Leap second: Earth's slowing rotation makes Saturday one second longer (+video)
Leap second: International timekeepers are adding an extra second to the official time to ensure that our clocks will keep pace with the Earth's rotation.
The world is about to get a well-earned long weekend but don't make big plans because it will only last an extra second.
A so-called 'leap second' will be added to the world's atomic clocks as they undergo a rare adjustment to keep them in step with the slowing rotation of the earth.
To achieve the adjustment, on Saturday night atomic clocks will read 23 hours, 59 minutes and 60 seconds before moving on to midnight Greenwich Mean Time.
Super-accurate atomic clocks are the ultimate reference point by which the world sets its wrist watches.
But their precise regularity - which is much more constant than the shifting movement of the earth around the sun that marks out our days and nights - brings problems of its own.
If no adjustments were made, the clocks would move further ahead and after many years the sun would set at midday. Leap seconds perform a similar function to the extra day in each leap year which keeps the calendar in sync with the seasons.
The grandly named International Earth Rotation and Reference Systems Service (IERS) based in Paris, is responsible for keeping track of the gap between atomic and planetary time and issuing international edicts on the addition of leap seconds.
"We want to have both times close together and it's not possible to adjust the earth's rotation," Daniel Gambis, head of the Earth Orientation Centre of the IERS, told Reuters.
Gambis said the turning of the earth and its movement around the sun are far from constant.
In recent years a leap second has been added every few years, slightly more infrequent than in the 1970s despite the long-term slowdown in the earth's rotation caused by tides, earthquakes and a host of other natural phenomena.
Adjustments to atomic clocks are more than a technical curiosity.
A collection of the highly-accurate devices are used to set Coordinated Universal Time which governs time standards on the world wide web, satellite navigation, banking computer networks and international air traffic systems.