Atomic clocks are the most precise devices to measure time. Atomic clocks such as rubidium, hydrogen, and cesium clocks are used by scientists and engineers when computing distances with Global Positioning Systems (GPS), measuring the rotation of Earth, precisely knowing the positions of artificial satellites, and imaging stars and galaxies.

The clock that is used as the standard for the second is the cesium-133 atomic clock. The measurement of the second is defined as the time it takes for 9,192,631,770 periods of microwave radiation that result from the transfer of the cesium-133 atom between lower-energy and higher-energy states. The second is currently known to a precision of 5 X 10^-16, or one second in 60 million years!