We don’t exactly, as told by Andrew Grant in Science News (March 8, 2014):
The international time standard — Coordinated Universal Time, or UTC — is a calculation, not a physical clock. That makes timekeeping a bit fuzzy. The International Bureau of Weights and Measures near Paris collects time readings from about 200 atomic clocks in more than 50 international labs and then, once a month, averages the results.
Judah Levine, the official U.S. timekeeper for more than 40 years, does his best to align U.S. clocks with UTC. Several times a day, he sends a file to the bureau with the time he thinks it is, based on the cesium clock at the National Institute of Standards and Technology in Boulder, Colo.
Around the 10th of each month, he gets an e-mail response telling him how wrong he is, based on the previous month’s data. Levine then tweaks his clock’s ticking rate — usually by a nanosecond per day. “I have to make an estimate of where I am today,” he says, “because I won’t know until a month from now.”
For a work biography of Judah Levine, click here. For images of or relating to Judah Levine, click here.
For a work biography of Andrew Grant, click here. For images of or relating to Andrew Grant, click here.
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