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Julian Date The Julian Date (JD) is the Julian day number plus the Decimal Fraction Of The Day that has elapsed since noon. 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. The term ''Julian date'' is also used to refer to:
The use of ''Julian date'' to refer to the day-of-year (ordinal date) is usually considered to be incorrect. Alternatives
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.
::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.
:: RJD = JD - 2400000
::TJD = JD - 2440000.5 :or ::TJD = (JD - 0.5) mod 10000
::DJD = JD - 2415020
::RD = JD - 1721424.5 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 — Scaliger chose this because it pre-dated all historical dates. ''Note:'' although many references say that the "Julian" in "Julian day" refers to Scaliger's father, , who introduced the Julian calendar in 46 BC . In his book ''Outlines of Astronomy'', first published in 1849 , the astronomer John Herschel wrote: 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 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. Astronomer s 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 Clock s. 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 (except for JD) are integer divisions, meaning the remainder in the division is discarded. The months January to December are 1 to 12. Astronomical Year Numbering is used, thus 1 BC is 0, 2 BC is −1, and 4713 BC is −4712.'' For a date in the Gregorian calendar (at noon): For a date in the Julian calendar (at noon): For the full Julian Date, not counting Leap Seconds (divisions are real numbers): So, for example, 1 January 2000 at midday corresponds to JD = 2451545.0 The day of the Week can be determined from the Julian day number by calculating it Modulo 7, where 0 means Monday. See also Footnotes
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