UTC
Co-ordinated Universal Time.

How do you keep time around the World?

Everyone around the world needs to keep to an agreed timescale. On the 1st January 1972, Coordinated Universal Time (UTC) was adopted as the official time for the world. The International Bureau of Weights and Measures (BIPM) acts as the official time keeper of atomic time for the world. Some 65 laboratories with 230 atomic clocks are used to calculate this composite timescale. The time measured by the rotation of the Earth about its axis slowly drifts away from UTC and leap seconds are inserted, as required, to keep UTC to within 0.9 seconds of the Earth's time.

Leap Seconds

A leap second was added to the national time scale at the very end of December 1998. Co-ordinated Universal Time (UTC), creating much media interest. This was the twenty-second leap second to be added to the time scale since the practise was introduced in 1972. You may be forgiven for asking "Why bother?"; after all we add a whole day every fourth year - a leap year.

The reason is fundamentally the same: to keep man-made scales for recording time in step with the natural rhythms of the heavenly bodies. Without a leap year of 366 days, the calendars would get out of step with the natural time determined by the rotation of the earth.

History of TIME

Time impinges on almost everything we do, so much so that we take it for granted, However it is not just in this modern age that time has had such an important effect on our lives, man has been preoccupied by time since the dawn of civilisation and it is probably the first thing that he measured: the day with the sun, the months with the moon and the year with the seasons. Certainly by the third millennium B.C. the sundial was being used to measure time; the shadow of a stick placed in the ground swept out an arc as the sun moved across the sky; this arc was divided into segments, so dividing the day into intervals of time. Water clocks were used to 'keep time' during the hours of darkness and on cloudy days. In these early days, timekeeping probably had more to do with religious rituals than secular needs. There were no further significant innovations in timekeeping until the fourteenth century A.D. when the mechanical clock was invented. However it was the sundial that continued to be the master clock giving the ultimate reference of time - solar time. The mechanical clocks enabled the solar day to be divided conveniently into hours and, as the clocks became more accurate, into minutes and seconds. However, as man improved his ability to measure time, he noticed that the time of day measured by the sundial varied regularly throughout the year by as much as 15 minutes from the time predicted by mechanical clocks. To get round this, a mean solar day was defined as being the average length of all individual solar days in a year, and the second was the 1/86,400 of the mean solar day.

It was, however, the pressure to expand trade and commerce in the 18th century that had the most profound effect on timekeeping. As most trade was by sea, sailors needed to be able to navigate reliably in order to carry merchandise quickly and safely between ports often separated by thousands of miles. Accurate timekeeping was the key to navigation. Sailors had learned to use the north star to determine their position or latitude north of the equator, but east-west navigation was more difficult as the earth was rotating. Anyone who has travelled to the USA will have changed their watches because of the time difference. In fact the local time measured by the sun changes by four minutes for each degree of longitude that you move to the west or east. If, instead of changing your watch when you travel, you leave it set at the time of your starting point and then compare it with the local time at your destination, you can work out how far east or west you have moved. It was this method that sailors used to determine their east-west position, but to do so they needed accurate clocks that would keep good time on board ship. In 1713 the British Government offered an award of £20,000 to anyone who could build a ship's clock or chronometer which could keep time to a few seconds a month. the prize was eventually won by john harrison almost 50 years later. sailors set their chronometers to the mean solar time at greenwich, which came to be called 'Greenwich Mean Time'.

Improvements in mechanical clocks continued well into the present century, the most sophisticated keeping time to a few seconds in five years. However, scientists eventually turned to atoms, the fundamental building blocks of matter, to develop even more accurate clocks. It was recognised that atoms had natural vibrations that could provide the beat of a clock just like the swing of a pendulum. The first atomic clock, using atoms of the silvery metal caesium, was built at the National Physical Laboratory in the 1950s. Atomic clocks today are accurate to 1 second in 300,000 years.

These superaccurate clocks showed that atomic time was more uniform than Greenwich Mean Time. This is because GMT is linked to irregularities in the earth's rotation. indirect fossil evidence suggests that the earth's rotation has slowed down by about three hours during the last 600 million years. Superimposed on this are much larger fluctuations in speed due to the changing distribution of the mass of the earth, rather like the spinning ice skater speeding up and slowing down as he first pulls in his outstretched arms towards his body and extends them again. In 1967 scientists worldwide agreed to measure time using the atomic second. However it was recognised that, as atomic time is not linked to the earth's rotation, it would gradually become more and more out of step with gmt. to get round this problem they came up with a new time scale which used the seconds of atomic time but which added or subtracted seconds to keep it in step with the earth's natural time. This new scale for measuring time is called Co-ordinated Universal Time (UTC) and the added or subtracted seconds are leap seconds. It is the International Earth Rotation Service in Paris which measures the rotation of the earth and determines when a leap second is needed.

Atomic time and UTC are kept through the collaboration of about forty laboratories worldwide, each having several atomic clocks. It is the National Physical Laboratory which collaborates on behalf of the UK and has the responsibility for maintaining the national scale of UTC. NPL broadcasts time signals from a transmitter at Rugby. Anyone with a suitable receiver can set their clock directly against the national atomic clock. Some domestic clocks are now available with an in-built receiver.

As mentioned earlier, we take accurate timekeeping for granted. Our present technology-based society would collapse without very accurate clocks. Even the most unpunctual person will be dependent on the timekeeping provided by an atomic clock when making a phone call, since these clocks control the effective operation of national and international telecommunications networks. In another critical application, radio signals from atomic clocks situated in satellites enable ships equipped with special receivers to locate their position to a few tens of metres. So trade and commerce are dependent on accurate timekeeping today just as they were a few hundred years ago.

Scientists worldwide, are working to develop the next generation of clock to meet the demand for even more accurate timekeeping. Prototype clocks being developed use laser beams to manipulate atoms of caesium so that their natural vibrations can be measured more accurately.

However it may be the celestial bodies that once again hold the key to ultimate timekeeping. Rotating stars called pulsars which emit pulses of radio waves at very precise intervals in the same way as a flashing lighthouse could be the master clock of the future.

Acknowledgements

NPL, National Physical Laboratory