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The Julian calendar, introduced by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar.{{#invoke:Footnotes|sfn}} It took effect in 45 BC (709 AUC), shortly after the Roman conquest of Egypt. It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII. The Julian calendar gains against the mean tropical year at the rate of one day in 128 years. For the Gregorian the figure is one day in 3,226 years. The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%.

The Julian calendar has a regular year of 365 days divided into 12 months, as listed in Table of months. A leap day is added to February every four years. The Julian year is, therefore, on average 365.25 days long. It was intended to approximate the tropical (solar) year. Although Greek astronomers had known, at least since Hipparchus, a century before the Julian reform, that the tropical year was a few minutes shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the year gained about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was corrected by the Gregorian reform of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, years evenly divisible by 100 are not leap years, except that years evenly divisible by 400 remain leap years.<ref>Introduction to Calendars. (15 May 2013). United States Naval Observatory.</ref> Consequently, the Julian calendar is currently 13 days behind the Gregorian calendar; for instance, 1 January in the Julian calendar is 14 January in the Gregorian. Old Style (O.S.) and New Style (N.S.) are sometimes used with dates to indicate either whether the start of the Julian year has been adjusted to start on 1 January (N.S.) even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.) rather than the Gregorian (N.S.). Dual dating uses two consecutive years because of differences in the starting date of the year, or includes both the Julian and Gregorian dates.

The Julian calendar has been replaced as the civil calendar by the Gregorian calendar in all countries which formerly used it, although it continued to be the civil calendar of some countries into the 20th century. Among the last countries to convert to the Gregorian calendar were Russia (in 1918) and Greece (in 1923).<ref name="Norby">Nørby, Toke. The Perpetual Calendar Version 29 February 2000</ref> As of 1930, all countries that were using the Julian calendar had discontinued it. Most Christian denominations in the West and areas evangelized by Western churches have also replaced the Julian calendar with the Gregorian as the basis for their liturgical calendars. However, most branches of the Eastern Orthodox Church still use the Julian calendar for calculating the dates of moveable feasts, including Easter (Pascha). Some Orthodox churches have adopted the Revised Julian calendar for the observance of fixed feasts, while other Orthodox churches retain the Julian calendar for all purposes.<ref>Towards a Common Date of Easter. (5–10 March). World Council of Churches/Middle East Council of Churches Consultation, Aleppo, Syria.</ref> The Julian calendar is still used by the Berber people of North Africa,<ref>The Manipulation of Time - Calendars and Power in the Sahara</ref> and on Mount Athos. In the form of the Alexandrian calendar, it is the basis for the Ethiopian calendar, which is the civil calendar of Ethiopia.


Julian calendar sections
Intro  Table of months  Motivation  Context of the reform  Julian reform  Leap year error  Month names  Year numbering  New Year's Day  From Julian to Gregorian  Eastern Orthodox usage  See also  Notes  Bibliography  External links  

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