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Put a little extra tick into your tock

As a chronometer, the Earth just isn’t up to scratch and every so often a 'leap second' is added to compensate. Image montage: Ben Gilliland

Those of you booking a holiday at the end of June are in for a treat. The world’s official time keepers have decided to add a ‘leap second’ to 2012. Like a ‘leap year’, a ‘leap second’ is added to bring our clocks back into sync with the rotation of the Earth and, thanks to that, your holiday will be one second longer.

The length of a day is determined by the Earth’s rotation and one full rotation equals one full day. But the speed of the Earth’s rotation isn’t constant – ocean tides pulled back and forth by the Moon’s gravity, churning molten materials deep in the Earth’s bowels, earthquakes and even friction from the wind all add up and force the planet to give up a tiny bit of its rotational energy. In other words, it slows down and our clocks need to compensate for this.

That’s not to say that the planet isn’t a good time keeper – in 200 years a day has only lengthened by two milliseconds. But, over millennia, all those tiny increases add up and in 400million years or so, a day will be 26 hours long.

The custodians of humanity’s timekeeping is a group called the International Earth Rotation and Reference Systems Service (IERS). These ‘time lords’ use a global network of radio telescopes called the Very Large Baseline Interferometry (VLBI) network to measure the speed of the Earth’s rotation to within a millionth of a second.

In general, one leap-second is added every year or two but unusual activities in the Earth’s core since 1999 has meant that only two leap-seconds have been added in this time (the last was added in 2008).

But why should we care? Would it matter if we let the odd millisecond slide by? True, you and I can’t perceive these variations and, even if we could, it wouldn’t really matter. But things like satellite navigation systems have to be able chart the passage of time so accurately that these tiny changes do make a difference.

So enjoy the extra second at midnight on June 30 and spend it wisely.