Why Do We Have Leap Years? The Science and History Behind the Extra Day

This year, February will be a little longer than usual due to a leap year. In 2024, Leap Day falls on Friday, Feb. 29, making the year 366 days long instead of the regular 365. But why do leap years occur?

What is the Purpose of a Leap Year?

Leap years exist because, while the world follows a 365-day Gregorian calendar, it actually takes the planet a little more than a year to orbit the sun. According to NASA, it takes Earth 365 days, 5 hours, 48 minutes, and 46 seconds to orbit the sun. The nearly six extra hours are accounted for by adding leap years. Without this adjustment, calendars and seasons would gradually fall out of sync and impact agricultural cycles based on the seasons.

NASA explained, "If we never had leap years, all those missing hours would add up into days, weeks, and even months. Eventually, in a few hundred years, July would actually take place in the cold winter months!"

Why is Leap Day in February?

Leap Day falls in February due to ancient Roman history. The Romans originally had a 10-month calendar, with the calendar not divided into months during winter. Eventually, the Romans established January and February, with February having the fewest days. Julius Caesar then adjusted the calendar to line it up with the sun and added Leap Day via decree. However, the difference in time wouldn't be fully accounted for until Pope Gregory XIII adopted the Gregorian calendar in 1582. This calendar considers all years divisible by four as leap years, with the exception of century years, which must be divisible by 400 to be leap years.

When is the Next Leap Year?

Leap years occur every four years, with the exception of century years that cannot be divided by four. The next leap year will be in 2028, with Leap Day observed on Tuesday, Feb. 29. Following that, the next leap year will be in 2032, with Leap Day falling on Sunday, Feb. 29.