The first practical cesium atomic frequency standard was built at the National Physical Laboratory in England in 1955. The frequency of the cesium reference was established or measured relative to astronomical time.
The cesium atom’s natural frequency was formally recognized as the new international unit of time in 1967. A second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom’s resonant frequency, replacing the old second that was defined in terms of the Earth’s motions. The interval of a second quickly became the physical quantity most accurately measured by scientists.
As of January, 2002, the National Institute of Standards and Technology primary cesium standard was capable of keeping time to about 30 billionths of a second per year. Called NIST-F1, it is the 8th of a series of cesium clocks built by the National Institute of Standards and Technology.
I have to report on this for one basic reason. My wife keeps her clocks set at radically different times. For example, the clock in our bedroom is set 20 minutes ahead. In our kitchen one of the clocks is set 3 hours ahead. Therefore, it really doesn’t matter how accurate a second can be. As Mark Twain describes the issue with inaccurate timepieces:
My watch slowed down to that degree that it ticked like a tolling bell. I began to be left by trains, I failed all appointments, I got to missing my dinner; my watch strung out three days’ grace to four and let me go to protest; I gradually drifted back into yesterday, then the day before, then into last week, and by and by the comprehension came upon me that all solitary and alone I was lingering along in the week before last, and the world was out of sight.
I have no idea what time it really is. I must be late for something.