I unit of time. It is defined by taking the fixed numerical value of the cae- sium [cesium] frequency, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s-1. So, that was a quick history of how the world decided to use atomic clocks as our source of the length of the second. The next question to address is, "Which is the precise second of the day/hour that you are currently experiencing?" 1.3.1 What Is TAI? International Atomic Time, or from the French, Temps Atomique International (TAI), is the world's time as determined by the weighted average of a worldwide network of atomic clocks (over 400 in 80 national laboratories). The scientists in these laboratories use numerous techniques to measure very precisely the time difference between clocks, allowing each lab to compare the time of its clock with that of the other labs' clocks. BIPM publishes these comparisons monthly in its Circular T (more on that in the following section). TAI is what is known as a monotonic timescale, meaning that since it was aligned with universal time on 1 January 1958 00:00:00 (its "time zero," known as its epoch), it has never jumped time (as in some form of leap event) and continues to count forward one second at a time. It is uniform and very stable, meaning that it is not exactly in step with the slightly irregular rotation and orbit of the earth. For this reason, another timescale beyond TAI was needed, one which more closely aligns with our day and years (although the differences are small). 1.3.1 What Is UTC? To have a timescale that reflects the realities of our world (look at what happened to the calendar before the leap year problem was corrected), scientists developed UTC, or Coordinated Universal Time. UTC is identical to TAI except that it is a discontinuous timescale because it occasionally has leap seconds inserted to ensure it reflects changes in the earth's rotation and therefore aligns with our days and years. Leap s