Julian Day Calendar
Related Category: Astronomy: General
system of astronomical dating that allows the difference between two dates to be calculated more easily than conventional civil calendars with their uneven months.
Julian Day calendar
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Julian Day: {sometimes Julian Ephemeris Day} [JD] the astronomers' scale of date and time. Used in dialling, for example, for the accurate calculation of the EoT and sun's declination.
Julian Day
The Julian Day is the number of days since the year -4712. The Julian Day begins at 12:00 Noon Greenwich mean time.
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Julian day. A system of counting days from noon 1st January 4713 BC. The name has nothing to do with Julius Caesar but was invented by the mathematician Scaliger who named it in honour of his father, Julius Scaliger.
Julian Day -- A unit of time within the Julian Dating System where the number of ephemeris days that have elapsed since 12h ephemeris time on January 1, 4713 B.C. JD for 1970 January 1 is 2440588.
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Julian Day (JD) Astronomers simplify their timekeeping by merely counting the days, and not months and years. Each date has a Julian Day number (JD), beginning at noon, which is the number of elapsed days since January 1st, 4713 B.C.
Julian day
The Julian date is the interval of time in days and fractions of a day, since January 1, 4713 BC Greenwich noon, Julian proleptic calendar. In precise work, the timescale, e.g., Terrestrial Time or Universal Time , should be specified..
Julian day (NASA SP-7, 1965) The number of each day, as reckoned consecutively since the beginning of the present Julian period on January 1, 4713 B.C.
Interestingly, the Julian day was not named for Julius Caesar, like that calendar was - it was named for Julius Caesar Scaliger, the father of Joseph Scaliger, the French humanist who combined the solar cycle (28 years), Metonic cycle (19 years), ...
Three thousand days in the variation of R Leonis are plotted as visual magnitudes versus the running count of Julian Day number, which begins January 1, 4713 BCE. JD 2452500 is August 13, 2002, 2454750 October 10, 2008.
John Herschel originated the use of the Julian day system in astronomy and made several important contributions to the improvement of photographic processes (Cyanotype).
See Julian Day. calendar day (NASA SP-7, 1965) The period from midnight to midnight. The calendar day is 24 hours of mean solar time in length and coincides with the civil day unless a time change occurs during the day.
For most day-to-day activities on Earth, people don't use Julian days, but the Gregorian calendar, which despite its various complications is quite useful.
This applies for the Julian day. Virtually the only possible variation is using a different reference day, in particular one less distant in the past to make the numbers smaller.
An arbitrary calendar is not synchronized to either the Moon or the Sun; an example is the Julian day used by astronomers.
The fractional part represents the Universal Time. One quirk of the system is that a Julian day begins at noon, rather than midnight. The Julian date for midnight Jan. 1, 1995 is 244 9718.5; for midnight Jan. 1, 1996 the Julian date is 244 0083.5.
 See also: Period, Time, Second, Noon, Sun
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