We only get an extra day in the year, a “leap day,” every four years… ish. And it all has to do with science, politicians, and the church. Sound familiar? Leap Year started way back in ancient times, but the same things are influencing our daily lives today. Let’s review how Leap Year got started.
The Science
It takes the Earth 365 and 1/4 days to orbit around the sun. (365.242190 days if you want to be exact.) It takes our Earth 24 hours to rotate on its axis and complete one day. Since the time of Earth’s orbit isn’t divided evenly with the time of one day, we have some extra time building up each year. A “common year” is really 52 weeks plus one day. If we didn’t account for it, the seasons would start to drift, and we’d be celebrating Thanksgiving when the tulips are blooming. So, the Roman Emperor Julius Caesar introduced a Leap Year in BCE 45 to keep the Roman Calendar aligned with the solar calendar (which is based on Earth’s relationship to the sun.) Which gets us into the politics.
The Politics (and the science)
Ancient Hebrew, Buddhist, and Chinese calendars were based on the position of the moon as well as the earth’s relationship to the sun. Hence, they were called “lunisolar.” One year measured by lunar cycles was 11 days off from one year measured by the earth’s orbit. So, the people in control of such things would add extra time, called “intercalary months” into the calendar to make up the difference. The ancient Roman calendar had 10 months plus a loosely defined winter period called “Mercedonius.” (Ugh, imagine not knowing how long winter would be and depending on politicians to decide!) They eventually added January and February to extend the year but still had Mercedonius. In fact, consuls decded when to add Mercedonius, and they manipulated the calendar for political gain. (Again, sound familiar?)
By the time Julius Caesar became emperor of Rome, the Roman calendar and the solar year had fallen far out of line. When Caesar had traveled in Egypt, he took a liking to the way they set up their calendar, which was a 365 day year with one extra day added every four years. But, it wasn’t a perfect fix.
Enter The Church
By the 16th Century, the Roman Catholic Church noted a significant shift. The holy day of Easter was slipping away from its regular time, the first Sunday after the first full moon following the vernal equinox, by roughly ten days. So then Pope Gregory XIII called for an updated calendar, one that dictated that Leap Years would not happen EVERY four years. If a year was divisible by 100, but NOT by 400, that Leap Year was skipped. So, 2000 was a leap year, but 1700, 1800, and 1900 were not. And that’s how the Gregorian Calendar, the one we follow today, was made.
Why is it called Leap Year?
As I said earlier, a common year is 52 weeks + one day. That means as the years pass, the day of your birthday shifts forward one day. If your birthday was on Saturday one year, it will be on Sunday the next year. However, with the addition of the Leap Day in Leap Years, the day of most people’s birthday “leaps” forward an extra day. Hence, the name. People born on Leap Day are called “Leaplings.”
Myths and Legends
Leap Day has some myths and legends associated with it. One legend says that in 5th Century Ireland, St. Bridget complained to St. Patrick that women never got to propose marriage. So, St. Patrick allowed that on Leap Days, women could propose. (I have not confirmed this myth with research.) Apparently, in some countries, Leap Day has a negative connotation, much like Friday the 13th. In Greece, people think Leap Day is bad luck, and no one gets married on Leap Day.
Learn more about Leap Year in this post I wrote, or by visiting History.com or The National Air and Space Museum article on the Science of Leap Year.
Thanks for getting nerdy with me!
I would HATE to have no idea when seasons were coming — I need the order! It’s interesting how each part of the Leap Day idea evolved over time. Almost collaboration?
Yes – more proof that it is hard to get something totally “right” and our ideas evolve over time. 😉