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You probably know that this year, 2012, is a leap year, and that
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means that this year we get an extra day on February 29th.
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So instead of having 365 days this year there'll be
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366. Great,
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an extra day--who really cares? Well, people
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born on February 29th on some previous leap year, also known as leaplings,
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they care because they finally get to celebrate their real birthday.
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But for the rest of us it's just a day like any other day--whoop dee doo! So why
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do we go to all the trouble
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to have a leap year?
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Well, I'll explain.
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Now we all understand a day.
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One full day is actually how long it takes for the Earth
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to spin around exactly once.
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And a year
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is how long it takes for the Earth to orbit the sun
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exactly once.
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So while the Earth
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orbits around the sun in a full year
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it spins around 365.25 times.
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In other words, one full year equals 365.25 days.
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This is called the astronomical year.
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But here's the problem: Our calendar year is only 365 days,
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and that's because
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there's really no way to have a .25 day.
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And so those extra .25 days, they just keep accumulating,
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and what they do is make it so that the stars slowly drift out of sync
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with our calendar. So this is where Leap Year comes in
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to save the day.
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Every year we set aside those .25 days, until the fourth year when
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they equal one full day. And then on that fourth year we put that extra day on
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February 29th and we call it Leap Day. Bam! We're back in sync.
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That's why February 29th exists. Cool.
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But the real interesting thing is how we humans figured this all out.
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It really was the Egyptians who first figured out Leap Year. They noticed by
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watching the stars--
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specifically the Sirius star--
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that the astronomical year was actually 365.25 days, and
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they
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noticed this by seeing the Sirius star
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slowly drifting out of sync.
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But the western world wasn't so fast to figure this all out. It wasn't until many
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centuries later when Julius Caesar, with the help of an astronomer, discovered just
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like the Egyptians first did that the year is really 365.25
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days.
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And they created the Julian Calendar with the Leap Year that we know and love
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to fix that problem. Well done, Julius.
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Well...not so fast. You see,
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if you want to get really exact about it, the astronomical year is actually
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365.2422 days which is 11 minutes
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14 seconds shorter
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than the Julian Calendar.
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And that means in 128 years from now,
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if we use the Julian Calendar,
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we'll be off again by one full day.
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So today, we use a revised version of the Julian Calendar. It's called the
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Gregorian Calendar because Pope Gregory
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initiated it. The Gregorian Calendar is just like the Julian Calendar, but it's got
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a few more rules.
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So while every fourth year is a leap year, every year that's divisible by
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100
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are now no longer leap years. And that means that years 1700, 1800
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and 1900--
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those were not leap years, even though they normally would be
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And here's another rule: If the year is also divisible by 400, then it
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is still a leap year,
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which means that the year 2000-- that
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WAS a leap year.
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And with all those complicated rules, our calendars can stay in sync with the
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stars
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for millennia to come.
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But...
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one more thing.
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Did you know that the earth's rotation is slowing at a rate of .005
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seconds per year?
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And that means in about 2 billion years we're gonna have to have to add
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one more leap year to keep us in sync.
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But don't worry--
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we've got plenty of time to revise the calendar and fix that.