How does one arrive at the exact number of cycles of radiation a cesium-133 atom makes in order to define one second?

WWVH’s chief objective is to broadcast Coordinated Universal Time (UTC) — the official time for the world — throughout the Pacific region. These signals help residents, from Alaska to Australia and from California to China, coordinate, calibrate and synchronize their clocks, networks and equipment, which are vital to telecommunications, internet connections and a wide array of government and consumer services …..

Colorado radio station WWVB marks half a century as the nation's official time broadcaster on July 5. Most people aren’t even aware the station exists, but it has a rich and fascinating history. Its future is uncertain, however, as newer technologies threaten to make it obsolete.

These radio stations don’t play news, talk shows or music. But they do broadcast very, very accurate time signals.