Every four years, February 29 appears on the calendar like a distant relative dropping in for a visit: it’s regular enough to be expected, but just infrequently enough that it’s often a surprise. However, leap days play an important role in keeping our calendars on track, and it’s all thanks to Julius Caesar.
Most years, the calendar is made up of 365 days. However, the Earth’s orbit around the sun is actually slightly longer than that, about 365 ¼ days—making the calendar year slightly shorter than a solar year. Which, if you’re trying to establish an accurate system for measuring the passage of time and changing seasons, throws a small wrench in the works, Deanna Conners writes for EarthSky.org.
Julius Caesar was confronted with this dilemma back in 46 B.C.E., when he was developing what came to be known as the Julian Calendar. On the advice of an Alexandrian astronomer named Sosigenes, Caesar decreed that every four years an extra day would be added to the calendar to keep it on track, Conners writes. That way, the calendar would make up for the discrepancy and account for the slightly awkward length of the solar year.
At first, Caesar’s solution seems simple and straightforward: “bank” those extra quarter-days for a few years, then spend them on a leap day. But as the centuries rolled on, people began to notice something was off. The Julian Calendar, which had been adopted throughout the former Roman Empire, was speeding ahead of the solar year. By the end of the 16th century, the calendar year had drifted as much as 10 days ahead, CNN reports.
The problem? The solar year is actually slightly shorter than Caesar and Sosigenes thought.
“It’s not exactly a quarter of an extra day; it’s a little less,” physicist Judah Levine tells Rachel Wise for Quartz. “And so adding one day every four years was too much.”
To be fair to Caesar and Sosigenes, they were only off by a matter of a few decimal points—astronomers now know that a solar year is actually 365.24219 days long. But while it may not seem like much, under the Julian Calendar, that slight error led to a discrepancy of about 11 minutes a year, Wise writes. So in 1582, Pope Gregory decided to reset the calendar and take those calculations into account with something called “the century rule.”
“If a leap year falls on a century, a year ending in double zeroes, you only add a leap day if it’s divisible by 400,” Levine tells Wise. “For that reason 1900 wasn’t a leap year but 2000 was.”
Pope Gregory was also responsible for setting leap day as February 29, instead of adding it on to the end of the year. The calendar won’t skip another leap year until 2100, but that little adjustment has kept our calendars fairly accurate for over 400 years.