Buzz
At some point during elementary school, your science teacher likely told you that there are 365 days in a year because that’s the time it takes for Earth to make one full orbit around the sun. What they may not have clarified, though, is that it’s not exactly 365 days—it’s actually closer to 365.2421 days.
So, to ensure our calendar year begins precisely when Earth completes a full orbit, we need to account for an extra quarter of a day each year, or one day every four years. History.com reports that the Egyptians had already adopted this system before Europe caught up in 46 B.C.E., when Roman leader Julius Caesar and astronomer Sosigenes teamed up to create what we now know as the Julian calendar, which consists of 12 months, 365 days, and an extra “leap day” every four years on February 29.
However, rounding 0.2421 up to 0.25 each year caused an issue, as it didn’t quite add up to a full day every four years. This small difference led to the calendar year starting a day earlier than Earth had completed its orbit after 128 years. By the 14th century, the calendar year was beginning 10 days before Earth had finished its orbit around the sun.
In 1582, Pope Gregory XIII attempted to correct the mistake by suggesting that we occasionally skip a leap day. His Gregorian calendar, which we still follow today, dictates that leap days be omitted during years divisible by 100 but not by 400. For example, the year 2000 had a leap day because it’s divisible by both 100 and 400; however, the year 2100 will not, as it’s divisible by 100 but not by 400.
Gregory XIII’s fix to Caesar’s overcompensation is still a bit of an under-correction, meaning we’ll likely have to reassess our leap day system in around 10,000 years.
