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MLA Full: "The Mysterious Leap Second." YouTube, uploaded by SciShow, 29 February 2012,
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Leap Day's got nothing on the Leap SECOND! Hank explains why a second is being added to 2012 and why some are upset about it.

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Hello, and happy Leap Day! It's February 29th, or it will be, or it was. The day when we celebrate--we don't actually celebrate anything.

Leap Day is more of an acknowledgement that it takes more than 365 days--365 days and about six hours--for us to orbit the sun. So every four years we need to correct for those six hours by adding a full day, because four times six is twenty-four, I think.

It's not exact, but it's close, and if you want to know how we get more exact, you can watch CGP Grey's video on the leap year, I'll put a link to it somewhere nearby me.

But that's not actually what this video is about. In addition to those twenty-four hours, we're also going to be getting a leap second later this year. 2012 is a leap second year. What does that mean? Well, it means that on June 30th at 7:59 and 59 seconds, PM, eastern standard time, a second will be inserted at the U.S. naval observatory's master atomic clock facility in Washington, D.C. Cause we have one of those. It's important stuff. It's also being inserted at other timekeeping facilities across the world.

This isn't the first time that this has been done. Since 1972 a second has been added to the world's official timekeeping devices twenty-four times, most recently on December 31st 2008. But why are we doing it, and why are some people so upset about it?

We measure time based on the Earth's rotation relative to the sun. This time is called mean solar time, and it's defined very simply. It takes the amount of time that it takes the Earth to spin, and divides that into twenty-four hours and divides those hours into minutes and seconds et cetera. So in a mean day we have twenty-four hours. We've defined it that way. There shouldn't be any extra seconds. But, uh, turns out that the Earth's rotation is slowing down ever so slightly which means that the actual spin of the Earth is falling somewhat behind what mean solar time predicts.

Yeah, the Earth is slowing down. It's nothing you need to be concerned about, it's basically thanks to the scumbag moon up there which is literally dragging on us, and giving us nothing in return. Except, like, the tides, which are actually pretty cool and, you know, something for werewolves to get all excited about. But it's actually the scumbag moon's tidal action that's adding the torque and friction to our rotation which means that a day in 2012 is one to two milliseconds longer than it was in 1912.

That might not seem like a lot, but it adds up, and since we can't adjust the Earth's rotation (yet), we have to adjust our clocks. Earth's official timekeeping is done by atomic clocks, which are not radioactive, people who have told me that, you're crazy. They actually just measure the oscillations of an atom, it doesn't have anything to do with radioactivity.

And since 1967, a second has been defined as 9,192,631,770 of those oscillations. Very excited that I got that. Oh yeah.

This way of measuring time, called international atomic time, is way, way, way, way, way, way, way more consistent than mean solar time. The United Kingdom's national atomic clock, for instance, is expected to remain accurate within one second for the next 138 million years. So that's a bridge we can cross when we come to it, which you know, we probably won't come to it.

So some people argue, probably rightly, that it's unnecessary to do this extra leap second thing and that it's a big pain in the butt. The extra second has to be added manually to a lot of computer systems that depend on precise timekeeping, and there's a ton of telecommunication and navigation systems that have to be re-synced when these changes happen and it's, uh, it's a nightmare for those people. But the pro leap second people say that eliminating it would create way more problems, especially down the road, like 5,000 years from now when, at this rate, the Earth's rotation would be like an hour behind what the atomic clocks say.

Even by 2100 the difference would be a couple minutes or so. The power to break this stalemate ultimately rests with the International Telecommunication Union, a bureau of the U.N. which literally defines what time is, and last month, its members voted to postpone a decision on whether to dump the leap second until 2015 saying that more study was needed. And I'm like, holy crap, guys! You have one job! Do it!

In the mean time, we have the leap second, and I'm, you know, what are you gonna do with it this year? You get an extra second! So enjoy it.

If you have other ideas for things that we should be doing on this show, questions that you would like us to answer, please see us in the Youtube comments below or, of course, on Facebook or Twitter. Goodbye.