The Atomic Clock
Within the atom, when electrons jump from one place to another, they absorb energy, which is later released in the form of a wave. All kinds of waves have one thing in common: they cause a movement to be repeated at intervals.
Sea waves, for example, make a small buoy bob up and down with a regular motion.
The waves that are emitted from atoms are so regular and occur so many times in a second that they can be used as a standard of time. Different atoms produce many different wave movements, and the most suitable of these has turned out to be the element caesium. In 1967 a certain wave emitted from the caesium atom was adopted as an international standard of the second. One second was defined as 9,192,631,770 vibrations of this particular wave.
Caesium clocks can be built anywhere in the world and will give exactly the same measurements and remain extremely accurate: a caesium clock will not gain or lose more than one second in 6,000 years. More accurate clocks are contemplated using hydrogen atoms, which will even improve on the accuracy of a caesium clock by as much as a hundred times.
With such clocks, which do not depend on the motion of the earth, sun or planets, irregularities in the rotations of these heavenly bodies have been discovered. It is now known, for example, that our solar year can alter by as much as three seconds. It was, in fact, irregularities such as this which forced the early timekeepers to adopt an average, or mean day. Hence the need for Greenwich Mean Time.