Mathematics Throughout History

Time and Math in History

How does your clock automatically change?

If you ever wondered how your more recent clocks—analog and digital—change without any twisting of the watch stem on your part, it’s merely a matter of radio control. Inside your timepiece are an antenna and a radio receiver, which allows the watch to synchronize with the atomic clock in Boulder, Colorado, through the National Institute of Standards and Technology. The NIST operates the radio station WWVB, and with its high power transmitter (50,000 watts) broadcasts a timing signal 24 hours a day, 7 days a week. All of the broadcast frequencies are in the high frequency radio spectrum, a part that is often called “shortwave;” the time is kept to within less than 0.0001 milliseconds of Coordinated Universal Time (UTC). Your watch or clock catches the signal, and decodes the time code bits into time, day of the year, daylight savings time, and even leap year and second changes. All you have to do is select the time zone so the clock can convert the signal’s UTC to your time.

When the signal gets to your watch, it may be a bit “inaccurate,” as the signal sometimes depends on atmospheric conditions or even distance from the NIST—and we mean a bit. In fact, it may vary as much as a millisecond if the signal is bouncing around between the Earth and the ionosphere. In general, the accuracy is usually off by less than 10 milliseconds. That means you really don’t need to complain—that translates to 1/100th of a second. No doubt you’ll still make the meeting, meet up with your friends after work, and even make it to the baseball game on time!


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