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Leap Day: The History Of Why We Do Leap Day And Where It Started

Ancient History | March 1, 2021

The Shepherd 's calendar, 1579, February. Two shepherds in a field with their sheep. (Culture Club/Getty Images)

Leap Day comes around every four years, but why? Why do we tack an extra day onto February every now and then? Just for fun? Some kind of time travel accident? It couldn't be science, could it? It turns out it is.

The Problem With Calendars

Somewhere along the path of human history, people noticed that the Sun and Moon do some predictable things. The Moon goes through its phases, and the Sun sometimes stays longer in the sky and sets in different places. When early civilizations figured that out, they realized they could use the info to mark the passage of the days so they'd know when to plant their crops and hold religious ceremonies and so forth. We hear a lot about the different calendars created by early civilizations, like the Mayans, but we follow a different, perhaps less accurate calendar. Why we have leap year is actually the result of this inaccuracy.

Pope Gregory XIII. (Unknown author/Wikimedia Commons)

Leap Day

In antiquity, there were several different calendar systems, so one of the greatest accomplishments of Julius Caesar was the development of a standardized calendar for use across his vast empire. He sought the advice of the noted astronomer, Sosigenes, who told him the solar year lasted 365 and one-quarter days, so Caesar simply divided the number of days by twelve to create twelve months. There was only one problem: It was off by 11 minutes and 14 seconds annually. That might not seem like a lot of time, but year after year, it really adds up. By the 1500s, the solstices and equinoxes weren't happening when the calendar said they would, and the whole system was a mess.

Pope Gregory XIII was particularly vexed that no one could give him a definitive date for Easter, which is celebrated on the first Sunday after the first full moon after the Spring Equinox, so he commissioned a whole new calendar system. Fortunately, science had come a long way since the days of Julius Caesar, so the Gregorian calendar was based on a more precise 365.2421-day solar year. There was still a remainder to contend with, roughly equal to one-quarter of a day, so Gregory and his astronomy buddies decided to add one full day at the end of the shortest month to the calendar every four years.

While this was a solid, workable solution, it didn’t completely solve the problem. The math actually works out best if leap years occur every four years except on centennial years (i.e., the first year of a century) that are not divisible by 400. That would add way too much confusion to an already confusing system, however, so from time to time, the people at the International Earth Rotation and Reference System Service need to step in. They check the calendar against the solar and lunar alignments to make sure we're all in sync in our timekeeping, and when things get off, they throw in a leap second to get the calendar back on track.

Saint Brigid as depicted in Saint Non's chapel, St Davids, Wales. (Wolfgang Sauber/Wikimedia Commons)

Bachelor's Day

The rarity of leap year made it the subject of folklore and superstition. In Ireland, for example, Leap Day was known as Bachelor's Day, the one time it was socially acceptable for women to propose marriage to men. It was associated with Saint Bridget, who was said to have asked Saint Patrick to permit women to pop the question. In compromise, Ol' Patty selected one day a year it would be allowed, choosing February 29 because he knew it only occurred every four years.

Tags: ancient history | calendar | february

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Karen Harris

Writer

Karen left the world of academic, quitting her job as a college professor to write full-time. She spends her days with her firefighter husband and four daughters on a hobby farm with an assortment of animals, including a goat named Atticus, a turkey named Gravy, and a chicken named Chickaletta.