Please turn on JavaScript. Media requires JavaScript to play. Advertisement As 2008 turns to 2009 at the end of this month, an extra second will be added to every clock. But who decides exactly what time it is? Professor Brian Cox meets the man in charge of all our timekeeping - the world's director of time. Time is something we all take for granted. Morning turns to evening; autumn drifts into winter and another year becomes history as the earth completes one more journey around the sun. But what is time? How do we measure its passing? Does it always tick at the same rate? Did it have a beginning, and will it ever end? FIND OUT MORE... Horizon: Do You Know What Time It Is? is broadcast on BBC Two on Tuesday, 2 December at 2100 GMT Or watch it later on the BBC iPlayer These are questions that might seem better placed in a philosophy course, but in fact they are immensely important, not only for understanding our place in the universe but also for the functioning of the 21st Century world. From financial transactions to satellite navigation, we rely on everyone on the planet agreeing on a unique, precise timestamp. Get the time wrong and money and lives could be lost. The responsibility for ensuring we all keep the right time rests with Dr Dennis McCarthy, the world's director of time. More than anyone, Dr McCarthy appreciates the need for the world's population to be synchronised. But for those who don't spend their working day checking atomic clocks, why is knowing the time so important? Think for a moment about how the GPS satellite navigation system works. There is a network of over 30 satellites orbiting earth that broadcast a high-precision time-stamp down to the GPS system in your car. These signals travel at the speed of light, which is very nearly one foot every thousand-millionth of a second - or one nanosecond (for the more metrically minded, that's around 30cm, which is far less elegant. If there is a God, he built the universe using imperial measurements). Wind resistance By measuring the time delay between all the different signals, your GPS can work out your position relative to all the satellites, and therefore your position on earth, assuming that all the clocks remain precisely in sync. The atomic clock is actually putting out an electronic signal which is essentially analogous to the ticking of a pendulum clock

Dr Dennis McCarthy, director of time The GPS system is accurate to better than 16ft (5m), which means that everyone in the world using GPS must agree on the time to within 16 nanoseconds. How difficult is this? Until 1967 the second was defined using the motion of the earth. This is perhaps in accordance with our intuition. The earth rotates once on its axis every 24 hours, and there are 3,600 seconds in one hour. That would be fine if the earth kept good time, but in fact it doesn't. The earth's rotation rate changes every day by thousands of nanoseconds, and this is due in a large part to wind. When winds push against mountain ranges, they can either speed up or slow down the rate of spin of the solid ground, transferring that spin into the atmosphere. Over the course of thousands of days, these changes in the rate of spin of the earth can result in the earth's rotation getting "out of sync" with the high-precision atomic clocks that we use to keep the GPS system ticking over. Leap second That's where Dr McCarthy comes in. Based in the US capital, Washington DC, Dennis McCarthy's job is to keep an eye on the effects of small variations in the earth's rotation and add or subtract "leap seconds". The next one will be on 31 December this year - December 2008 will last one second longer than December 2007, when no leap seconds were added. One of the atomic clocks at the Washington Observatory The director of time works for the equally grandly named International Earth Rotation and Reference Systems Service. It receives data on the earth's rotation from a series of observatories around the world that plot the earth's exact position relative to a grid constructed from extremely bright, distant astronomical objects known as quasars. These are distant galaxies, some over 10 billion light years away, powered by super-massive black holes which devour entire star systems and shine with the light of a trillion suns. Because they are so distant, their position in the sky is absolutely fixed relative to the earth and they form a very steady and precise reference system relative to which we can measure the earth's rate of spin, and thereby keep our clocks in sync with the Earth's rotation. "We have a number of clocks at the naval observatory located all over the grounds," says Dr McCarthy, as he gives me a tour of the Washington facility. 9bn ticks a second "The atomic clock is actually putting out an electronic signal which is essentially analogous to the ticking of a pendulum clock... which might tick once every second or once every couple of seconds," says Dr McCarthy. "This thing is providing us something which is going nine billion times per second so it provides us with a very fine definition of the time." As if this wasn't enough, there is a more fundamental problem for global timekeeping. In 1905, Albert Einstein's theory of relativity showed that there is no such thing as absolute time. Every clock, everywhere in the universe, ticks at a different rate relative to every other clock. For GPS, this is an enormous issue because it turns out that the clocks on the satellites drift by almost 40,000 nanoseconds per day relative to the clocks on the ground because they are high above the earth's surface (and therefore in a weaker gravitational field) and are moving fast relative to the ground. Forty-thousand nanoseconds is 40 thousand feet, so you see the problem. Einstein's equations first written down in 1905 and 1915 are used to correct for this time-shift, allowing GPS to work, planes to navigate safely and you to get to your relations' house at the other end of the country without getting lost this Christmas. There is a complex and wonderful industry behind the seemingly simple job of keeping the world in sync. Add your comments on this story, using the form below. Interesting article, but for the stupid push towards the Imperial system. If I remember well, on one of the last UK attempts to launch a probe, it missed its target by billions of miles because some British scientist measured acceleration in inches per second. If you want to grow up and stay up to date in science, stop promoting the imperial system. Or it will keep your research at imperial level.

Osylc, London "...which is very nearly one foot every thousand-millionth of a second - or one nanosecond (for the more metrically minded, that's around 30cm, which is far less elegant. If there is a God, he built the universe using imperial measurements)" What utter nonsense - "very nearly one foot", "that's around 30cm". How very precise and scientific! And God does not use imperial measurements - it's just dumb humans.

Ian, Glos, UK "The GPS system is accurate to better than 16ft (5m), which means that everyone in the world using GPS must agree on the time to within 16 nanoseconds." Errm, no it doesn't. Obviously it doesn't. Only the satellites have to agree, users simply compare the differential between the signals, their knowledge of the actual time is irrelevant.

Stew, Phnom Penh, Cambodia This very interesting article omits saying anything about the fact that the rotation of the earth is slowing down, due to the action of the tides. Also if we harness the energy of the tides to produce electricity for mankind's needs, we are actually taking the Earth's kinetic energy of rotation for the purpose, making it slow down even more. But this energy would otherwise simply be expended in causing erosion of coasts. Perhaps our activities are actually good for the environment here.

David Butcher, Luton, UK "December 2008 will last one second longer than December 2007, when no leap seconds were added" - that's amazing considering that 2008 had 29 days in it in 2008 but not 2007.

Curph, London 2008 is a leap year it's a day and one second longer than 2007

Awen Saunders, Portland, Oregon, USA The question is what to do with the extra second this year? Steve, Sunny Poole. UK Wow this was a big waste of 180 seconds of my life

Simon T, Milton Keynes The article reminded me of a graffiti message: "Time is natures way of stopping everything happening at once."

Ian, Cairns Australia Name

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