Most people know a leap year adds February 29 every four years. Fewer know that 1900 was not a leap year, 2000 was, and 2100 will not be. The rules exist because Earth’s orbit does not fit neatly into whole days.
Key Takeaways
- The tropical year, from one spring equinox to the next, is about 365.2422 days.
- The Julian calendar added a leap day every four years, which overcorrected slightly.
- The Gregorian calendar, introduced in 1582, skips leap years in century years not divisible by 400.
- Different countries adopted the Gregorian calendar at very different times.
The problem
If a calendar had 365 days every year, it would drift by almost a quarter of a day per year relative to the seasons. After a century the equinox would fall about 24 days off.
Julius Caesar’s fix
The Roman calendar had become a mess by the 1st century BC. On the advice of the astronomer Sosigenes of Alexandria, Julius Caesar introduced the Julian calendar in 46 BC with a 365-day year and a leap day every fourth year, averaging 365.25 days. To realign the calendar, 46 BC was stretched to 445 days, sometimes called the “year of confusion”.
A small overcorrection
365.25 is about 11 minutes longer than the tropical year. Over centuries this added up. By the 1500s the spring equinox had drifted about 10 days earlier on the calendar, which mattered to the Church because Easter’s date depends on it.
The Gregorian reform
In 1582 Pope Gregory XIII introduced the Gregorian calendar. Two changes:
- Ten days were dropped. Thursday, October 4, 1582 was followed by Friday, October 15, in countries that adopted it immediately.
- A new leap year rule: years divisible by 4 are leap years, except years divisible by 100, unless they are also divisible by 400.
So 1600 and 2000 were leap years; 1700, 1800 and 1900 were not; 2100 will not be. This averages 365.2425 days, close to the tropical year.
Uneven adoption
Catholic countries like Spain, Portugal and Italy switched in 1582. Britain and its colonies, including what became the United States, did not switch until 1752, when 11 days were dropped: September 2 was followed by September 14. Russia switched in 1918 after the revolution, and Greece in 1923.
This is why George Washington’s birthday is listed as February 22, 1732, though under the calendar in use at his birth it was February 11, 1731, because the new year also changed from March 25 to January 1.
Leap seconds
Earth’s rotation is not perfectly regular, so leap seconds have occasionally been added to atomic time. In 2022 the General Conference on Weights and Measures voted to stop adding leap seconds by 2035.
Leap day traditions
There is a tradition, linked in folklore to Ireland, that women may propose marriage on February 29. People born on leap day, sometimes called leaplings, often celebrate on February 28 or March 1 in common years.
Frequently Asked Questions
Will the Gregorian calendar drift? Very slowly: about one day every few thousand years.
Why February? February was the last month of the early Roman year, and it was already the short month.
Do other calendars use leap years? Yes. Many lunar and lunisolar calendars add leap days or months.
The bottom line
Leap years keep the calendar in step with the seasons. The century rule fixes the small overcorrection of adding a day every four years.
