NIST-7 is one of the two most accurate timekeepers in the world and it resembles a ten-foot-long silver cannon. The clock sits in a National Institute of Standards and Technology (NIST) laboratory nestled at the foot of the Rocky Mountains, and is aligned due west. That way the sun moves across the sky parallel to the clock itself, and even the interaction of the solar radiation on Earth's magnetic field does not disturb the clock's accuracy. Since NIST scientists started building clocks in the 1950s (when the institute was known as the National Bureau of Standards), the accuracy of their timepieces has improved steadily by a factor of ten every seven years or so. Among the most demanding users are astrophysicists, who are timing the phenomenally punctual pulsations of stars known as millisecond pulsars. NASA also needs the clocks to time the navigation commands it sends off to deep-space probes. People who work with telecommunications, the global-positioning system, security, and defense also need to send or receive signals with billionth-of-a-second accuracy. Since 1967, the officially sanctioned length of a second has been defined by atomic standards: A second is equal to 9,192,631,770 oscillations of the radiation emitted or absorbed by atoms of cesium 133 when they undergo what is known as hyperfine transition.
There are also attempts to build mercury clocks that will, in theory, be accurate to one second in 30 billion years! However, according to the general theory of relativity, the phenomenal accuracy of these clocks would be subject to small changes in the Earth's mass resulting from particle radiation and meteor strikes. Therefore, two phenomenally accurate clocks will not indicate the same time if they are not exactly at the same location on Earth.