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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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