and astronomer PierreSimon Laplace first expressed the time of day as a decimal fraction added to calendar dates in his book, Traité de Mécanique Céleste, in 1799.^{[19]} Other astronomers added fractions of the day to the Julian day number to create Julian Dates, which are typically used by astronomers to date astronomical observations, thus eliminating the complications resulting from using standard calendar periods like eras, years, or months. They were first introduced into variable star work by Edward Charles Pickering, of the Harvard College Observatory, in 1890.^{[20]}
Julian days begin at noon because when Herschel recommended them, the astronomical day began at noon. 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 doubledated most nighttime observations with both Egyptian days beginning at sunrise and Babylonian days beginning at sunset. This would seem to imply that his choice of noon was not, as is sometimes stated, made in order to allow all observations from a given night to be recorded with the same date. When this practice ended in 1925, it was decided to keep Julian days continuous with previous practice.
The Julian day number can be calculated using the following formulas (integer division is used exclusively, that is, the remainder of all divisions are dropped):
The months (M) January to December are 1 to 12. For the year (Y) astronomical year numbering is used, thus 1 BC is 0, 2 BC is −1, and 4713 BC is −4712. D is the day of the month. JDN is the Julian Day Number, which pertains to the noon occurring in the corresponding calendar date.
The algorithm is valid at least for all positive Julian Day Numbers.^{[21]} The meaning of the variables are explained by the Computer Science Department of the University of Texas at San Antonio.
It is worth noting that this algorithm does not follow the NASA^{[22]} or the US Naval Observatory^{[23]}  the convention in these systems being that the Gregorian Calendar did not exist before the date October 15, 1582 (Gregorian). This algorithm effectively backdates the Gregorian calendar onto the Julian calendar for dates before the introduction of the Gregorian calendar. Thus any calculations made with this formula before October 15, 1582, will not agree with these previous ephemerides.
You must compute first the number of years (y) and months (m) since March 1 −4800 (March 1, 4801 BC):
The value 'a' will be 1 for January and February, and 0 for other months. And 'm' will be 0 for March and 11 for February.
All years in the BC era must be converted to astronomical years, so that 1 BC is year 0, 2 BC is year −1, etc. Convert to a negative number, then increment toward zero.
Then, if starting from a Gregorian calendar date compute (note the floor brackets!):
Otherwise, if starting from a Julian calendar date compute:
Note: (153m+2)/5 gives the number of days since March 1 and comes from the repetition of days in the month from March in groups of five:
Mar–Jul:  31 30 31 30 31 
Aug–Dec:  31 30 31 30 31 
Jan–Feb:  31 28 
For the full Julian Date (divisions are real numbers):
So, for example, January 1, 2000, at 12:00:00 corresponds to JD = 2451545.0
The US day of the week W1 can be determined from the Julian Day Number J with the expression:
W1  0  1  2  3  4  5  6 

Day of the week  Sun  Mon  Tue  Wed  Thu  Fri  Sat 
The ISO day of the week W0 can be determined from the Julian Day Number J with the expression:
W0  1  2  3  4  5  6  7 

Day of the week  Mon  Tue  Wed  Thu  Fri  Sat  Sun 
This is an algorithm by Richards to convert a Julian Day Number, J, to a date in the Gregorian calendar (proleptic, when applicable). Richards does not state which dates the algorithm is valid for.^{[25]} All variables are integer values, and the notation "a div b" indicates integer division, and "mod(a,b)" denotes the modulus operator.
variable  value  variable  value 

y  4716  v  3 
j  1401  u  5 
m  2  s  153 
n  12  w  2 
r  4  B  274277 
p  1461  C  −38 
D, M, and Y are the numbers of the day, month, and year respectively.
