Julian day

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Julian day is used in the Julian date (JD) system of time measurement for scientific use by the astronomy community. Julian date is recommended for astronomical use by the International Astronomical Union.

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Julian Date

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 although it is widely used that way in the earth sciences and computer programming.

The Julian date (JD) is the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon, Julian proleptic calendar.[1] In precise work, the timescale, e.g., Terrestrial Time (TT) or Universal Time (UT), should be specified.[2]

The Julian day number (JDN)[3] is the integer part of the Julian date (JD).[2] The day commencing at the above-mentioned epoch is JDN 0. Negative values can be used for preceding dates, though they predate all recorded history. Now, at 14:15, Thursday December 30, 2010 (UTC) the Julian day number is 2455561.

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