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The '''Julian calendar''', named after [[Julius Caesar]], has twelve months in a year and a leap day every four years. Thus a Julian year is exactly 365.25 days. The astronomical year is slightly shorter, causing the Julian calendar to "drift" out of sync with the seasons by one day every 128 years. This defect is corrected in the [[Gregorian calendar]], a modified version of the Julian calendar created by Pope Gregory XIII in 1582. The Julian calendar was used in Britain and its colonies until 1752, when they switched to the Gregorian calendar. George Washington was born in colonial Virgina on February 11, 1731 by the Julian calendar, but on February 22, 1732 according to the Gregorian calendar. Russia used the Julian calendar until 1918.

The '''Julian calendar''' is a calendar that was formerly in widespread use named after Roman dictator [[Julius Caesar]]. It has twelve months in a year and a leap day every four years. Thus a Julian year is exactly 365.25 days. The astronomical year is slightly shorter, causing the Julian calendar to "drift" out of sync with the seasons by one day every 128 years. This defect is corrected in the [[Gregorian calendar]], a modified version of the Julian calendar now in general use. The Julian calendar was used in Britain and its colonies until 1752, when they switched to the Gregorian calendar. George Washington was born in colonial Virgina on February 11, 1731 by the Julian calendar, but on February 22, 1732 according to the Gregorian calendar. Russia used the Julian calendar until 1918.

The calendar was designed by Sosigenes of Alexandria and instituted in Rome by Caesar as dictator in 46 BC. The previous Roman calendar featured the insertion of intercalary days by an official called the pontiff, who could abuse this authority for political purposes. By Caesar's time, the calendar was dramatically out of sync with the seasons. Sosigenes designed the calendar so that the spring equinox would occur each year on March 25, but the calendar soon drifted out of alignment. The Gregorian calendar is aligned so that the equinox falls on March 21.

The calendar was designed by Sosigenes of Alexandria and instituted in Rome by Caesar as dictator in 46 BC. The previous Roman calendar featured the insertion of intercalary days by an official called the pontiff, who could abuse this authority for political purposes. By Caesar's time, the calendar was dramatically out of sync with the seasons. Sosigenes designed the calendar so that the spring equinox would occur each year on March 25, but the calendar soon drifted out of alignment. The Gregorian calendar is aligned so that the equinox falls on March 21.