EXAMPLE 9 Really accurate time

NIST’s atomic clock is very accurate but not perfectly accurate. The world standard is Coordinated Universal Time, compiled by the International Bureau of Weights and Measures (BIPM) in Sèvres, France. BIPM doesn’t have a better clock than NIST. It calculates the time by averaging the results of more than 200 atomic clocks around the world. NIST tells us (after the fact) how much it misses the correct time by. Here are the last 12 errors as we write, in seconds:

In the long run, NIST’s measurements of time are not biased. The NIST second is sometimes shorter than the BIPM second and sometimes longer, not always off in the same direction. NIST’s measurements are very reliable, but the preceding numbers do show some variation. There is no such thing as a perfectly reliable measurement. The average (mean) of several measurements is more reliable than a single measurement. That’s one reason BIPM combines the time measurements of many atomic clocks.