Happy Leap Day! Brought to you by Julius Caesar
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February 29 appears on the calendar every four years. It's like a distant relative. It drops in for a visit. It's regular enough to be expected. But it's also infrequent. It's often a surprise.
Leap are important. They keep our calendars on track. It's all thanks to Julius Caesar.
The calendar is usually made up of 365 days. But Earth's orbit around the sun is slightly longer than that. It's about 365 1/4 days. This makes the calendar year slightly shorter than a solar year. This throws a small wrench in the works. That's if you're trying to establish an accurate system for measuring the passage of time. And measuring the changing seasons. That's what Deanna Conners wrote. She wrote it for EarthSky.org.
Julius Caesar was confronted with this dilemma. That was back in 46 B.C.E. He was developing a calendar. It came to be known as the Julian calendar. Caesar said that every four years an extra day would be added to the calendar. This was on the advice of an Alexandrian astronomer. His name was Sosigenes. This would to keep it on track, Conners writes. The calendar would make up for the difference. It would account for the slightly odd length of the solar year.
Caesar's solution seems simple at first. It seems straightforward. You "bank" the extra quarter days. You do this for a few years. Then you spend them on a leap day. But the centuries rolled on. People began to notice something was off.
The Julian calendar was adopted. It was used throughout the former Roman Empire. But it was speeding ahead of the solar year. The calendar year had drifted. This was by the end of the 16th century. It was as much as 10 days ahead. That's according to CNN.
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." That's what physicist Judah Levine told Rachel Wise. She was writing for Quartz. "And so adding one day every four years was too much."
Caesar and Sosigenes were only off by a few decimal points. Astronomers now know that a solar year is actually 365.24219 days long. This may not seem like much. Not under the Julian calendar. But that slight error led to a problem. It was off by about 11 minutes a year. That's according to Wise. Pope Gregory decided to reset the calendar. This was in 1582. He took those calculations into account. He came up with an idea. It was 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." That's what Levine told 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. That's instead of adding it on to the end of the year. The calendar won't skip another leap year until 2100. That little adjustment has kept our calendars fairly accurate for over 400 years.