Rewrite this sentence for readability: 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 accuracy of atomic clocks is based upon the resonance of caesium 133, which was used to define (in 1958 as the length of time for 9,192,631,770 cycles of vibration as they pass through a magnetic field.