The new calendar was thought up by an advisor to Pope Gregory XIII, called Christopher Clavius. He suggested trimming back the calendar a little by adjusting the way leap years were added. Remembering that every fourth year was a leap year, Clavius suggested the rule should be that any year number that was divisble by 4 should be a leap year but that centenary years (1600, 1700, 1800 etc.) should NOT. That put a one-day brake on the calendar every 100 years.
Clavius went one step further still because he calculated that that was a tiny bit too large a correction. His final tweak was to let years that could be divided by 400 (1600, 2000, 2400 etc.) break this rule and be leap years.
Since this correction meant that the calendar would be accurate to better than a day for over three thousand years, Calvius thought this was a neat solution and so too did Pope Gregory. The change to use this new 'Gregorian calendar' was made first in Catholic countries in 1582 but the UK didn't change to it until 1752. |
