WASHINGTON – 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,” said 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 2 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 said. 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 can’t 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 30 cm every billionth of a second (which is a nanosecond). So if the clocks in GPS satellites and your GPS receiver drift just 1 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 640 meters. 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.
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 kilometers, 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,” said 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 said, 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.
But there are many other planetary processes that either bog down or rev up the Earth’s spin. Any large-scale interaction between parts of the Earth, such as ocean winds or movements of the planet’s molten iron core against the solid mineral mantle, subtly speeds up or slows down the Earth’s rotation.
Any process that redistributes lots of mass upward or downward will slow or speed the rotation, much as a spinning ice skater slows or quickens her rotation by drawing in or extending her arms. Evaporation, precipitation, melting, changing wind patterns, earthquakes and volcanic eruptions are among the processes that can do this.
“In recent decades, the Earth’s rotation has been speeding up,” O’Brian said. “This is almost certainly a temporary effect, based on weather, climate changes, changes in the crust or flow of magma.”
For geophysicist Richard Gross of the Jet Propulsion Laboratory in Pasadena, California, even the tiniest changes in the Earth’s rotation are more than merely interesting. “To navigate a spacecraft through space and get it to Mars, you need to precisely know how Earth is oriented with respect to Mars,” said Gross, whose works contributes to JPL’s interplanetary navigation.
He noted that even microsecond changes in Earth’s rotation produce enough of a shift in the Earth-Mars orientation to throw the navigational precision off by a kilometer or more. And that could mean the difference between putting a Martian lander down successfully on a predetermined flat surface or sending it tumbling into a ravine. To help Gross get better at mastering these subtleties, he and a colleague calculated a few years ago that the Sumatran earthquake of Dec. 26, 2004, repositioned enough of the Earth’s mass to snip 6.8 microseconds from the length of the day.
“We have come full circle,” O’Brian said. The rotation of the Earth had long been the most accurate measure of time for humanity, but now such technologies as atomic clocks and GPS devices make it possible to measure tiny variations in the Earth’s rotation.
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