Leap second: An extra second to be added to last minute of today

Updated: 2015-06-30
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On June 30, at 23:59:59 GMT precisely, the world's clocks will add an extra second to the day, bringing the total number of seconds for 2015 up to 31,536,001
Blink and you'll miss it but time will stand still tonight. 
A second will be added to clocks at 23:59:59 in co-ordinated universal time – the worldwide time standard – which is an hour behind British Summer Time. 
The adjustment is necessary because measuring 24 hours by the spinning of the Earth – astronomical time – is slightly inaccurate due to the gravitational pull of the Moon.
Today our high-tech world relies on super-accurate atomic clocks, which don’t lose time as easily, so the ‘leap-second’ as it is known, will allow both measures to catch up with one another. 
The extra second is needed because the Earth's rotation is gradually slowing down by around two-thousandths of a second per day, and it needs to catch up with atomic time.
Atomic time uses vibrations within atoms to measure time and is said to be the most reliable because atoms resonate at extremely consistent frequencies. 
So leap seconds are occasionally used to help 'Earth time' catch up to 'atomic time' as the former is slower by about two thousands of a second per day. 
To keep them in sync, it is necessary to occasionally jump Earth's time back - for mathematical reasons similar to adding leap years. 
The decision to do so is made every time Earth time is slower by about half a second, making it about half a second quicker instead.
'At the time of the dinosaurs, Earth completed one rotation in about 23 hours,' says Daniel MacMillan of Nasa's Goddard Space Flight Center. 
'In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds.' 
This year will be the 26th time since 1972 that a leap second will have been added. 
It is the 26th occasion the adjustment has taken place since leap-seconds were introduced in 1976, and it could cause computer systems to falter. When the last one occurred in 2012, a number of high-profile websites, including business networking site LinkedIn, were disrupted. 
And in Australia, more than 400 Qantas flights were delayed when staff were forced to switch to manual check-ins. 
On June 30, not everyone will add the leap second in the same way or at the same time. 
In some systems, the computer clock shows 60 seconds instead of rolling over to the next minute, or showing the 59th second twice. 
As a result the computer sees a leap second as time going backward, causing a system error and the CPU to overload.
For computers that don't crash, processes based on precise timing, such as the amount of time a valve opens to add a chemical to a mix, may be off by half a second. 
But it remains to be seen whether this will create large-scale problems, as some computer scientists have predicted. 
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