So scientists have been interested in finding things in nature that are regular and dependable. The Earth goes around the sun in about 365 days, the moon goes through a cycle about every month, and, until recently, the geyser Old Faithful shot out steaming water about every 15 minutes every day. But scientists want absolute accuracy. Investigating the natural vibrations of atoms seems to be the best the answer. An atom vibrates at very regular intervals in nanoseconds.
All natural elements absorb and release electromagnetic radiation at a certain fixed frequency.
In simple terms, atomic clocks are electronic devices that can measure time by counting the number of times atoms vibrate. Because many different types of elements are used for atomic clocks, the clocks work in different ways. Cesium atomic clocks are one of the most accurate and most commonly used types of atomic clocks. The picture above is a very old cesium atomic clock. T o give you an idea about the size of a laboratory Cesium atomic clock, it is about the size of a railroad flatcar.