A Second Is Defined by 9,192,631,770 Ticks of a Cesium Atom

The second is the international unit of time, and today it is defined by atoms. The U.S. National Institute of Standards and Technology explains that the second is currently defined using cesium atoms, which absorb and emit microwave radiation with a specific frequency. Atomic clocks count 9,192,631,770 of those microwave oscillations, and the elapsed time interval is called a second. The advantage, according to NIST, is that atomic clocks let us define the second, the international unit of time, in a precise and measurable way that does not change over time. The results are remarkable. NIST says commercially available cesium clocks keep time to within 1/3,000,000 of a second per year. The ideas of the minute and the second themselves are ancient, and NIST notes they began in astronomy, not timekeeping.