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The second (symbol: s) (abbreviated s or sec) is the base unit of time in the International System of Units (SI).
The second (s) For thousands of years the Earth's rotation was our most stable timekeeper. However, the quartz and atomic clocks invented during the 1930s and 1950s are even better timekeepers, and show that the Earth does not rotate steadily but wobbles. Since 1967 the definition of the second has been related to the movement of electrons in a caesium atom: The second is 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 caesium 133 atom.