Peter Reuell in the Harvard Gazette:
A professor emeritus of physics who recently died at 96, Ramsey’s work lay the foundation for the development of the atomic clock, a device that allows scientists to measure time more precisely than ever, and which is a critical component in global positioning systems (GPS).
Just as a grandfather clock counts the oscillations of a pendulum to keep time, atomic clocks use the movement of atoms — which oscillate at precise frequencies — to measure time. Using the devices, a second is no longer measured as a fraction of the time it takes the Earth to revolve around the sun, but as the time it take a cesium-133 atom to oscillate 9,192,631,770 times.
The advantage of such clocks is in their previously unheard of accuracy. Since every cesium-133 atom oscillates at the same frequency, clocks can be built that neither lose nor gain a second in millions of years. Such precision is critical in a number of scientific fields. Atomic clocks are used to track satellites in deep space, to test Einstein’s theory of general relativity, by astronomers seeking to use multiple radio telescopes to capture images of objects light-years away, and by geologists, who use GPS to track the movement of earthquake fault lines.
More here.