The atomic clock, the most accurate of timekeeping devices, is based on the measurement of changes in the energy states of atoms. The energy change involved in the most common forms of the atomic clock occurs when the atom absorbs energy, causing an electron to alter its spin characteristics and, subsequently, its magnetic field. The unique frequency (number of complete oscillations per second) of the radiation absorbed by an atom when it undergoes such an energy change is a periodic phenomenon analogous to the swing of a pendulum and may thus be used as a time standard. Because this frequency is largely independent of all normal external conditions, such as air pressure and magnetic fields, the atomic clock is a highly stable device. Atoms especially suitable for atomic clocks include cesium, rubidium, and hydrogen.

The need for such precision, accuracy, and consistency is evident when you think of stellar navigation. Being off by one second could cause a rocket or missile to end up miles away from an intended rendezvous or target.

The U.S. government maintains several atomic clocks in various parts of the country. The NIST (National Institute of Standards and Technology) maintains an atomic clock in Boulder, Colo., and Kauai, Hawaii. And the USNO (U.S. Naval Observatory) atomic clock is maintained in Washington, D.C.

Related Links:

• The Evolution of Time Measurement from the National Institute of Standards & Testing.
  
• Download Software to Synchronize Your Clock With the Atomic Clock from NIST's Automated Computer Time Service (ACTS).
  
• How Stuff Works: The Atomic Clock from the award-winning source of credible, unbiased, and easy-to-understand explanations of how the world actually works.