Why Do We Have Leap Year?

Even though the standard calendar year is 365 days, Earth actually takes 365 days 5 hours 48 minutes and 46 seconds to go completely around the sun. (This is called a solar year.) In order to keep the calendar cycle synchronized with the seasons, one extra day is (usually) added every four years as February 29.

Where did this practice come from? Who came up with the idea to add an extra day to the year?

What is the Julian calendar?

There’s a very long (and complicated) story behind the history of the leap year. We could start with the early Egyptians (and other ancient civilizations) and their use of intercalation, or the practice adding extra days or months to the calendar, usually done for the sake of aligning time with solar and lunar schedules.

But we’re going to fast forward for the sake of brevity and leap right into an explanation of how Julius Caesar affected timekeeping.

The Julian calendar (established by Caesar in 46 BCE) introduced the Egyptian solar calendar to the Roman world, standardized the 365-day year, and was the first attempt at using a leap year to match the calendar year to the solar year. However, this first version of a leap year was different than the one we know today: the Julian calendar didn’t have a February 29, rather February 24 was doubled (lasting 48 hours) every four years.

But this didn’t fix the problem. There were still 11 minutes and 14 seconds unaccounted for. This doesn’t sound like much, but the seasons had shifted 10 days by the 16th century. In 1582, Pope Gregory XIII created a slightly modified calendar to try to make the calendar match the solar year as closely as possible so the seasons wouldn’t shift again in the future. In order to accomplish this, the Pope’s new calendar had to have some oddly specific rules to account for all of the math going on.

What is the Gregorian calendar?

Called the Gregorian calendar, this new calendar system said that no century year (like 1900) would be a leap year except for centuries divisible by 400 (like 2000). In order to put the seasons back to where they should be, the Pope eliminated October 5 through October 14, 1582. The calendar moved directly from the fourth to the fifteenth that year to align the dates with the seasons again. It feels almost like science fiction to think that 10 full days were removed from the calendar in the year 1582. At this point, however, the extra day was still accounted for with a longer February 24 every four years.

Eventually, adding an extra day at the tail end of February evolved through custom and time—it is occasionally referred to in official records—and was recognized officially in England with the passage of the Calendar (New Style) Act 1750. February 29 became the extra day we know today.

But where does the phrase leap year come from?

The earliest recorded use of the English phrase leap year comes from the late 1300s, in the form of the Middle English lepe yere, used in the context of the Julian Calendar.

The word leap in leap year refers to the figurative “leap” forward of a day that occurs during it because of the extra day. During non-leap years (known as common years), each day of the year moves forward one day of the week because a year is 52 weeks and one day long. For example, March 15 was a Tuesday in 2022 and a Wednesday in 2023. If 2024 was a common year, March 15 would be a Thursday but it “leaps” past Thursday and is a Friday instead because 2024 is a leap year.

This figurative use of leap to refer to the “leaping” of days has been recorded as early as the 10th century in Old English texts.