Our calendar, the gregorian calendar, take its name from the pope Gregoire XIII who made the rules built during the XVIth century. The target was to ameliorate a much more ancient one : the Julian calendar, which name comes from Jules César who created it 16 centuries before. This calendar should be an improvement of a more ancient one, supposed to date to the foundation of Roma !

In this calendar, the 'normal' duration of a year is made longer up to 365 days by increasing the length of the 12 months and the insert is reduced to one day every 4 years, reducing the duration of a quadriennal cycle to 1,461 days (3 years of 365 days and 1 year of 366 days). The year duration is thus estilated to 365 days 1/4, close to the real duration. But the duration of the julian year is too long of a bit more than 11 minutes and au fil du temps, the calendar finished up to put back of several days.

With time, the Julian calendar moved back in relation to the seasons, until it was 10 days late in the XVIth century, and the pope Gregoire XIII added a calendar reform to a breviary reform.

The new rules was inspired by religious considerations, then the calendar was not wedged on the equinox date as it was at the beginning of the julian calendar (25th of march), but on the one of the year 325 (21st of march).

To do that, 10 days were deleted in the year 1582, where the 4th of october was immediately followed by the 15th of october. To avoid new drifts, the overrate of the julain year was corrected by canceling 3 days every 400 years, by ignoring the leap year rule in the secular years, but not for the one divisible by 400 (1600 and 2000 are leap year, 1700, 1800, 1900 and 2100 are not).


©Alain Opériol - 1991-2010 (www.encinaal.fr)