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The world's clocks are set by cesium, whose atoms vibrate at a rate that never varies.

Physics Facts, sourced to Royal Society of Chemistry.

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What it means

Cesium was chosen because it has a single outer electron and a convenient transition frequency in the microwave range, which made it practical to build a clock around in the nineteen fifties. Every satellite navigation fix you have ever taken depends on this: the system works by comparing arrival times of signals, and an error of a millionth of a second is a position error of hundreds of meters.

The claim comes from Royal Society of Chemistry.

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