: The definition for emphemeris time contains the discussion on how the particular number of cesium transition counts was arrived at. "After three years of comparisons with lunar observations it was determined that the ephemeris second corresponded to 9192631770 cycles of the cesium resonance."  It took them three years to determine the count that fit emphemeris time, it was the best average arrived at in the 3 year period. It is my contention that the SI base units, including the meter are a poor choice for scientific calculations, as they were defined without the use of  physical science or mathematical constants. It is another argument for anti-metrication.