A naval atomic clock is a highly accurate timekeeping instrument maintained by a national navy. Navies use atomic clocks for accurate navigation information, electronic data transmission and general timekeeping. Many naval atomic clocks, including those maintained by the United States Navy, are also used by other organizations that require accurate time measurements.
Naval atomic clocks, like all atomic clocks, keep time by measuring changes in electron energy states in certain atoms. The U.S. Naval Observatory's Master Clock, for instance, measures electron energy state transitions in cesium atoms when the electrons interact with specific radio frequencies.
The change in a radio wave's frequency after it interacts with an electron is a set, constant quantity, making atomic clocks far more accurate than either traditional mechanical or quartz clocks. While mechanical and quartz clocks can vary a second or more per day, the most accurate atomic clocks only vary about one ten-billionth of a second each day. This means that these clocks do not lose or add single seconds for millions of years.
Naval atomic clocks bear little resemblance to traditional clocks. These machines take up large amounts of space and require very tight environmental and electronic controls for the highest accuracy. The U.S. Naval Observatory's Master Clock is actually dozens of different interconnected atomic clocks occupying numerous rooms at the Washington, D.C. facility.