A caesium standard is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The word standard has several meanings: Classically, standard referred to a flag or banner; especially, a national or other ensign carried into battle; thus standard bearer indicates the one who bears, or carries, the standard. ... In telecommunication, a frequency standard is a stable oscillator used for frequency calibration or reference. ... Hyperfine structure is a small perturbation in the energy levels (or spectral) of atoms due to the proton-electron dipole moment interaction. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Atomic mass 132. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

By the definition of the SI second, the frequency of the radiation in a transition between the two hyperfine ground states corresponds, in the absence of external influences (e.g., the Earth's magnetic field), to a frequency of 9,192,631,770 Hz. The International System of Units (abbreviated SI from the French language name Système International dUnités) is the most widely used system of units. ... The second (symbol: s) is the SI base unit of time. ... The magnetosphere shields the surface of the Earth from the charged particles of the solar wind. ... The hertz (symbol Hz) is the SI unit of frequency. ...

The number 9,192,631,770 was chosen so that the cesium second equaled, to the limit of human measuring ability in 1960 when it was adopted, the existing standard ephemeris second based on the earth's orbit around the sun. Because no other measurement involving time had been as precise, the effect of the change was less the the experimental uncertainty of all existing measurements. Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third-closest planet to the Sun. ... The Sun is the star at the centre of our Solar system. ...

Source: from Federal Standard 1037C and from MIL-STD-188 Federal Standard 1037C entitled Telecommunications: Glossary of Telecommunication Terms is a U.S. Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ... MIL-STD-188 is a series of U.S. military standards relating to telecommunications. ...

See also

• Atomic clock
• Rubidium standard