B1 Intermediate US 81 Folder Collection
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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 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.
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How leap year works

81 Folder Collection
Shinichiro published on January 20, 2020
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