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September 24, 2013

With Earth spinning more slowly, time isn't flying as fast as before

Don't forget to set your clocks ahead two thousandths of a second before you go to sleep tonight. Same thing goes for bedtime tomorrow. And every day after that, because that is how much slower the Earth turns on its axis each day now than it did a century ago.

All of those sub-eyeblink slowdowns each century have been adding up, too. For Jurassic-era stegosauruses 200 million years ago, the day was perhaps 23 hours long and each year had about 385 days. Two hundred million years from now, the daily dramas for whatever we evolve into will unfold during 25-hour days and 335-day years.

"We naively think there always has been 24 hours per day," says Thomas O'Brian, chief of the Time and Frequency Division of the National Institute of Standards and Technology (NIST). "But that is not the case."

For all but the past 60 to 70 years, those extra milliseconds adding to each day did not matter one whit. The boss still can't tell if you arrive at work two milliseconds after 9 a.m. And twice a year, those accumulating micromoments essentially vanish when most of us adjust our clocks with the start or end of daylight saving time.

Except for one thing: Those micromoments don't actually vanish, and in an era of intense technology, they now matter a whole lot.

"We have become critically dependent on incredibly precise timekeeping," O'Brian says. Technologies such as smartphones, GPS devices and the power grid rely on thousands of separated elements — such as satellites, cell towers, generating stations, computers, electrical switches and countless computers — that cannot get more than a millionth of a second out of sync with one another before bad stuff happens.

Consider GPS signals between satellites and receivers on the ground. Those are radio signals that move at the speed of light, which means they travel about one foot every billionth of a second (which is a nanosecond). So if the clocks in GPS satellites and your GPS receiver drift just one millionth of a second — a thousand nanoseconds — out of sync with each other, the system will not pinpoint your location more precisely than within about two-fifths of a mile. If the synchronization drifts off by one thousandth of a second, the system couldn't tell you for sure if you were in Washington or Boston.

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