The Modern Calendar Turns 425

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On February 24 1582, Pope Gregory XIII enlisted the help of astronomers and mathematicians to correct the Julian calendar, which was 10 days out of sync with the seasons. The adjustment would bring the date for Easter back into realignment with the vernal equinox (commonly referred to as the first day of spring).

Because it takes the Earth a little more than 365 days to travel around the Sun, calendars eventually shift out of line with the seasons unless adjustments are made. The Julian system, introduced by the Romans in 46 BC, was advanced for its time, inserting an extra (“leap”) day to the calendar every four years. But this still added 11 minutes every year. The Gregorian correction made a small change in the rule about leap years.*

To adjust the calendar to the seasons, 10 days were removed from October 1582 (October 5 became October 15). Most Catholic countries in Europe adopted the Gregorian calendar immediately. Britain and the American colonies reluctantly adopted the new system in 1752. By then, a jump of 11 days was necessary to correct the imbalance. A legal degree turned midnight on September 2, into September 14.

Anyone living at the time of the skip had to adjust their thinking as well as their birth dates. To avoid confusion, dates were referred to as “Old Style” or “New Style.” Officials throughout the British Empire were also forced to quell taxpayers’ fears that they would have to fork over money for days that never existed. A surviving story, most likely folklore, claims some people believed their lives were being shortened and rioted to get their eleven days back.

*No century year is a leap year unless it is exactly divisible by 400 (example: 1600, 2000); years evenly divisible by 4,000 are designated as common (not leap) years. This will keep the calendar accurate to within one day in 20,000 years.