Happy New Year (and one second)
It is nearly the start of another new year, but this year there will be an extra pip just before midnight. The pips (heard often on BBC Radio Four) mark the five seconds leading up to the hour with the final, longer, pip marking the start of the new hour. This year there will be seven pips instead of the normal six because we have to add a leap second in order to keep Coordinated Universal Time (UTC) in line with time measured by the Sun.
The reason we have leap seconds in the first place is due to the very gradual slowing down of the Earth's rotation over time. The second is defined as "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 cesium 133 atom" (USNO) and is measured by a collection of caesium clocks around the world.
Earlier in 2005 the suggestion that these leap seconds should be abolished caused quite a debate. Why should anyone care? It's just one second, after all.
It matters for radio astronomy. In order for interferometry to work correctly astronomers need to know very accurately where their telescopes are and the exact time at which each measurement was made so that the signals can be combined later. If time is measured differently at different telescopes then it all goes horribly wrong, we lose vital information and can no longer make a map of the sky (which is the whole point). You might still be thinking "so what?" Well, it matters for satellites too. One of the arguments for abolishing leap seconds is the difficulty that systems such as the GPS satellite network have in incorpating the changes into their systems. Many systems like this require the changes to be added in by hand rather than automatically. The proposals are still being discussed and the debate is ongoing, but I will be listening at midnight.
Right now I'm off to listen to Jool's Hootenanny.... Happy New Year.