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.