As the electrons absorb or release energy, they move up or down to different orbits. Lasers are used to push the atoms together into a ball, which cools them. The ball is then projected a metre upwards through magnets and a microwave chamber, which heats the atoms. Any that have absorbed energy glow as they fall back down to where they came from, past a detector10: Each atom is like a mini solar system, with electrons orbiting a central nucleus.

Accurate to within a second every 300 years, it established a new standard, far more reliable than the rotation of the Earth9: The modern atomic clock uses the gas caesium-133 in a vacuum. They made good cheap watches, but were soon surpassed in accuracy8: Time changed for ever when the first atomic clock was built at the National Physics Laboratory in Middlesex in 1955. They count the seconds using the vibration of electricity through crystals, and have nothing to do with the sun. Since most of the vessels at sea already used charts with Greenwich as 0 degrees longitude, this was declared the prime meridian7: The position of Greenwich as the centre of time was first threatened when quartz clocks were made in the 1920s. But in 1852 a master clock was set up to send electrical pulses from Greenwich across the country. Despite some local opposition, soon all British public clocks were set by it6: In October 1884, delegates from 25 countries met in Washington to decide on a system of time zones for the whole world. This called for a clock that stayed accurate at sea in any temperature and regardless of motion.

H4 was invented by John Harrison5: Until the coming of the railways it did not matter that noon in London was 11.44am in Plymouth. This made possible the large public clocks that would be built into towers in town centres across the world, starting with Italy in the 14th century4: The need for sailors to know where they were led to agreement that the position of the sun at noon in Greenwich, London, should be considered 0 degrees longitude. A globe inside the tower rotated once a day3: The next step forward was to use a system of falling weights to drive the mechanism, instead of water. The same principle powered the first known mechanical clock, which was built in a tower 30ft high by the astronomer and inventor Su Song in China in 1088. So if you saw someone pop their cork half a minute early last night, it was probably one of them.

Ahead of their time or out of step? Only time will tell.From sundials to atomic clocks1: Our understanding of the passage of time has always been based on the movement of the sun and Earth. The ancient Egyptians had sundials dividing daylight into 10 parts plus dusk and dawn. The rotation of the Earth, “moving” the sun from directly overhead on one day to the same position on the next, has formed the basis of time-keeping ever since. This includes Greenwich Mean Time, whose status as a universal standard for the world is under threat. But everyone agrees that 24 hours equals one day2: The Greeks were the first to use water clocks regulated by a constant drip from a stone vessel. It sympathises with astronomers, whose work tracking distant stars would be thrown into chaos. The Royal Astronomical Society said last week: “Over a few decades, when the error might grow up to half a minute or so, one can imagine the arguments that lawyers and insurance companies might have about whether an event had occurred before or after midnight.”Nevertheless, the heads of American technology corporations tend to get their way, and they seem to think of Greenwich Mean Time and the rotation of the Earth as quaint, old-fashioned, a bit wobbly – and 32 seconds slow.