Leap second

By David Ord

…… Just hang on a second!

That is exactly what we will be doing in 2015 as the Jan 5th edict from the International Earth Rotation and Reference Service (IERS) requires us to insert a ‘leap second’ at the end of June this year. ‘Leap seconds’ are used to adjust the time scale Coordinated Universal Time (UTC) to the apparent motion of the Sun, i.e. to Earth’s rotation.

UTC itself is the time standard used to determine local times in time zones worldwide and is based on the combined output of around 30 highly precise atomic clocks. This statistical time scale is called International Atomic Time or TAI.

A normal day consisting of 86,400 seconds defines each second as one 86,400th of the time taken for the Earth to rotate about its axis. In atomic time, the second is defined as the time it takes for a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times. The accuracy of the latter is typically within 1 second every 20 million years, but can be up to 15 times better than that.

Unfortunately, the Earth’s rotation does not begin to compare with atomic time keeping in terms of its rotational reliability. Indeed, the speed of the Earth’s rotation differs from day to day and from year to year and even the differences are inconsistent. For example, in 2001, the discrepancy to atomic time was .02 seconds, but in 2011 was .28 seconds. Hence there is a need to monitor these fluctuations in time and make the necessary adjustments. There have been 27 ‘leap seconds’ added since 1972.

cesium clock

The American NIST-F2 Cesium atomic clock accurate to 1 sec in 300 million years

However, there are proposals afoot to stop the adjustment of UTC to the Earth’s rotation and use only the International Atomic Time (TAI). This move is being largely advocated by the French, with some American support pushing for a decision at the next World Telecommunication Conference later this year in Geneva.

The British Government opposes the move to banish ‘leap seconds’ and stands firmly behind Greenwich Mean Time.

Now, this is not the first time that we have had a slight difference of opinion with the French about who should be the Time Lords of planet Earth. For nearly 130 years, Greenwich has been the home of time and the moment when the sun passes its highest point is noon, Greenwich Mean Time. Today, even Astronauts on the ISS ignore both Washington and Moscow time zones in favour of the GMT standard. But it was not always thus.

Britain has a proud heritage of keepers of time; though not necessarily renowned for time-keeping. The first mechanical clock in Dunstable Priory was made in 1283; the world’s oldest working clock at Salisbury Cathedral dates back to 1386 and then there is John Harrison, who invented the first ship’s chronometer.

Harrison’s clocks were accurate within 0.06 seconds everyday. As fantastic as this invention was for a seafaring nation, the fact that it also began the concept of standard time was rather overlooked.

Before the telegraph and railways, every town had its own local time. The fact that it was noon in London and 11:45am in Exeter did not create any major problems. But with the railways came a timing nightmare and indeed, it was Great Western Railways who first introduced a standard ‘Railway Time’ in 1840. The standard chosen by them was Greenwich, which had already been home of the Royal Observatory for over 165 years and had kept the most precise astronomical records in the world.

By 1855, 90 percent of all towns in Great Britain were using Greenwich Mean Time as a standard. Some towns refused, others procrastinated and it took a Government decree in 1880 to officially force the time standard throughout the country.

In 1884, at the International Meridian Conference in Washington, the British GMT was adopted as the benchmark for world time and the Greenwich Meridian was agreed as the zero point for longitude. Not everyone agreed though.

The French were miffed that their proposal that the meridian should run through Paris was not accepted. They ignored the International consensus and proceeded to follow their own Paris Mean Time. This rather ineffectual stance against the rest of the world and common sense lasted for around 20 years. In 1911, the French Government with extreme reluctance announced that ‘Paris Mean time will be retarded by 9 minutes and 21 seconds’. This effectively brought the French into line with GMT – but they could not bring themselves to mention Greenwich in the announcement.

First electric clocks, then quartz based followed by atomic clocks pushed the accuracy of timekeeping way beyond the consistency of the Earth’s rotation. In April last year, the National Institute of Standards and Technology (NIST) of America announced the Cesium-133 NIST-F2 atomic clock. It was certified as the most accurate clock in the world at 1 second in 300 million years and is used by the US Naval Observatory to control the GPS navigation system.

Still at an experimental stage, the strontium lattice atomic clock which neither gains nor loses a second in 5 billion years has been reported.

blue cloud

The ‘Blue Cloud’ heart of the Strontium Lattice clock, our most accurate timepiece

Precise timekeeping underpins much of our modern world. GPS, for instance, needs accuracy of about a billionth of a second in order to keep us from getting lost. Satellites rely on high precision timing coming from atomic clocks. GPS, in turn, is used for synchronizing digital networks such as cell phones and the NTP servers that provide the backbone of the internet.

Variations in the pull of the Sun and the Moon and changes in Earth’s molten core create very small changes to the Earth’s rotation and these irregularities cause major discrepancies in our modern digital world. To avoid Atomic Time getting out of synch with the Earth’s rotation, the fix was to introduce ‘leap seconds’. We have managed to survive the ‘leap second’ fixes since 1970.

The problem is that as we become more and more reliant upon digital devices, the impact of inserting ‘leap seconds’ increases. Because the adjustments are sporadic, there is a danger that they are botched, overlooked or the effects underestimated. If so, then there is the possibility that electronic systems that depend on accurate time and timing could be compromised. This could affect mobile phone networks, air traffic control, financial trading systems and network servers such as the internet.

During the last ‘leap second’ correction in 2012, the Qantas and Virgin Australia airline booking system crashed, as did servers run by Amazon, Mozilla, Foursquare, Reddit and Linkedin.

Google on the other hand developed a procedure to gradually add a couple of milliseconds during a period prior to the ‘leap second’ change and called it the ‘Leap Smear’. This ensured that none of their clients were affected – so the ‘leap second’ can be handled.

However, the limited chaos that was caused has fuelled the French proposal to stop ‘leap second’ corrections and therefore effectively abandon time based on the Earth’s rotation in favour of Atomic time only.

Not that the French are still bearing any grudges over the adoption of GMT in 1884, but it is worth pointing out that it has been particularly difficult to find any ‘entente cordial’ with them on the subject of time.

Those with an eye for detail who are reading this article may have noticed that the acronym for Coordinated Universal Time is not as you might expect CUT but UTC. This is because the French refused to accept the acronym and preferred their acronym TUC for ‘Temps Universel Coordonne’. UTC was proposed and accepted by the rest of the world as a compromise.

The official British view is that the duration of the ‘atomic day’ is meaningless to society and that we should continue to measure time in a meaningful way. In other words a day should remain ‘one turn of the earth’ and not 794,423,384,92800,000 cycles of cesium-133 radiation..

The proposal to ditch the ‘leap second’ has appeared at the World Telecommunication Conference in 2003, 2005 and 2012; with no conclusion reached. It is on the agenda for a decision again later this year. It will not be an easy one and there may be many in disagreement which ever way it goes. Perhaps they will have second thoughts (sic) and defer the decision once again.

One thing is for sure, let us enjoy the extra second this year, while we still can..

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