A later, still more ingenious, timer from the West consisted of a lens which at midday would concentrate the sun's rays on a charge of gunpowder in a tiny cannon. By 1800, a mechanical timer had been invented that, after a delay, would set off a bomb.
The regular movements of the stars and the sun made it possible to measure longer periods of time. The sundial has an obvious drawback in countries where direct sunlight is unreliable; it also has to be adjusted for different latitudes. But, from the earliest Egyptian sunsticks, it developed into a sophisticated timekeeper, with variants such as nocturnals, to show time by the moon, and transportable, pocket sundials. A modern one can be seen outside the National Maritime Museum in Greenwich, but there are limits to their accuracy and utility.
In Europe, the impulse behind the development of the first mechanical clocks seems to have been the medieval Church's desire to regulate times of prayer. The earliest cathedral clocks simply rang a bell at intervals, but the late 14th-century clock in Wells Cathedral has a dial showing the hours. The idea of time as an ever-rolling stream (which is how it appears in an eggtimer) is starting to be replaced by that of time as a succession of discrete fractions: hours, minutes, seconds, endlessly repeated. But the earliest clocks were still so inefficient that they had to be put right every day by reference to a sundial.
In the mid-17th century, in Holland, Christiaan Huygens patented the first pendulum clock, allowing a huge increase in accuracy. These early mechanical clocks and watches were accurate enough for most uses, but there was a need for a far more precise instrument for the purpose of navigation. If a ship possessed a clock that would give the same time as that at Greenwich (or some other predetermined point), it would be possible to discover a ship's longitude by calculating the difference in the positions of the sun, moon or stars from their known positions at the fixed point.
Because there seemed little hope of making a mechanical timekeeper that could withstand the changes in temperature and buffetings of a sea voyage, the Royal Observatory was set up in 1675 to produce a complete Nautical Almanach, which would allow longitude to be found by astronomical observation and tedious calculation.
Meanwhile, ships were wrecked or delayed, and their crews at risk of scurvy from prolonged voyages. Partly in response to the continuing loss of life and assets, Parliament offered a prize of pound;20,000 for the invention of an accurate ship's chronometer and set up a Board of Longitude to judge it. John Harrison, the son of a village carpenter, devoted his lifetime to solving the problem, which he did finally in 1762 with his fourth model. Each of his attempts made significant improvements in clock design.
It was not until 1884 that an international conference agreed to site the prime meridian, marking zero longitude, at Greenwich - which is why the new millennium will officially start there (before that, each country had its own meridian). From 1833, British sailors going down the Thames could set their clocks by a red ball on the roof of the Royal Observatory which is still raised and then dropped at 1pm every day.
Until the mid-19th century, only sailors and Londoners were interested in Greenwich time. Other parts of the country kept local time, which might differ by as much as half an hour as one travelled towards the west coast of Ireland. But the coming of the railways brought a need for conformity, and from 1852 the Royal Observatory set up an electrical clock to send time pulses down the telegraph wires. Times were standardised throughout the country.
Timekeeping became essential for factory workers who were paid by the hour. Officers in the first World War were issued with wristwatches which they would synchronise before an attack. Social events increasingly ran to time and people were expected to be punctual.
Electric clocks, then quartz clocks and finally atomic clocks, freed timekeeping from mechanical instruments. And a modern atomic clock is accurate to within one second in three million years - which means that had one been set to work at the creation of the universe, it would not have lost or gained more than about one hour and 25 minutes.
Thanks to Maria Blyzinsky of the Royal Observatory in Greenwich