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Julian day is the 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),<ref>Dershowitz & Reingold 2008, 15.</ref> a date at which three multi-year cycles started and which preceded any historical dates.<ref>Both of these dates are years of the Christian or Common Era (which has no year 0 between 1 BC and 1 AD). Astronomical calculations generally include a year 0, so these dates should be adjusted accordingly (i.e. the year 4713 Before Christ becomes astronomical year number −4712, etc.). In this article, dates before October 15, 1582 are in the (possibly proleptic) Julian calendar and dates on or after October 15, 1582 are in the Gregorian calendar, unless otherwise labelled.</ref> For example, the Julian day number for the day starting at 12:00 UT on January 1, 2000, was 2,451,545.<ref>McCarthy & Guinot 2013, 91–2</ref>

The Julian date (JD) of any instant is the Julian day number for the preceding noon in Greenwich Mean Time plus the fraction of the day since that instant. Julian dates are expressed as a Julian day number with a decimal fraction added.<ref name="IAU">"Resolution B1" 1997.</ref> For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2,456,293.520833.<ref>US Naval Observatory 2005</ref>

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. 2018 is year 6731 of the current Julian Period. The next Julian Period begins in the year 3268 AD.


Julian day sections
Intro   Terminology    History    See also    References   

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