The Gregorian Calendar
The Gregorian Calendar is a revision of the Julian Calendar which was instituted in a papal bull by Pope Gregory XIII in 1582. The reason for the calendar change was to correct for drift in the dates of significant religious observations (primarily Easter) and to prevent further drift in the dates.
The important effects of the change were:
- Drop 10 days from October 1582, to realign the Vernal Equinox with 21 March
- Change leap year selection so that not all years ending in "00" are leap years.
- Change the beginning of the year to 1 January from 25 March
Despite a frequently repeated factoid these days, there is no special treatment of years which are divisible by 4000. The main aspect that gets any attention these days is the leap year rule. The change in the number frequency of leap years (by dropping 3 every 400 years) slightly changes the average year length to something closer to reality.
Adoption of the new calendar was essentially immediate within Catholic countries. In the Protestant countries, where papal authority was neither recognized not appreciated, adoption came more slowly.
England finally adopted the new calendar in 1752, with eleven days removed from September. The additional day came because the old and new calendars disagreed on whether 1700 was a leap year, so the Julian calendar had to be adjusted by one more day.
The Gregorian year length gives an error of one day in approximately 3,225 years.
