On time. Photograph: Sergio Dionisio/APLondoners who are familiar with the "Underground minute", or the phenomenon of time actually slowing down as a train approaches a tube station, will have no trouble grasping the concept of the "leap second". Briefly, 2006 will arrive a second later on Sunday because the earth is not keeping up with our system of timekeeping.
The friction of the tides means that the rate at which the world is spinning on its axis is slowing. Days are now about two milliseconds longer than they were at the beginning of the nineteenth century. So the International Earth Rotation and Reference Systems Service occasionally deploys a leap second in order to regulate "sun time" with "clock time".
Before considering exactly how to use that extra second - and bear in mind it represents only 1/3,600th of the "extra" hour we enjoy every year when the clocks go back - you might like to ponder what exactly a second is.
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