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Why Do We Have Leap Years?

We've reached the end of February once again, but don't change your calendars to March just yet. The year 2020 is a leap year (also known as an intercalary year or bissextile year), which means that good ole February has 29 days rather than the usual 28. The reason why is cosmic. Although the calendar year we observe has 365 days, it actually takes the Earth 365.242189 days (that's 365 days, five hours, 48 minutes, and 45 seconds) to complete a full orbit around the Sun. To make up for that disparity, an additional day is added every four years so that the heavenly bodies are in sync.

This isn't just to satisfy the neurotic tendencies of astronomers and calendar-makers, however. That quarter of a day would eventually add up — meaning that the seasons wouldn't align with the time of year they're associated with. After a century, for instance, we'd be about 24 days off.

Flat lay of calendar with cup of coffee and paper clips
Credit: northfolk/ Unsplash

Of course, there's more to it. Not every fourth year is a leap year since a complicated if/then scenario excludes a few of them. A year is not a leap year if it's divisible by 100 unless it's also divisible by 400. This is because the difference between a leap year and astronomical year is slightly less than a quarter of a day, so every once in a while a fourth year is skipped. It's rare, though — 1800 and 1900 weren't leap years and 2100, 2200, 2300, and 2500 won't be either. For the next eight decades, however, we won't have to worry about this.

In the meantime, here's an easy way for the forgetful to remember when a long February is upon us: Leap years always fall on presidential election years in the U.S. As for why February was chosen, the answer is as simple as it is practical: It's the only month without 30 or 31 days, which makes it uniquely qualified to bear that burden for us.