About the Gregorian calendar
The Gregorian calendar, also called the Western calendar and the Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582; the decree, a papal bull, is known by its opening words, Inter gravissimas. The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries.
The motivation for the Gregorian reform was that the Julian calendar assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost exactly 11 minutes shorter. The discrepancy accumulates at the rate of about three days every four centuries, resulting in the equinox being on March 11 (a cumulative error of about 10 days since Roman times), and moving steadily earlier in the Julian calendar, at the time of the Gregorian reform. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady movement in the date of the equinox undesirable.
The Gregorian calendar reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory’s time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter. The reform was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio). Lilius’ proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years: this part of the proposal had been suggested before by, among others, Pietro Pitati. Lilio also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.
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