Atomic clock

The second is defined as 9,192,631,770 periods of the caesium-133 atom, and is currently realised at the National Physics Laboratory (NLP) to an accuracy of one second in 15 million years.

Scientists are currently working on technology to increase this accuracy to 1 second in 10 billion years.

The standard way of counting the passing of seconds is by the use of an atomic clock. There are internationally agreed time-scales which set the beginning of each new day and the calendar. Greenwich Mean Time (GMT) was established as the first global time scale in 1984. The current atomic clock global time scale is UTC or Co-ordinated Universal Time. UTC was adopted as the official time for the world in 1972. The official keeper of atomic time is the International Bureaux for Weights and Measures.

NPL uses its atomic clock to contribute to the determination of UTC along with the atomic clock of 65 laboratories worldwide.

UTC is a compromise between the times defined by the atomic clock and the time based on the earths rotation about its axis. The seconds of UTC are counted using an atomic clock, allowance is made to keep UTC within 0.9 seconds of the Earths rotation by inserting leap seconds at the end of each quarter. Leap seconds are inserted to take account of the speeding up or slowing down of the rotation of the Earth.

Twenty two leap seconds have been added between January 1st 1972 and January 1st 1999 either at the end of June or December. Without the addition of leap seconds, the sun would be seen overhead at midnight (rather than noon) after approximately 50 000 years.