National Physical Laboratory

Why do we need leap seconds? (FAQ - Time)

Atomic clocks are now over a million times more regular than the Earth's rotation - the Earth has become inadequate as a basis for accurate time-keeping.

Since 1972, instead of dividing the mean solar day into 86 400 seconds, the world's time scale, known as Coordinated Universal Time (UTC), has been based on counting seconds from atomic clocks. But astronomers and navigators would still like the time to remain consistent with the 'traditional' positions of the stars, sun and moon to within a fraction of a second.

As UTC is running about one second per year faster than time based on the Earth's rotation (Greenwich Mean Time, or Universal Time), an extra second is occasionally inserted in the UTC time scale to let it fall behind and start catching up again. This positive leap second, when required, is added immediately before midnight UTC either at the end of June or at the end of December. The positive leap second is second number 60 in a 61-second minute. A negative leap second would remove the final second (number 59) before midnight UTC to give a 59-second minute. Negative leap seconds are unlikely to be needed in the foreseeable future.

Further information about the leap second and other time-related issues

Last Updated: 18 Feb 2015
Created: 27 May 2010

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