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.
Smart alecks will reply that it's a 60th of a minute, and strictly speaking they're right. But a second was defined more precisely by the 13th General Conference of Weights and Measures in 1967:
"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."
The machine first used by Louis Essen and Jack Parry to standardise atomic time is in the Science Museum and pictured here.
But why divide a minute into 60 seconds and an hour into 60 minutes in the first place? The sexagesimal system is thought to have been invented by the Sumerians, and there are a number of interesting theories as to why they chose a base of 60.
