The History of Leap Year

Leap Day on a calendar

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A leap year is a year with 366 days, instead of the usual 365. Leap years are necessary because the actual length of a year is nearly 365.25 days, not 365 days as commonly stated. Leap years occur every four years, and years that are evenly divisible by four (2020, for example) have 366 days. This extra day is added to the calendar on February 29.

However, there is one exception to the leap year rule involving century years, like the year 1900. Since a year is actually slightly less than 365.25 days, adding an extra day every four years results in about three extra days being added throughout 400 years. For this reason, only one out of every four century years is considered a leap year. Century years are only considered leap years if they are evenly divisible by 400. Therefore, 1700, 1800, 1900, and 2100 were not leap years. But 1600 and 2000 were leap years.

Julius Caesar, Father of Leap Year

Julius Caesar was behind the origin of leap year in 45 BCE. The early Romans had a 355-day calendar and to keep festivals occurring around the same season each year, a 22- or 23-day month was created every second year. Julius Caesar decided to simplify things and added days to different months of the year to create the 365-day calendar; the actual calculations were made by Caesar's astronomer, Sosigenes. Every fourth year following the 28th day of Februarius (February 29) one day was to be added, making every fourth year a leap year.

In 1582, Pope Gregory XIII further refined the calendar with the rule that leap day would occur in any year divisible by four as described previously.

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Bellis, Mary. "The History of Leap Year." ThoughtCo, Apr. 5, 2023, Bellis, Mary. (2023, April 5). The History of Leap Year. Retrieved from Bellis, Mary. "The History of Leap Year." ThoughtCo. (accessed June 4, 2023).