字幕表 動画を再生する 英語字幕をプリント 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.