Sullivan, Parker and other NIST physicists record precise seconds from the cesium fountain clock for about a month, and then an average of all these tiny time frames is calculated to come up with the most perfect count of "one-one-hundred."

This count, averaged with another from the French atomic fountain clock, keeps the beat for what Parker calls the ensemble clock. The ensemble clock is an average of reported times of 200 less exact, atomic clocks in several dozen participating countries and is compiled every month by the French International Bureau of Weights and Measures.

Once the ensemble clock is compared against the tick-tock set by the two cesium fountains, the result is called International Atomic Time, or TAI.

Still, the process is not over. After this adjustment is made, the Bureau of Weights and Measures distributes a publication called Circular T that states the difference between the two atomic fountain clocks and the ensemble clock. From this, about 50 worldwide time-keeping agencies synchronize their watches accordingly.

The TAI is so exact, it keeps a steadier pace than the Earth's spin. And that still is the ultimate determinate of time.

Earth's rotation is altered by a surprising process. It's the Moon that slows the Earth's rotation little by little. The distance between our planet and its satellite grows greater all the time. This effects us like an extending arm on an ice skater.

But the effect is slight. It would take 2,000 years before an hour was lost between the Earth's sluggishness and the TAI's perfect time, Parker says.

To avoid seeing the Sun overhead at 1 p.m. in the year 4002, and more importantly for astronomers, to keep the stars in their places in the sky, an extra second is added to the International Atomic Time about every 18 months. With this adjustment, Universal Coordinated Time, or UTC, is created and distributed worldwide.

UTC is the time kept at the internationally accepted zero longitude at Greenwich, England. Eastern Standard Time, in the United States, is always exactly five hours behind UTC.

But to make these tiny adjustments, a measure of the Earth's sluggishness has to be made and distant stars are used as reference points. United States Naval Observatory astronomer Tom Johnson in Washington, D.C., charts the locations in the sky of some of the farthest known stars. When it appears that Johnson's reference stars move 0.9 seconds of a degree in the sky, he knows it's time to add a second to the TAI.

"Essentially, these stars don't move in comparison to us," said Johnson. It's the Earth that's changing its speed.

Johnson and astronomers involved with the International Earth Rotation Service in France, decide when to add another second to International Atomic Time. Currently, there is a difference of 32 seconds between International Atomic Time and Universal Coordinated Time, or UTC.

So, while Sullivan and Parker continue to fidget with their atomic fountain clock, a timepiece so exact the Earth has to conform to its beat, the noonday Sun above Boulder sits above, exactly like the noonday Sun in Greenwich, England and airplanes are exactly where they're calculated to be, and the stars hold their place in the sky.