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? 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.
