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Why the Leap-Second Is Important for Keeping Atomic Clocks Attuned to Nature

In January 2012, international delegates gathered at the headquarters of the International Telecommunications Union (ITU) in Geneva. Their brief was to discuss whether we should change a basic foundation of our culture, our biology and our evolution. Should our days and nights continue be regulated by the rising and setting of the sun, as it has been since the first bugs found themselves stranded on the shores of the primordial sea? Or should an increasingly arbitrary definition of the basic unit of time, as paced out by a soulless atomic clock, rule the measure of our lives?
        Atomic clocks were first demonstrated in 1949, but by the mid 1960s they had advanced to the point where they were the most accurate clocks in world; more accurate than what had been the master clock by which all other clocks and watches on the planet had been regulated up until then - the rotating earth itself. In fact, compared to the exquisitely precise measured beat of the atomic clock, the earth was revealed to be quite a poor time-keeper. So in 1967 one second of time was redefined as being 9,192,631,770 vibrations of the Caesium-133 atom, rather than one 86,400th part of the mean solar day.
        In 1972, when atomic clocks took over the role of the worlds master time-keepers, the time was originally set like any other clock to match the movement of the sun as it rose and set. Man's most advanced technical triumph in keeping track of time was built to agree with the humble sundial - surely man's oldest method of tracking the passage of the day. Since that day, atomic clocks have become vital in regulating the increasingly complex technology that forms the infrastructure of our lives. Internet web servers and email providers all need to work in synchronized harmony. GPS satellites know the correct time to within a billionth of a second so they can keep us found to within a few yards of where we actually are. Mobile phone networks passing on our text messages and digitized voices also need to know the time to high precision.
        But there is a problem. The earth's rate of rotation is gradually slowing down. The main reason for this is the moon. As the moon goes around the earth, it pulls at the waters of the great oceans of the world and moves them around, creating the tides we see in ports and at the sea shore. But sloshing all that water around and dragging it over the ocean bottom creates heat due to friction. The heat produced is eventually radiated out into space, but the energy to create that heat has to come from somewhere. And that somewhere is the rotational energy of the earth - the potential energy stored up in the planet by virtue of it rotating like a gyroscope. Another consequence of this tidal friction is that the earth's rotational energy is being transferred to the moon's orbital energy, so the moon is gradually getting further away from the earth. A hundred million years ago, back in the days of the dinosaurs, the day was about 4 minutes shorter than it is now. Ten thousand years ago, back in the days of stone age man, the day was about a quarter of a second shorter than it is now. Back in 1967, the day as marked out by the atomic clocks was 0.006 seconds shorter than it is now.
        Now .006 of a second does not sound like a lot. But if the day as measured by atomic clocks is shorter by six milliseconds than the day as measured by the rising and setting of the sun, the difference between the atomic clocks and the sun as it arcs through the sky soon becomes significant. The lapse of six months or so will result in the two 'clocks' separating by a second. Since 1972, the solution has been to effectively 'stop the atomic clock' for a second to allow the earth to catch up, and for the past forty years inserting these 'leap-seconds' has meant that the time as kept by atomic clocks has been kept in synchronisation with the earth's rotation to within one second.
        But inserting these leap-seconds into all the world's atomic clocks, all at the same moment, creates the potential for mischief. A bug in a program somewhere could mean that the clock which (say) regulates the stock exchange transactions in a major financial centre might have a hiccup and throw electronic financial dealings around the world into turmoil. To prevent the possibility of this happening, there was a proposal at the ITU meeting in Geneva that we do away with leap-seconds and just allow our clock time to drift away from 'natural' time as shown on a sundial.
        But the obvious flaw in this argument is that if we do away with leap-seconds, the time would eventually come when the earth lags so far behind time as regulated by atomic clocks that we would be having lunch as the sun rose! That is clearly a nonsense, so there would have to be some sort of reckoning at some stage in the future to bring the atomic clocks back into line with the natural day as experienced by the rising and setting of the sun. Atomic clock time would eventually have to be brought back into line with the sundial.
        There was a counter-proposal at the ITU meeting that it is better to leave things as they are and carry on inserting a leap-second once or twice a year, than wait for several hundred years to insert a leap-hour. The technology is in place now to deal with the problem by inserting leap-seconds. By waiting several hundred years to insert a leap-hour, we would be presuming on un-tried technology in as yet un-built atomic clocks, and that has far more potential for mischief than continuing with the current system.
        At the International Telecommunications Union meeting, the "Lets prevent a problem before it happens," argument was led by the United States, with France and Germany in tow. But they did not prevail over the "If it ain't broke, don't fix it," case as led by the United Kingdom, with China and Canada amongst the countries who were of similar mind. Each argument has its merits, but there was no clear cut case which would bring the large majority of undecided countries in favour of one or the other. In the end, it was decided to keep things as they are and wait to see if a convincing argument for change emerged at some time in the future.
        So, for now, the humble sundial in the garden will continue to be a good guide to when it is time for lunch!


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