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• To understand leap year, you have to know this one fact: Earth rotates 365.24219 times during one full orbit of the sun.

• That's right, a year is not 365 days.

• It's 365.24219 days, which means our 365-day calendar is off by a quarter of a day, or about six hours every year.

• It's pretty annoying.

• Julius Caesar thought so too.

• Back in 45 BC, he came up with a solution.

• For three years, we ignore that extra quarter of a day, then every four years, we add those four quarter days back together and stick an extra day at the end of February.

• Without that leap day, the gap between the calendar year and the true year would gradually increase.

• After 100 years, the calendar would be off about about 25 days.

• So, why does this matter?

• Billions of years ago, something smashed into Earth and knocked its axis about 23 degrees.

• And thus, were born the seasons.

• Winter when we're tilted away from the sun, and summer when we're tilted toward it.

• And we're used to these seasons lining up with our calendar.

• It's handy for all sorts of record-keeping.

• But without leap year, our calendar would become disconnected from the seasons.

• After 700 years or so, Christmas would show up in the middle of summer.

• We'd have Thanksgiving in the spring, St. Patrick's Day in the fall, and Labor Day in the winter.

• Chaos!

• It's still not a perfect system though.

• Leap year actually overcorrects slightly, so every 100 years we skip a leap day.

• But that puts the calendar slightly behind again, so we skip skipping leap day every 400 years.

• And even still, after a few thousand years, we'll be off again by a day.

To understand leap year, you have to know this one fact: Earth rotates 365.24219 times during one full orbit of the sun.

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# How leap year works

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Shinichiro posted on 2022/05/05
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