Like most people at work, physicist Tom Parker watches the clock. But unlike most people, it's Parker's job. He keeps an eye on the world's most accurate timepiece, the F-1 Cesium Fountain Atomic Clock. Parker's effort is crucial to everything from running the Internet to managing spacecraft and making accurate astronomy observations.

Yet telling time on Parker's $200,000 clock, which behaves less like a Rolex, and more like a toy-sized factory, is rather difficult. In fact, it's so complex, Parker checks the time with this clock only every few months.

Tucked away in a glass case at the Time and Frequency Division of the National Institute of Standards and Technology (NIST) in Boulder, Colorado, a vertical, meter-long copper tube is shielded by thin layers of material for protection against Earth's magnetic field.

The hodge-podge aesthetics betray this chronometer's significance: The F-1 Cesium Fountain Atomic Clock, in partnership with another much like it in Paris, synchronize all the clocks of the world by defining the length of a second as accurately as possible to keep the modern world running like, well, clockwork.

The fancy clock in Boulder answers the obvious question, "What time is it?" But that question really means answering this one: "Where is the Earth in its daily rotation?" The NIST helps the continuously improving international effort to keep accurate time so that the stars and the Sun are in the same position at the same time, year after year after year.

Precision in time keeping has been crucial ever since mariners navigated by the position of stars, but it's important for modern reasons as well.

Much of astronomy and science depends on accurate timekeeping. The underlying reason is that time is inextricably linked to distance. While a trip between two points takes a certain amount of time, the time it will take depends on the distance traveled. This is true for cars on a highway or bits of data beamed between satellites, or light traveling between stars and planets.

A radio signal sent from a Mars probe, for example, will take a certain amount of time to travel to Earth. The astronomer whose clock can most accurately measure the time a signal was sent, and the time it was received, will best know where that instrument is in space.

Keeping accurate time is also important for most anything that relies on computer communications, including the Internet and aircraft and missile navigation. Without a unified timekeeper, the modern world would stop in its tracks, or at best it would operate in a very sloppy and dangerous fashion.

"If a plane and an air traffic controller's time were off by 10 nanoseconds, the plane could actually be 10 feet away from where it was thought to be. That could be disastrous," explained Donald Sullivan, the lead scientist at the Time and Frequency Division. A nanosecond is one-billionth of a second.

For all the importance of exact timekeeping, Sullivan and his team of scientists turn on the cesium fountain clock only every three months or so.

The clock's purpose is to measure the length of a second as it is internationally defined: the moment that passes while a cesium atom oscillates between its two lowest levels of excitement 9,192,631,770 times.

To get the cesium shaking, cesium gas is introduced into the copper tube and all six laser beams force the atoms into a tight ball, until they're almost completely still -- very near the temperature of absolute zero. At this temperature, the atoms are at the lowest level of excitation.

When this level is obtained, all but the bottom laser turn off, and the cesium atoms are tossed up through the tube into the microwave-emitting cavity.

Then the bottom laser turns off and the atoms fall. The laser switches on, and the process repeats again and again. This up and down action lends the fountain clock its name.

It's in the microwave chamber that the atoms are set abuzz. The energy transferred by the microwave radiation energizes the atoms and causes the cesium to vibrate and give off light, a process called florescence. Readings from a light detector let the scientists know exactly how the atoms are moving, and until the characteristic light of cesium atoms wiggling at their lowest natural resonance appears, the scientists keep tuning the microwave emission. But when it does appear, the moment is measured as the atoms wiggle 9,192,631,770 times.

A truly perfect second has passed.