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  • You probably know that this year, 2012, is a leap year, and that

  • means that this year we get an extra day on February 29th.

  • So instead of having 365 days this year there'll be

  • 366. Great,

  • an extra day--who really cares? Well, people

  • born on February 29th on some previous leap year, also known as leaplings,

  • they care because they finally get to celebrate their real birthday.

  • But for the rest of us it's just a day like any other day--whoop dee doo! So why

  • do we go to all the trouble

  • to have a leap year?

  • Well, I'll explain.

  • Now we all understand a day.

  • One full day is actually how long it takes for the Earth

  • to spin around exactly once.

  • And a year

  • is how long it takes for the Earth to orbit the sun

  • exactly once.

  • So while the Earth

  • orbits around the sun in a full year

  • it spins around 365.25 times.

  • In other words, one full year equals 365.25 days.

  • This is called the astronomical year.

  • But here's the problem: Our calendar year is only 365 days,

  • and that's because

  • there's really no way to have a .25 day.

  • And so those extra .25 days, they just keep accumulating,

  • and what they do is make it so that the stars slowly drift out of sync

  • with our calendar. So this is where Leap Year comes in

  • to save the day.

  • Every year we set aside those .25 days, until the fourth year when

  • they equal one full day. And then on that fourth year we put that extra day on

  • February 29th and we call it Leap Day. Bam! We're back in sync.

  • That's why February 29th exists. Cool.

  • But the real interesting thing is how we humans figured this all out.

  • It really was the Egyptians who first figured out Leap Year. They noticed by

  • watching the stars--

  • specifically the Sirius star--

  • that the astronomical year was actually 365.25 days, and

  • they

  • noticed this by seeing the Sirius star

  • slowly drifting out of sync.

  • But the western world wasn't so fast to figure this all out. It wasn't until many

  • centuries later when Julius Caesar, with the help of an astronomer, discovered just

  • like the Egyptians first did that the year is really 365.25

  • days.

  • And they created the Julian Calendar with the Leap Year that we know and love

  • to fix that problem. Well done, Julius.

  • Well...not so fast. You see,

  • if you want to get really exact about it, the astronomical year is actually

  • 365.2422 days which is 11 minutes

  • 14 seconds shorter

  • than the Julian Calendar.

  • And that means in 128 years from now,

  • if we use the Julian Calendar,

  • we'll be off again by one full day.

  • So today, we use a revised version of the Julian Calendar. It's called the

  • Gregorian Calendar because Pope Gregory

  • initiated it. The Gregorian Calendar is just like the Julian Calendar, but it's got

  • a few more rules.

  • So while every fourth year is a leap year, every year that's divisible by

  • 100

  • are now no longer leap years. And that means that years 1700, 1800

  • and 1900--

  • those were not leap years, even though they normally would be

  • And here's another rule: If the year is also divisible by 400, then it

  • is still a leap year,

  • which means that the year 2000-- that

  • WAS a leap year.

  • And with all those complicated rules, our calendars can stay in sync with the

  • stars

  • for millennia to come.

  • But...

  • one more thing.

  • Did you know that the earth's rotation is slowing at a rate of .005

  • seconds per year?

  • And that means in about 2 billion years we're gonna have to have to add

  • one more leap year to keep us in sync.

  • But don't worry--

  • we've got plenty of time to revise the calendar and fix that.

You probably know that this year, 2012, is a leap year, and that

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What Is a Leap Year?

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    Vincent Chang posted on 2013/08/15
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