We tick by the cesium clock.

The beat of the cesium clock in another, as are the orbits of the satellites that remain, the wild grand siderial swing of the stars themselves.

In effect, the meter is defined to be the distance traveled by light in 0.000000003335640952 second, as measured by a cesium clock.

On-board cesium clocks provide the local clock source.

The "Paul trap" that made this possible also led to such practical applications as the development of the cesium clock, used for precise measuring of time.

What happened to the cesium clock?

A few years later, with the invention of the cesium atomic clock, an alternative offered itself.

The original outer satellites are still there, their atomic batteries and cesium clocks ticking faithfully away though their computers have long crashed.

But this cesium clock, the experts promise, will neither gain nor lose a second in 6 million years.

This is generated by an atomic cesium clock or a satellite-derived clock by a device in the same central office as the network element.