Gregorian calendar

The Julian calendar drifted. By the 16th century, Easter—supposedly fixed to the spring equinox—had wandered ten days from its astronomical target. Pope Gregory XIII commissioned a correction in 1582: skip ten days (October 4 was followed by October 15) and revise the leap year rule to prevent future drift. The result was a calendar accurate enough that it will not need adjustment for over 3,000 years.

The Julian calendar's error was small—roughly one day every 128 years—but cumulative. Julius Caesar's astronomers in 45 BCE had overestimated the solar year by about 11 minutes. Sixteen centuries later, the accumulated error threatened the Church's ability to calculate Easter correctly, a matter of theological importance.

Gregory's solution modified the leap year rule: century years would be leap years only if divisible by 400. Thus 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not. This dropped three leap days every 400 years, compensating almost exactly for the Julian calendar's excess.

Adoption was not immediate. Catholic countries implemented the Gregorian calendar quickly—Spain, Portugal, and Italy in 1582. Protestant countries resisted a papal reform; Britain and its colonies waited until 1752. Russia retained the Julian calendar until 1918; the Orthodox Church still uses Julian reckoning for liturgical purposes.

The staggered adoption created chaos for historians: events in different countries might have different dates despite occurring simultaneously. The "Old Style" and "New Style" designations track which calendar applied. George Washington's birthday is February 22 (Gregorian) or February 11 (Julian), depending on which system you use.

The Gregorian calendar's accuracy reflects sophisticated astronomical knowledge available by 1582: the solar year was measured precisely enough that the correction could be calculated correctly. What Caesar's astronomers got slightly wrong, Renaissance observations fixed.