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In 1967, an atomic standard was adopted for second, the SI unit of time. One standard second is defined as the time taken for 9 192 631 770 periods of the radiation corresponding to unperturbed transition between hyperfine levels of the ground state of cesium - 133 atom. Atomic clocks are based on this. In atomic clocks , an error of one second occurs only in 5000 years. From 60 sec is 1 min from this 60 minutes is hour

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13y ago

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