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The big hunk of rock you're on right now - you know, the Earth - is spinning. We call one full rotation a day, and it takes 24 hours, or 86,400 seconds. Well, sort of. As NPR's Nell Greenfieldboyce reports, tomorrow is predicted to be a tiny bit shorter than usual.
NELL GREENFIELDBOYCE, BYLINE: The nation's headquarters for precise timekeeping is the U.S. Naval Observatory in Washington, D.C. Outside, there's a big, black digital clock with red numbers. Inside, I met up with Dennis McCarthy. He's sort of retired now, but for years, he held the title director of time. He says the planet's spin gets affected by all kinds of things.
DENNIS MCCARTHY: You driving here today moved your mass from someplace to here, and that changed the moments of inertia of the Earth, and you could possibly have affected the rotation of the Earth.
GREENFIELDBOYCE: That's an extreme example, but stuff moving around on Earth does have an effect on the speed of its rotation. You've got water melting from the polar ice caps and going into the oceans, weather systems in the atmosphere, plus the movements of the planet's liquid core.
MCCARTHY: The Earth's rotation has been variable. We've known about the rotation of the Earth being variable for about a hundred years.
GREENFIELDBOYCE: McCarthy says in the long term, the Earth's rotation has been slowing down because of the influence of the moon, and that's not going to change. That's been going on for millions of years.
MCCARTHY: Although the slowing down is continuing, there are departures from that general pattern.
GREENFIELDBOYCE: Recently, the Earth's spin has been speeding up a tiny bit, leading to some shorter days. Earlier this month, July 9 was short by a little over a millisecond. Tomorrow has been predicted to be similarly short with more short days to come in August. Nick Stamatakos is head of the Earth Orientation Department at the U.S. Naval Observatory. He says a millisecond here or there may not sound like much but consider this. In one second, the Earth's equator rotates the length of about four football fields.
NICK STAMATAKOS: So the Earth's moving pretty fast, so any little variations will accumulate, and it's an issue.
GREENFIELDBOYCE: It matters for things like precise navigation, launching missiles, GPS. That's why researchers are constantly measuring the speed of the Earth's spin. They do it by pointing telescopes at quasars, incredibly luminous cores of distant galaxies that are so far away, they act as fixed, unmoving points in space. Now, in addition to tracking the spinning of the planet, scientists keep time in another way. They count the seconds of the day with atomic clocks. Stamatakos says atomic time is very, very accurate.
STAMATAKOS: It's very repeatable, and it keeps going. And the Earth is the one that's a variable.
GREENFIELDBOYCE: To keep atomic timekeeping in sync with the Earth's spin, the world's timekeepers have sometimes added in leap seconds. The recent spate of quick spins and short days has raised the prospect of a negative leap second, basically subtracting a second from atomic time. That's never been done before, and it's debatable whether computer systems are set up to even be able to handle that concept.
STAMATAKOS: There would be some consternation - some programs that would have to be changed.
GREENFIELDBOYCE: He says even regular old leap seconds can cause disruptions. That's why they've fallen out of favor in recent years, and why an international group of scientists is now debating how closely atomic time really needs to track Earth's inconsistent spinning.
Nell Greenfieldboyce, NPR News.
(SOUNDBITE OF NATHANIEL DREW X TOM FOX'S "WEIGHTLESSNESS") Transcript provided by NPR, Copyright NPR.
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