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Gregorian calendar(grĕ-gor -ee-an, -goh -ree-) The calendar that is now in use throughout most of the world and that was instituted in 1582 by Pope Gregory XIII as the revised version of the Julian calendar. The simple Julian four-year rule for leap years was modified so that when considering century years only one out of four, i.e. only those divisible by 400, were to be leap years: 1700, 1800, and 1900 were not leap years. There are therefore 365.2425 days per year averaged over 400 years. This greatly reduced the discrepancy between the year of 365.25 days used in the Julian calendar and the 365.2422 days of the tropical year, which had resulted in the accumulation of 14 days over the centuries.
The revision came into effect in Roman Catholic countries in 1582, the year being brought back into accord with the seasons by eliminating 10 days from October: Thursday Oct. 4 was followed by Friday Oct. 15. The vernal equinox, which would have occurred on Mar. 11 and which had originally fallen on Mar. 25 in Julius Caesar's time was thus adjusted to Mar. 21. Gregory also stipulated that the New Year should begin on Jan. 1. Non-Catholic countries were slow to accept the advantages of the Gregorian reform. Britain and its colonies switched in 1752 when an additional day had accumulated between old and new calendars: Sept. 2, 1752 was followed by Sept. 14 and New Year's Day was changed from Mar. 25 to Jan. 1, beginning with the year 1752. The very slight discrepancy between the Gregorian year and the tropical year amounts to about three days in 10 000 years.
(New Style), the calendar system that replaced the insufficiently accurate Julian calendar (Old Style). In contrast to the latter, the Gregorian calendar specified that century years were to be leap years only if they were evenly divisible by 400. The Gregorian calendar was first adopted by a number of European countries in 1582. The USSR adopted it in February 1918.