The lecture on timekeeping will be at 7 p.m. Tuesday at the Jefferson Lab's CEBAF auditorium, 12000 Jefferson Ave., Newport News. The event is free and open to the public.
For more information about time, see the U.S. Naval Observatory's Web site at www.usno.navy.mil
NEWPORT NEWS - You know the story.
Your watch says 8:59 a.m. Your kitchen clock says 9:01 a.m. But in your car, it's already 9:03 a.m.
So just how late are you going to be?
Time is a funny thing, says Chris Ekstrom, a physicist with the U.S. Naval Observatory in Washington. Depending on how you measure it, it changes. And when motion and temperature mess with those measurements, you've got two watches with two different times.
Enter the atomic clock, Ekstrom's area of expertise.
"The quartz crystals in today's watches and clocks drift off, but the atom is much more accurate," Ekstrom said. "After about a second, the highest quality quartz crystals and the atomic clocks are the same. But after 10 seconds, the atomic clock is already much better."
On Tuesday, Ekstrom will be at the Thomas Jefferson National Accelerator Facility to talk about the history of timekeeping, from ancient sundials to pendulums to atomic clocks.
"It's amazing where we've come," he said, "when you think that for a long fraction of history, knowing nighttime hours wasn't useful."
The USNO is the site of the Master Clock System, which keeps the official time for the country's navigation and communications systems. The Master Clock draws on dozens of atomic clocks to keep its accuracy.
So what is an atomic clock?
Until the late 1920s, timepieces depended on the swinging of pendulums to tick off seconds, minutes and hours. Pendulums were replaced by the more regular vibrations of quartz crystals.
But temperature can speed up or slow down those vibrations, Ekstrom said. In fact, here's a fun trivia fact: Your watch will be more accurate if you keep it on your wrist, where it is exposed to a constant body temperature.
"If you put it on your night stand, it's going to be much less regular," Ekstrom said.
Atomic clocks don't have that problem. From research that picked up speed after World War II, scientists have used the fact that when an atom changes from a higher energy state to a lower one, it puts out a frequency wave that's identical for every atom of the same type.
Shining microwaves on atoms can set that frequency, and clocks based on it will not drift off by more than one second in 100,000 years. Ekstrom's research involves making atomic clocks that are even more stable.
Applications of the technology are many. Satellites in the Global Positioning System, which let navigators locate their exact position and set their routes, carry at least two atomic clocks.
Emergency vehicles use GPS to pinpoint their destinations. Trucking services use it to keep track of fleets. Telecommunications companies use it to synchronize digital networks. In Japan, GPS links are being installed in cars, letting drivers find exactly where they are and get directions.
At least one company is selling atomic watches over the Internet, for prices ranging from $149 to $199.
All this is a far cry from the USNO's first Master Clock - a time ball that was installed on a telescope dome in 1845. The time ball dropped every day at noon, and the residents of Washington set their clocks according to it.
"It's a long history," Ekstrom said. "Needless to say, we've come a long way."
Submitted: Saturday, December 12, 1998 - 12:00am