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How Do Some Clocks Set Themselves?

With Daylight Saving Time set to start, take a look inside the radio-controlled clocks that adjust automatically

A growing number of clocks automatically synchronize with a radio signal and don't have to be adjusted for Daylight Saving Time. How do they work?

As Daylight Savings Time approaches, you’ll be seeing many reminders to shift your clocks an hour forward just before going to sleep on Saturday night. This got us thinking about the clocks that set themselves. Available widely for as little as $10 or $15, these radio-controlled clocks are increasingly popular, as they adjust automatically to time shifts and will work virtually anywhere in the continental United States. You may well own one of them already. But you may not know how they work.

This clock’s low-tech appearance conceals an elaborate system for keeping it precisely in tune with what the National Institute of Standards and Technology deems official time: a clock calibrated by the movement of a clump of cesium atoms in Boulder, Colorado. Housed at the NIST’s Physical Measurement Laboratory, this is the official atomic clock, and it keeps time for the entire country.

The NIST's cesium fountain atomic clock, in Boulder, Colorado

The sophisticated apparatus—known as NIST-F1—is the latest in a line of high-tech atomic clocks and was officially adopted as the U.S.’s time standard in 1999. The accuracy of NIST-F1 is continuously improving, and as of 2010, scientists calculated that its uncertainty had been reduced to the point that it will neither gain or lose a second over the course of 100 million years.

This degree of accuracy is achieved by a complex technological setup. In 1967, the International Bureau of Weights and Measures officially defined a single second as the time it takes a single cesium atom to transition between energy levels a given number of times—that is, cesium’s natural resonance frequency. NIST-F1 is known as a cesium fountain atomic clock because it uses a fountain-like array of lasers to manipulate cesium atoms and detect this frequency as accurately as possible.

Inside the device, six powerful lasers are aimed at a gas containing cesium atoms, slowing down their movement and cooling them down to temperatures just millionths of a degree above absolute zero. Next, a pair of vertical lasers push the clumped ball of cesium atoms about a meter upward in the cavity, which is filled with microwave radiation. As it drifts back downward, another laser is pointed at the atoms and detects how many were altered by the microwaves. Scientists calibrate the microwave frequency to maximize the number of atoms affected.

The NIST uses this measure of cesium’s resonance frequency as the official second for the U.S. primary time standard. But how does it get to your radio-controlled clock? The official time standard is sent to WWVB, NIST’s shortwave radio station in Fort Collins, Colorado. Once per minute, WWVB uses five antennas to broadcasts a digital code indicating the official time—including the year, date, hour, minute and whether Daylight Savings Time is in effect—across the country.

Most radio-controlled clocks are programmed to receive this signal once per day with built-in receivers and calibrate their time accordingly. Experts say that your radio-controlled clock will work best when positioned near a window facing the source of the broadcast, Fort Collins. Many other countries have their own official time broadcasts, based on other atomic clocks.

A clock that stays accurate for 100 million years is pretty good, right? Not for NIST. In 2010, they announced advances in developing a new “quantum logic clock,” which keeps time based on a single atom of aluminum. The new clock will neither gain nor lose a second over 3.7 billion years, the researchers report, giving it the title of the world’s most precise clock.

So this year, if your clock automatically jumps an hour ahead at 2 a.m. Sunday, remember that an intricate setup of lasers and atoms thousands of miles away is the reason why. We’ve sure come a long way from watching sundials and winding watches.


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