TIME KEEPS ON TICKING
I was intrigued by "Time Machine" [November]. Although the apparent intent of the Clock of the Long Now project is to make us more aware of long-term thought, several comments in the article reflect a misunderstanding of the technical concept of "accuracy." The lead-in paragraph says that the clock is designed to "run with perfect accuracy." The term "accuracy" is a qualitative expression that attempts to describe the degree to which a measurement is "inaccurate." In the field of metrology, this imperfection is best described in statistical terms, the accepted term being "uncertainty of the measurement," or the degree to which the "true" value is not known. Even the NIST-F1 cesium fountain atomic clock is not perfect; it attains an uncertainty level of about 5 x 10–16 second. This is equivalent to about +/– 1 second in 60 million years.
Kurt SolisHouston, Texas
