10. Greenwich Mean Time

How did the name of a pleasant part of London situated by the River Thames become synonymous with international time keeping? The reasons go back into history. Thousands of years ago, people had no reason to divide their lives into hours and minutes. Their time was the movement of the sun, which created day and night, and the rhythm of the seasons. But, as human societies became more complex, this pleasant state of affairs—or so it appears to modern clock watchers—was a handicap to trade and effective government. Gradually, a 24-hour cycle was introduced, based on the point when the sun was in the middle of the sky—which became midday. But, because the earth rotates, midday in one town might be twenty minutes after midday in another town a hundred miles away. So each area of the world kept its own time.

When transport was slow and a journey could take several days, twenty minutes difference meant nothing. But when the first railways were built—allowing people to travel much faster—these time differences became a problem. A man might set his watch at a station, and set off on a train for a meeting—only to find that his destination was twenty minutes ahead, and that he had missed his appointment.

The solution was obvious—a national standard time, so that every town could set their watches and clocks by it. The electric telegraph was the key to success, for a simultaneous signal could be sent along it to any part of the country. So, in 1852, the first signal went out from the astronomers of Britain's Royal Observatory—which was then situated at Greenwich. From then on, Britain followed Greenwich Mean Time.

The word "Mean" here has nothing to do with explanations or selfishness. It refers to something which is in the middle—an average. When the noonday sun at its highest point was directly over a particular place (the Meridian Line in Greenwich) the astronomers defined this as noon, Greenwich Time. This Greenwich Time was used for the whole country, so it became the "Mean" by which time was calculated in Britain. The Meridian Line already existed as a map reference point. It was used to help sailors calculate distances at sea by determining longitude. The Greenwich meridian Line defined 0° longitude.

Britain managed quite well with Greenwich Mean Time, but the development of many different national times around the world meant that a country could still be out of step with its neighbours. So in 1912 an international conference decided that Greenwich Mean Time would be used throughout the world. Local time in various countries could then be calculated for the purposes of international travel and communications, as hours ahead of or behind Greenwich Mean Time.

In 1948 the Royal Observatory was moved away from the London air to a village in the South of England called Herstmonceux. Instead of changing the "G" in "GMT" to an "H" the astronomers simply added 81 seconds to their calculations.

Recently scientists discovered a way of measuring time that does not depend on the position of the sun, instead, they make use of the regular properties of the atom, in a device known as an atomic clock. Today, the information from eighty of these atomic clocks, situated all over the globe, is sent to a central station in Paris. The results are averaged, and a signal is sent out to all the countries of the world. The result is known as Universal Co-ordinated Time. But those who calculate time and distance using Greenwich Mean Time need not worry. In practice, it differs from Universal Co-ordinated Time by less than a second. So, the world can continue to use GMT for many years to come.