Atomic clock is the most accurate type of clock. The best atomic clocks would not lose or gain more than a second in tens of millions of years. An atomic clock keeps time by counting the energy changes of atoms (basic units of matter). When an atom of a certain element gains or loses energy, it vibrates at a certain frequency (rate). This frequency is identical from atom to atom. Thus, timekeeping based on the frequency is nearly perfect. The first experimental atomic clock was built by the U.S. National Bureau of Standards in 1949. In 1967, atomic clocks became the basis for world timekeeping when the second was defined as the amount of time a cesium atom takes to make 9,192,631,770 energy transitions. Global Positioning System (GPS) satellites carry atomic clocks.
See also Time.
