What time is it?

In a story about how the rotation of the earth has slowed 2/1000 of a second over the last century, science writer Ivan Amato explains how dependent our technology is on extremely accurate time-keeping.  For example, if the clocks in your GPS device and the satellite it connects to are out of synch by as little as 1/1000 of a second, your location would be thrown  off by hundreds of miles.

From Why, in the Jurassic era, an Earth day may have been only 23 hours long. – The Washington Post.:

“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.

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.”