If Atomic Clocks Are So Accurate Why Do They Need Adjusting From Time To Time?

Hydrogen maser clocks are the most accurate of atomic clocks – they lose less than one second of time in one and a half million years. Caesium clocks are used in international time keeping but, still, minute adjustments must still be made periodically because the rotation and movements of the Earth are not quite as regular.

The clocks must reflect real time as well as accurate time and the length of the year must be adjusted now and again so that the calendar is exactly synchronised with the Earth's rotation. This has a tendency to either speed up or slow down.

There have been 17 adjustments since 1972 to solve this problem. In 1992, the northern hemisphere's summer was one second longer than usual. An extra second was added to the world's time at precisely 23 hours, 59 minutes and 60 seconds on 30 June 1992. This was to address a discrepancy measured by the International Earth Rotation Service in Paris.