You Asked:

How does the clock in Fort Collins, Colo., work? — David, East Bethel, Minnesota

Marshall Brain Answers:

You have probably seen, and may actually own, one of the “atomic” clocks or wristwatches that automatically sets itself. You put in the battery and a few minutes later the clock automatically knows the correct time and date.

This is made possible by an atomic clock in Colorado. This atomic clock is nothing special by itself (see How Atomic Clocks Work for a description). What makes it unique is the nearby shortwave radio station. This single radio station has the ability to transmit the current time and date across the continental United States. This appears to be the antenna for the station:

This radio station is able to transmit such great distances because: a) it has a lot of power (70,000 watts) and a large antenna, b) it transmits at a very low shortwave frequency (e.g. 60 KHz), and c) it uses a very low data rate for the time message (one bit per second). It takes a full minute to transmit the current time and date. You can hear what the time message sounds like in this video:

This page contains more details on the WWVB radio station.