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Solar dating system now in general use. It was proclaimed in 1582 by Pope Gregory XIII as a reform of the Julian calendar. By the Julian reckoning, the solar year comprised 365 days. The addition of a leap day every four years was intended to maintain correspondence between the calendar and the seasons; however, a slight inaccuracy in the measurement of the solar year caused the calendar dates of the seasons to regress almost one day per century. By Pope Gregory's time, the Julian calendar was 10 days out of sync with the seasons; in 1582, to bring the vernal equinox (and thus Easter) back to its proper date, 10 days were dropped (October 5 became October 15). Most of Catholic Europe soon adopted the new calendar; Great Britain and its colonies (1752) and Russia (1918) followed much later. The Gregorian calendar differs from the Julian only in that no century year is a leap year unless it is exactly divisible by 400 (e.g., 1600, 2000). A further refinement, the designation of years evenly divisible by 4,000 as common (not leap) years, will keep the calendar accurate to within one day in 20,000 years.
This entry comes from Encyclopædia Britannica Concise. For the full entry on Gregorian calendar, visit Britannica.com.