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Leap Seconds

Today the International Telecommunications Union are voting on whether to abolish the leap second. This miniscule measure of time is added in to our time-keeping systems every so often to make sure they align more accurately with the time as measured by the spin of the Earth.

The original definition of the second was 1/86400 of a mean solar day which is related to the speed of the Earth’s spin about its axis. We might call this the “Earth second”. However the Earth’s spin in not regular. To begin with the Earth is slowing down by a couple of milliseconds per century due to tidal breaking. This breaking action is as a result of the drag of the Earth spinning beneath the tides created by the Moon. In effect the Moon is “stealing” energy from the Earth, increasing in its orbit about us while our spin slows.

In addition to this discrepancy the Earth is occasionally wobbled off course by major geological events, such as earthquakes. The 2004 Pacific earthquake which resulted in the Boxing Day Tsunami actually caused the Earth to speed up by over 2 milliseconds.

To avoid the problem of an irregular length of day – and therefore an irregular length of second – scientists adopted the much more regular SI second, which is the length of time it takes for 9,192,631,770 cycles of vibration of atomic caesium. This “atomic clock second” is accurate to one part in ten billion, and since 1972 this has been the international standard in timekeeping.

But time kept using the the SI second doesn’t match exactly with time kept based on the spin of the Earth, which after all is the time we experience every day. In order to make these two time signals match leap seconds are added every so often. Since 1972 25 leap seconds have been added. The last leap second was added at 23:59:59 on 31 December 2008, and the next one is due to be added at 23:59:59 on 30 June 2012. But leap seconds themselves are irregular, and are decided on by the ITU whenever the two time signals drift by more than 0.9 seconds.

The argument for abolishing these additional leap seconds is that it creates problems for modern computing and navigation systems that use the atomic clock second. Every time one of these irregular leap seconds is added the world’s hi-tech time keeping devices need to check and adjust by one second. It would be far simpler for us to use only “atomic clock seconds”.

However if we were to ditch the leap second then our civil time keeping would begin to drift with respect to “real” Earth time, so that in thousands of years time our clocks might read 8am just as the Sun is setting. This might seem like a minor concern right now – after all a millennium is a long time – but it’s something that astronomers and scientists do need to consider to avoid future problems. One alternative would be to introduce a “leap hour” to be introduced every few hundred years to keep the clock aligned with the real world.

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