The main variety of atomic clock uses caesium atoms cooled to temperatures that approach absolute zero. The timekeeping accuracy of the involved atomic clocks is important because the smaller the error in time measurement, the smaller the error in distance obtained by multiplying the time by the speed of light is (a timing error of a nanosecond or 1 billionth of a second (10 −9 or 1⁄ 1,000,000,000 second) translates into an almost 30-centimetre (11.8 in) distance and hence positional error).
The accurate timekeeping capabilities of atomic clocks are also used for navigation by satellite networks such as the European Union's Galileo Program and the United States' GPS. The system of Coordinated Universal Time (UTC) that is the basis of civil time implements leap seconds to allow clock time to track changes in Earth's rotation to within one second while being based on clocks that are based on the definition of the second. This definition is the basis for the system of International Atomic Time (TAI), which is maintained by an ensemble of atomic clocks around the world. It is defined by taking the fixed numerical value of the caesium frequency, Δ ν C s, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s −1. The second, symbol s, is the SI unit of time. This phenomenon serves as the basis for the International System of Units' (SI) definition of a second: Electron states in an atom are associated with different energy levels, and in transitions between such states they interact with a very specific frequency of electromagnetic radiation. It is based on atoms having different energy levels. The black units in the foreground are Microsemi (formerly Sigma-Tau) MHM-2010 hydrogen maser standards.Īn atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. The rack mounted units in the background are Microsemi (formerly HP) 5071A caesium beam clocks. Naval Observatory in Washington, D.C., which provides the time standard for the U.S.
The master atomic clock ensemble at the U.S.
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