Gregorian calendar

Gregorian calendar

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 365¹⁄₄ 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.

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Gregorian calendar - The system of dates used by most of the world. The Gregorian calendar was proposed by the Calabrian doctor Aloysius Lilius and was decreed by, and named after, Pope Gregory XIII on 1582-02-24. It corrected the Julian calendar whose years were slightly longer than the solar year. It also replaced the lunar calendar which was also out of time with the seasons. The correction was achieved by skipping several days as a one-off resynchronisation and then dropping three leap days every 400 hundred years. In the revised system, leap years are all years divisible by 4 but excluding those divisible by 100 but including those divisible by 400. This gives a mean calendar year of 365.2425 days = 52.1775 weeks = 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds. leap seconds are occasionally added to this to correct for irregularities in the Earth's rotation.