Julian day refers to a 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.) The Julian day number for today, 23 November 2012, is 2456255.
The Julian Date (JD) of any instant is the Julian day number for the preceding noon plus the fraction of the day since that instant. Julian Dates are expressed as a Julian day number with a decimal fraction added. The current Julian Date is 2456255.1564468.
The term Julian date may also refer outside of astronomy to the day-of-year (ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry, or to dates in the Julian calendar.
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. 2012 is year 6725 of the current Julian Period. The next Julian Period begins in the year 3268 AD.
Read more about Julian Day: Time Scales, Alternatives, History, Calculation
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