Julian calendar

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The Julian calendar began in 45 BC (709 AUC) as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year (known at least since Hipparchus).

The Julian calendar has a regular year of 365 days divided into 12 months with a leap day added to February every four years. The Julian year is, therefore, on average 365.25 days long.

The more modern Gregorian calendar eventually superseded the Julian calendar: the reason is that a tropical year (or solar year), which determines the cycle of seasons, is actually about 11 minutes shorter than 365.25 days. These extra 11 minutes per year in the Julian calendar caused it to gain about three days every four centuries, when compared to the observed equinox times and the seasons. In the Gregorian calendar system, first proposed in the 16th century, this problem was dealt with by dropping some calendar days, in order to realign the calendar and the equinox times. Subsequently, the Gregorian calendar drops three leap year days across every four centuries.

The Julian calendar remained in use into the 20th century in some countries as a civil calendar, but has been replaced by the Gregorian calendar in nearly all countries.[1] The Roman Catholic Church, the Eastern Rite Catholic Churches and Protestant churches have replaced the Julian calendar with the Gregorian calendar; however, the Orthodox Church (with the exception of Romania, Estonia and Finland) still use the Julian calendar for calculating the dates of moveable feasts. 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.[2] The Julian calendar is still used by the Berber people of North Africa, and on Mount Athos.

The notation "Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar. This notation is used in reference to dates from tsarist Russia (the country did not switch to the new calendar until 1918).

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