High Quality Content by WIKIPEDIA articles! The second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock. Early definitions of the second were based on the motion of the earth: 24 hours in a day meant that the second could be defined as 1?86?400 of the average time required for the earth to complete one rotation about its axis. However, nineteenth- and twentieth-century astronomical observations revealed that this average time is lengthening, and thus the motion of the earth is no longer considered a suitable standard for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

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