Despite the appeal of measuring a lifetime in daylights, sunsets, midnights, cups of coffee, inches, miles, laughter, or in strife, physicists still choose “seconds.” How refreshingly simple!

However, as you might expect, physicists have developed a precise way to define a second. Atomic clocks, such as the one shown in Concept 1 to the right, rely on the fact that cesium-133 atoms undergo a transition when exposed to microwave radiation at a frequency of 9,192,631,770 cycles per second. These clocks are extremely accurate. Thousands of years would pass before two such clocks would differ even by a second. If you are an exceedingly precise person, you might want to consider buying a wristwatch that calibrates itself via radio signals from an atomic clock. For now, though, you can visit a web site that displays the current time as measured by an atomic clock.

In addition to being used to measure a second, atomic clocks are used to keep time. The length of a day on Earth, measured by the time to complete one rotation, is not constant. Why? The frictional force of tides causes the Earth to spin more slowly. This means that the day is getting longer (does it not just feel that way sometimes?). Every fifteen months or so since 1978, a leap second has been added to official time-keeping clocks worldwide to compensate for increased time it takes the Earth to complete a revolution.