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September 24, 2013

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


The moment-to-moment monitoring and management by which electrical engineers maintain the flows of current in power grids, whose interconnected components can span thousands of miles, are possible only because of precisely synchronized clocks and high-speed communication by which even the most distant parts of the grid can keep track of each other's status. And forget about talking and texting on cellphones or Googling on your computer without superlatively timed handoffs of billions of signals between cellphone towers and perfectly timed transmissions of data packets crisscrossing the planet at lightning speed only to miraculously reassemble everywhere into coherent Web pages.

"If we relied on the Earth's length of day, we could not have any of this," says O'Brian, whose group at NIST develops, maintains and improves the supremely regular atomic clocks on which all other timekeeping ultimately is based.

An atomic second is defined, in techspeak, as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133 atoms." Translation: The cesium atoms behave like magnificently fast pendulums that never, ever waver the way the Earth's rotation does. It is because of those more than 9 billion oscillations per second that it is possible to synchronize clocks with better than millionths-of-a-second precision.

An Earth second, on the other hand, has been defined since early in the 19th century as 1/86,400th of a 24-hour day (60 seconds times 60 minutes times 24 hours = 86,400). The trouble for modern technologies, O'Brian says, is that the planet's length of day "is wandering unpredictably every day."

What's slowing the Earth down? The moon is the biggest and steadiest drag on the Earth's rotation. Think tides here. As the Earth turns, the moon's gravitation pulls on the Earth's oceans and some of the crust below. As the planet rotates, this tidal interaction with the moon acts like the rubber damper on a carnival wheel that slowly brings the rotating wheel to a halt. This accounts for the bulk of the roughly two-millisecond slowdown of Earth's rotation for each century's worth of this tidal drag.

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