Julian calendar

CalendarToday
Gregorian17 September 2019
Julian4 September 2019

The Julian calendar, proposed by Julius Caesar in 708 AUC (46 BC/BCE), was a reform of the Roman calendar.[1] It took effect on 1 January 709 AUC (45 BC/BCE), by edict. It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII.

The Julian calendar is still used in parts of the Eastern Orthodox Church, in parts of Oriental Orthodoxy and Anabaptism,[2] as well as by the Berbers.[3]

During the 20th and 21st centuries, the date according to the Julian calendar is 13 days earlier than (behind) the Gregorian date.

Year length; leap years

The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days. There is a simple cycle of three "normal" years followed by a leap year and this pattern repeats forever without exception. The Julian year is, therefore, on average 365.25 days long. Consequently, the Julian year drifts over time with respect to the tropical (solar) year (365.24217 days).[4]

Although Greek astronomers had known, at least since Hipparchus,[5] a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was largely 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, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years.[6] (Even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years).[4]

The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%, making the Julian 10.8 minutes longer. The accumulated effect of this difference over some 1600 years since the basis for calculation of the date of Easter was determined at the First Council of Nicea means for example that, from 29 February Julian (13 March Gregorian) 1900 and until 28 February Julian (13 March Gregorian) 2100, the Julian calendar is 13 days behind the Gregorian calendar; one day after (i.e. on 29 February Julian or 14 March Gregorian), the difference will be 14 days.