ABSTRACT
Modern timekeepers use atomic clocks as a de facto primary standard of time. Per this standard, a second is precisely the time for 9,192,631,770 orbital transitions of the cesium 133 atom. In actual practice, there is a slight discrepancy-86,400 atomic seconds is close to 3 ms less than a solar day, so when the discrepancy grows to about 1 s, a leap second is added to the atomic clock.
