Is time really of the essence?

Does knowing the precise hour, minute or second, and being in a particular place at the right time really matter that much any more?

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The barely noticed news that the most precise atomic clock in the world is capable of keeping time to within less than one second in 138 million years raises an interesting question: at the zenith of our ability to tell it, does time matter very much any more?

Knowing the precise hour, minute or second, and being in a particular place at the right time, has never seemed less important. Many of us work flexible hours, while news, shops, gyms and so on are available 24 hours a day.

And for those who make their living on the seas or in the skies, the black art of navigation - once so dependent upon time - has been supplanted by the black box of the GPS.

Once upon a time, however, horological accuracy was a matter of life or death.

Legend has it that shortly before a British fleet ran aground on the Isles of Scilly on the stormy night of October 22, 1707, with the loss of 2,000 lives, a sailor on the flagship revealed that, in defiance of regulations, he had been keeping his own log - and believed navigational disaster loomed.

If the story is true that Sir Cloudesley Shovell, the commander, had the man hanged instantly, then he lived to regret it, but not for very long. Within hours, HMS Association and three other ships were sunk. Only one man got off the flagship alive - and it wasn't Shovell.

The disaster was the result of an inability to keep time sufficiently accurately to gauge longitude - one's position east or west of the prime meridian that runs north-south, from pole to pole, through Greenwich in England.

For the purposes of navigation, the 360-degree circumference of the globe is divided into two halves of 180 degrees, east and west of Greenwich. Along the equator, each degree of longitude is equal to 60 nautical miles, or 111.32km (a distance that gets shorter, by known values, at higher or lower latitudes).

As the 360-degree Earth rotates once every 24 hours, 15 degrees is equal to one hour - which means that to find out exactly how far east or west you are from Greenwich, all you need is a sextant and a reliable timepiece set to London time. Take the noon sun sight where you are, say on the equator, and the difference between the two times gives your distance east or west of the prime meridian. So, if it is 3pm in Greenwich at your noon, then you are 45 degrees - or 2,700 nautical miles (5,000km) - west of Greenwich. Simple.

Only, get it wrong by just a minute and you will be 1.85km off course - more than enough to steer you onto the rocks on a stormy night off the Scillies.

In Shovell's time, navigators relied on dead-reckoning - educated guesswork, basically, which frequently proved fatal. Following the Scillies disaster, the British government offered a prize, equivalent to £2 million today, for a timepiece capable of remaining accurate on board ship to within 2.8 seconds day.

The prize was claimed, after three decades of work, by John Harrison, a cabinet-maker who pioneered a series of technological breakthroughs. The end result, in 1759, was H4, "the most important watch in the world", which saved countless lives and improved the efficiency of the maritime machine that powered the British Empire.

Harrison would have marvelled at the technology behind the UK National Physical Laboratory's CsF2 atomic clock, but the inventor of the pocket-sized H4 might also have regarded its finicky accuracy as overkill when set against its bulk and 2.4-metre height, which would have rendered it entirely unsuitable for use on board one of His Majesty's ships.

And only time will tell if CsF2 actually works as advertised. H2 proved its worth in many sea trials; the true accuracy of its descendant will remain essentially unproved until the year 138,002,011. You are advised not to hold your breath.