(Soundbite of a ticking clock)

DEBBIE ELLIOTT, host:

That's what you might consider seconds ticking away. But what is a second?

Unidentified Man #1: The second today is defined as the interval of 9,192,631,770 transitions of the hyperfine state of Cesium-133.

ELLIOTT: For us non-science types, that is to say a second is measured in vibrations of one atom of the element cesium. This week on Science Out of the Box, how do you measure the vibration of an atom? And what does that have to do with time?

This week, I visited the U.S. Naval Observatory to find out exactly how complicated telling time really is. The Naval Observatory runs the nation's Master Clock System, the one you call up if you want to know exactly what time it is.

Unidentified Man #2: U.S. Naval Observatory Master Clock. At the tone, Eastern Standard Time, 14 hours, 48 minutes, 10 seconds.

(Soundbite of beep)

Mr. JEFF CHESTER (Public Affairs Officer, Naval Observatory): This is the master clock building. This is where the actual current master clock system resides. And so this is where time, for all intents and purposes, originates.

ELLIOTT: That's observatory public affairs officer Jeff Chester. We're standing in a narrow hallway lined with tall plate glass windows. Racks of machinery hum behind the glass. These are atomic clocks which count those billions of atomic vibrations day in and day out.

There are more than 70 atomic clocks here. Why so many?

Mr. WARREN WALLS (Naval Observatory): It's like having a bunch of different watches on people. So what time do you have? What time do you have? What time do you have? And I go through and I mark down. And then I go through and do that again.

ELLIOTT: Warren Walls is the head of the observatory's master clock division. It's his job to keep track of what time all those atomic clocks are telling.

Mr. WALLS: And we use that collection then to come up with this best estimate of what absolute time is.

ELLIOTT: So tell me what time it is.

Mr. WALLS: Well, this is 19:46:51, 52, 53.

ELLIOTT: That's the kind of precision you couldn't get from a sundial or even your kitchen wall clock.

Chris Eckstrom(ph) is head of the observatory's clock development group.

Mr. CHRIS ECKSTROM (Naval Observatory): A clock is the combination of an oscillator and a counter. So an oscillator is just something that repeats and there are a whole class of oscillators we can use to keep time. It can be the rotation of the Earth. So we can notice when the sun's overhead. Or when the seasons are good for growing. And then if we just count them, we have a clock. And what time it is, is whatever's in our counter.

ELLIOTT: And for thousands of years, that's what people did. They measured time by the rotation of the Earth and the movements of the sun, the moon and the stars.

But the earth doesn't really keep good time. For one thing, it's slowing down by almost one second per year. In the '40s, scientists discovered that atoms make a much better standard. They vibrate very fast, very regularly, and most important, they never speed up or slow down.

So how do you get an atom to power a clock? Chris Eckstrom says fire a microwave beam at it.

Mr. ECKSTROM: And the atom with its response will tell us if our microwaves are too low in frequency, too high in frequency or just right. And then we trust that the atom is truth and we count those cycles and now you have an atomic clock.

So maybe a more accurate way of looking at the atoms and their response to the microwaves is they're like little bells. Now, if you had a bell and you yelled at it with a pure tone, at just the right pitch it would ring back at you. It would ring sympathetically. And that's pretty much what the atoms do.

ELLIOTT: Only this isn't sound. It's light. The bell in this case is a bunch of cesium atoms and the yell is a blast of microwaves.

Eckstrom and his colleagues tune the microwaves up and down the way you can change the pitch of a yell until they make the cesium atoms sing in harmony. At that point they know the microwave beam is tuned perfectly so it's vibrating at exactly the same frequency as the cesium atom. Remember: 9,192,631,770 times per second. A little math and you have a very accurate way of powering a clock.

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