What’s The Deal With Leap Year?

Hey, folks. Happy Leap Year!

*Crickets chirping*

…Yeah, okay, it’s not an actual holiday. But it does represent one of the most important and fascinating aspects about the Earth and our understanding of physics. It’s common knowledge that a year is 365 days; it’s what modern civilization uses to keep track of business performance, industry production, crop harvesting, population growth, radioactive decay, public transit, pizza deliveries, birthdays, Oscar acceptance speeches, and pretty much anything remotely affected by the passage of time. Needless to say, timekeeping is kind of important.

However, it’s inaccurate.

The 365 day per year model is based on the Gregorian Calendar, which was first instituted by Pope Gregory XIII in 1582. It was an update to the far older Julian Calendar, in an attempt to bring the actual day of Easter closer to the day the church thought it was supposed to be celebrated. It was shoehorned in at the end of February because, honestly, the Romans had a long history of treating the month like an afterthought. While altering the basis of time measurement must have been a huge headache for everyone involved – there are still several different calendars spanning various cultures, and Greece didn’t adopt the new calendar until 1923! – it also illustrated the big problem with timekeeping on Earth: it doesn’t divide into perfect increments. Earth’s orbit is 365.256 days. How do you add .256 of a day to a calendar? Hence why Leap Day happens every four years; the calendar skips over that .256, then multiplies by a whole number of those years to make up for it. .256 x 4 = 1.024, which is just enough to make an extra day and leftovers small enough that no one will really care…

For now, anyway.

Here’s the thing: How we measure Leap Years – and thus the passage of time – is going to have to change in the far future. The algorithm that the Gregorian Calendar uses is fine for our current civilization; it’s as accurate and easily applicable as it needs to be. But on long-term timescales – we’re talking tens of thousands of years – it won’t be able to keep up with the astronomy and physics it’s based upon. Thanks to the effects of the Moon’s gravity, Earth’s rotation is actually slowing down, creating longer days. We’ve already introduced Leap Seconds to make up for the discrepancies and inconsistencies in the planet’s rotation. That’s all assuming that nothing crazy happens with Earth’s orbit, or if it remains stable enough until humanity dies off and the sun goes red giant and destroys the planet in a few billion years.

…Happy Leap Year!

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