<P> Alarm clocks first appeared in ancient Greece around 250 BC with a water clock that would set off a whistle . This idea was later mechanized by Levi Hutchins and Seth E. Thomas . </P> <P> A chronometer is a portable timekeeper that meets certain precision standards . Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by means of celestial navigation, a precision firstly achieved by John Harrison . More recently, the term has also been applied to the chronometer watch, a watch that meets precision standards set by the Swiss agency COSC . </P> <P> The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years, and are used to calibrate other clocks and timekeeping instruments . Atomic clocks use the frequency of electronic transitions in certain atoms to measure the second . One of the most common atoms used is caesium, most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations . Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms . SI defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the Cs atom . </P> <P> Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe . </P>

Which of the following best defines a second as an si unit of time