Julian day

Julian day
(JD)
   a continuous count of days beginning with January 1, 4713 BC (-4712 CE), which is start of what is called the Julian period. The French scholar Joseph Justus Scaliger (1540-1609) introduced the Julian period in 1582 (the same year the Gregorian calendar was proclaimed), defining it to be 7980 years, the product of the 28-year cycle of the Julian calendar (after which the days of the week recur on the same dates), the 19-year Metonic cycle (after which the phases of the Moon recur on the same dates), and the 15-year indiction cycle (a unit of civil time in ancient Rome). It happens that 4713 BC is the last year in which all three cycles started simultaneously. In 1849 the British astronomer John Herschel introduced the Julian day as a means of providing an exact date for astronomical events independent of all calendars. The Julian day begins at noon Universal Time, and exact times of observations are expressed using decimal fractions of the Julian day. The first moment of the year 2004 CE, Universal Time, was JD 2 453 005.5. See also modified Julian day.

Dictionary of units of measurement. 2015.

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  • Julian day — JDN redirects here. For the military IT system, see Joint Data Network. For the comic book character Julian Gregory Day, see Calendar Man. Not to be confused with Julian year (disambiguation). Julian day is used in the Julian date (JD) system of… …   Wikipedia

  • Julian Day — Astron. a serial number equal to the number of days elapsed since January 1, 4713 B.C., proposed by Joseph Scaliger in 1582 and used in astronomical calculations: January 1, 1965, at noon, Greenwich Civil Time, was Julian Day 2,438,762.0. Abbr.:… …   Universalium

  • Julian Day — Astron. a serial number equal to the number of days elapsed since January 1, 4713 B.C., proposed by Joseph Scaliger in 1582 and used in astronomical calculations: January 1, 1965, at noon, Greenwich Civil Time, was Julian Day 2,438,762.0. Abbr.:… …   Useful english dictionary

  • julian day number — noun Usage: usually capitalized J : the number of a day in the Julian day calendar (as 2,436,934 for January 1, 1960) …   Useful english dictionary

  • 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 8.4 minutes (i.e. 507 seconds or 0.00587 days), that being the time it takes the Sun s light to reach… …   Wikipedia

  • modified Julian day — (MJD)    a count of days used by astronomers, space agencies, and others. Astronomers have long used the Julian day, a count of days beginning at noon Universal Time January 1, 4713 BC, as a means of specifying a date independent of all calendars …   Dictionary of units of measurement

  • julian day calendar — noun Usage: usually capitalized J : a system used especially by astronomers of numbering days consecutively from the arbitrarily selected point of the year 4713 B.C. instead of by cycles of days …   Useful english dictionary

  • Julian calendar — The Julian calendar began in 45 BC (709 AUC) as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year (known at… …   Wikipedia

  • Julian year (astronomy) — In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of 86,400 SI seconds each, totalling 31,557,600 seconds. That is the average length of the year in the Julian calendar used in Western… …   Wikipedia

  • Day count convention — In finance, a day count convention determines how interest accrues over time for a variety of investments, including bonds, notes, loans, mortgages, medium term notes, swaps, and forward rate agreements (FRAs). This determines the amount… …   Wikipedia

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