What a "second" means

         While many people may look at an electronic clock to illustrate the length of a second, the true nature of a second is much more complex. Numerous nations have collaborated for decades to determine precisely what a second should be. As of 1967, a second is defined to be the duration of "9,192,631,770 periods of radiation corresponding to a hyperfine transition of caesium-133 in the ground state" (Tal, 301).  With the use of well-crafted atomic clocks, metrologists across the world are able to measure a second to within nanoseconds of each other in the best cases.

        These atomic clocks measure the frequencies that correspond with specific atomic transitions. For a clock to be used in taking these measurements, it must first have an accurate frequency. This is determined by the clocks ability to tick according to the well-determined intervals associated with the atomic transitions. Additionally, these clocks must have stable frequencies. This is determined by the uniformity of ticks, or how well the ticks mark equal time intervals. Additionally, there is not a single clock that is used to measure the second. There are hundreds of these clocks in use around the world, and through significant analysis of their readings, the second is ultimately determined.