1.8.1 Time
The standard of time prior to 1960 was based on the mean solar day. As time-keeping accuracy and astronomical observations improved, it became apparent that the earth’s rotation is not perfectly steady. Meanwhile, physicists had been developing a device called an atomic clock. This instrument is able to measure, with incredibly high precision, the frequency of radio waves absorbed by atoms as they move between two closely spaced energy levels. This frequency can be reproduced with great accuracy at many laboratories around the world. Consequently, the SI unit of time—the second—was redefined in 1967 as follows:
An atomic clock at the National Institute of Standards and Technology is the primary standard of time.
One second is the time required for 9,192,631,770 oscillations of the radio wave absorbed by the cesium-133 atom. The abbreviation for second is the letter s.
Several radio stations around the world broadcast a signal whose frequency is linked directly to the atomic clocks. This signal is the time standard, and any time-measuring equipment you use was calibrated from this time standard.