The accuracy of atomic clocks is based upon the resonance of caesium 133, which was defined in 1958  as the length of time for 9,192,631,770 cycles of vibration as they pass through a magnetic field. The resonate constancy of caesium exceeds the accuracy of the Earth’s rotation around the Sun.
The accuracy of atomic clocks is based upon the resonance of caesium 133, which was used to define ( in 1958 ) the second as the length of time for 9,192,631,770 cycles of vibration as they pass through a magnetic field. The resonate constancy of caesium exceeds the accuracy of the Earth’s rotation around the Sun.
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defined
used to define (
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) the second