Why We Leap: The History of February 29

When you’re a busy working adult student, time matters, and every day counts—especially when you get an extra day.

Time-conscious Night Owls likely have big plans for squeezing as much as they can out of the extra day this month, but do you know why we have a Leap Day every fourth February (with a few exceptions)?

It all comes down to maintaining the seasons. Each year has 365.2422 days. If we did not have that extra day every four years, our seasonal calendars would shift by 0.2422 days every year and summer would eventually occur in January.

Hail Julius Caesar for conscripting his astronomer, Sosigenes of Alexandria, to solve the shifty calendar, which, at the time, had only 355 days. Initially, the Roman Senate tacked on the extra quadrennial day to February by doubling the length of February 23, which marked the rites and festivities to honor the god Terminus. Eventually, February 23 was restored to 24 hours and the Roman Senate added an extra day to February.

Due to a few math errors that affected Easter, Pope Gregory XIII also made modifications to the calendar in 1582, but maintained the overall calendar of antiquity.

That extra 0.2422 days is a tricky fraction to work with—it’s just shy of a quarter of a day, so while it’s almost accurate to say we have Leap Year every four years, that’s not quite right. Most years that are divisible by 4 are Leap Years, but years that are divisible by 100 are not Leap Years—unless they are also divisible by 400, in which case, they do get that extra day.

So 2016, divisible by 4, is a Leap Year. But 2100—divisible by 100—will not be. However, 2000 was, and 2400 will be, because they’re also divisible by 400.

And that’s why Night Owls have an extra day in February to prepare for next month’s goals, finish this month’s goals just under the wire, or relax because they forgot it was a Leap Year and finished everything on February 28.

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