Design for a pendulum clock, 1673
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Why build clocks?

The mechanical clock was invented for religious reasons. In medieval monasteries, monks were
expected to get up in the middle of the night to perform collective worship, in the service called matins. The monks needed some form of alarm to wake them up, and get them into the choir on time. They might use a simple water clock or a slowly burning candle, though this meant that a monk had to stay awake to watch it, and then ring a bell.

By the 13th century, monks were experimenting with mechanical timers, powered by falling weights, which rang bells. In the 1290s, we hear of the first true mechanical clocks, in Italy and Britain, which marked time passing and ran continuously. These also used a falling weight, which turned a toothed wheel, whose speed was regulated by a horizontal cross-bar. Swinging from side to side, the cross-bar alternately blocked and released the wheel’s movement. These early clocks were very inaccurate, and had to be corrected daily by comparison with a sundial.

The first mechanical clocks did not have faces, which few people would have known how to read in any case. They marked the passing of time by bells. It is from the Latin cloca, or bell, that we get our word "clock".

The world’s oldest surviving clock face, dating from the 1380s, is in Wells Cathedral. Besides having moving hands showing the time of day, it displays the motion of the sun in the sky and phases of the moon. Above its face are figures of four knights on horseback. Every quarter of an hour, the knights charge around a tower fighting a tournament, and one of them knocks another flat with his lance.

Pendulum clocks

In the 1580s, the brilliant Italian scientist, Galileo Galilei, was watching a lamp swinging from a long rope in Pisa Cathedral. Using his pulse to time each swing, Galileo discovered that this did not vary – no matter how far the lamp swung. In the 1630s, he came up with the idea of using a swinging weight, or pendulum, to control the speed of a clock. Galileo was planning to build a pendulum clock at the time of his death.

The first pendulum clock was built in 1656, by the Dutch astronomer and mathematician, Christian Huygens. It was immediately clear that it was a huge improvement on the earlier clocks. While these might lose or gain between 15 to 30 minutes a day, the new pendulum clock varied by only 10-15 seconds. Almost all existing clocks, including the one in Wells Cathedral, were now converted to run on pendulums.

Harrison's chronometer

The 18th century English inventor, John Harrison, spent more than 30 years improving the accuracy of clocks, in order to help navigators determine their longitude (east-west position) at sea. The simplest way of working this out would be to compare local time, by observing the sun at midday, with the time back home. For every hour's difference between the two times, a navigator knew he had travelled 15º east or west.

Few people believed that a clock with the required accuracy could be built. It could not use a pendulum, useless on a rolling ship, and it had to cope with extreme changes in temperature and humidity. Harrison built his clocks using springs and counterbalances, to compensate for the ship's movement. On his fourth attempt, in 1759, Harrison built a chronometer resembling a large pocket watch, now called H4. In 1761, it was tested on a ten-week voyage from England to Jamaica and back, and found to have lost just five seconds.

Greenwich time

On setting off on a voyage, a sea captain would set his chronometer by the time at the Royal Observatory at Greenwich, which is the origin of the Greenwich Mean Time we follow today. This is also why the whole world came to measure longitude in degrees east or west of the 0 meridian (line of longitude) running through Greenwich.

Because local time varies with longitude, there were noticeable differences in the times shown on clocks in the east and west of Britain. This only became a problem with the coming of the railways, in the 1830s and 40s, when it was necessary to make timetables. Railway companies usually chose to follow Greenwich time, diplomatically renamed “railway time". Stationmasters around the country had to set their clocks to “railway time”, using a leaflet giving conversions from local time, issued by the railway company.

It was only in 1880 that Parliament passed the Definition of Time Act, making Greenwich Mean Time standard for the whole country. This was transmitted throughout the land by telegraph signal and so all the clocks in Britain now showed the same time. In 1924, the first radio time pips were broadcast and, in 1936, the telephone speaking clock began its service.

Today's atomic clocks, which use the steady vibration of caesium atoms, are so accurate that they keep better time than the earth itself. Roughly every year a leap second must be added to clock time, in June or December, to match the less regular time of our imperfect planet.