Julian calendar

The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people (also known as the Berbers). For a quick calculation, between 1901 and 2099 the much more common Gregorian date equals the Julian date plus 13 days.

The Julian calendar was proposed in 46 BC by (and takes its name from) Julius Caesar, as a reform of the earlier Roman calendar, which was largely a lunisolar one. It took effect on 1 January 45 BC, by his edict. Caesar's calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years, until 1582 when Pope Gregory XIII promulgated a revised calendar. Ancient Romans typically designated years by the names of ruling consuls; the Anno Domini system of numbering years was not devised until 525, and became widespread in Europe in the eighth century.

The Julian calendar has two types of years: a normal year of 365 days and a leap year of 366 days. They follow a simple cycle of three normal years and one leap year, giving an average year that is 365.25 days long. That is more than the actual solar year value of approximately 365.2419 days (the current value, which varies), which means the Julian calendar gains one day every 129 years. In other words, the Julian calendar gains 3.1 days every 400 years.

Gregory's calendar reform modified the Julian rule by eliminating occasional leap days, to reduce the average length of the calendar year from 365.25 days to 365.2425 days and thus dramatically reducing the Julian calendar's drift against the solar year: the Gregorian calendar gains just 0.1 day over 400 years. For any given event during the years from 1901 through 2099, its date according to the Julian calendar is 13 days behind its corresponding Gregorian date (for instance Julian 1 January falls on Gregorian 14 January). Most Catholic countries adopted the new calendar immediately; Protestant countries did so slowly in the course of the following two centuries or so; most Orthodox countries retain the Julian calendar for religious purposes but adopted the Gregorian as their civil calendar in the early part of the twentieth century.

The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27- or 28-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 or 23 days after the first 23 days of February; the last five days of February, which counted down toward the start of March, became the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days. Some say the mensis intercalaris always had 27 days and began on either the first or the second day after the Terminalia (23 February).

If managed correctly, this system could have allowed the Roman year to stay roughly aligned to a tropical year. However, since the pontifices were often politicians, and because a Roman magistrate's term of office corresponded with a calendar year, this power was prone to abuse: a pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power.

Caesar's reform was intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This proved useful very soon after the new calendar came into effect. Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier. A century later, when Pliny dated the winter solstice to 25 December because the sun entered the 8th degree of Capricorn on that date, this stability had become an ordinary fact of life.

Although the approximation of 365+1⁄4 days for the tropical year had been known for a long time, ancient solar calendars had used less precise periods, resulting in gradual misalignment of the calendar with the seasons.