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International datelines notwithstanding, the Greenwich Meridian is the invisible line of longitude at which the year 2000 will officially begin. The photographer Brian Harris plots the line's course through England, while Charles Jennings tells its story
THE GREENWICH Meridian is the prime meridian in the world, the point at which the universal day starts, and at which the year 2000 will begin. It sounds like a God-given direction - Place the meridian here - or the result of some inescapable natural law; but in reality it is the product of accident and historical forces.

As ever, longitude is behind it all. But where do you put longitude 0 on the map? If the imaginary lines of longitude run from the North Pole to the South, and divide the earth up into 360 imaginary degrees, where do you start? Where do you put a line so that at any point of the globe you can fix your east-west position by working out how far you've travelled from your given starting point - a starting point, what's more, that's recognised by the rest of the world?

Hipparchus, the Greek astronomer, first had a go at fixing it on the island of Rhodes. Ptolemy, the Egyptian astronomer and geographer, then moved the prime meridian to the Canary Islands - the west-ern edge of the known world. Pope Alexander VI tried to set it just west of the Cape Verde Islands; while Philip II of Spain moved the meridian to the city of Toledo. In the 17th century, Cardinal Richelieu called an international conference to settle the matter - with the westernmost of the Canary Isles coming out as the chosen spot. This was certainly adhered to by the French for the next 100 years; but for the rest of the world things went on much as before, each nation tending to use whatever prime meridian it felt like. Things only began to change significantly with Nevil Maskelyne's Nautical Almanac of 1767. At the time, the most reliable way for mariners to work out their longitude in the middle of a large and featureless ocean was to examine the positions of certain known heavenly bodies with an instrument such as a sextant, and then look up the results in a table giving the positions of moon and stars for any given date. They could then work out their position from the initial longitude used by the maker of the table. Maskelyne's Nautical Almanac was the first reliable and practical almanac in the world, and - since Maskelyne was Astronomer Royal at the Greenwich Royal Observatory - it took the Greenwich Meridian as its starting point. Immediately, British navigators started to use it, alongside the British-made charts, which set longitude 0 at Greenwich. Other nations also adopted the Nautical Almanac, which meant that their charts had to have the same longitude arrangement as British ones.

But it was clear that some kind of agreement was needed to formalise the situation. The final outcome was decided at the International Meridian Conference, held in Washington DC in 1884. The United States offered to host the conference as it had "the greatest longitudinal extension of any country traversed by railway and telegraph lines". In due course, 41 delegates from 25 countries arrived. A month of wrangling and voting later, and the key questions had been answered. All the countries had voted in favour of the principle of a single prime meridian. Twenty-two out of 25 voted for Greenwich as the prime meridian. Twenty-three out of 25 voted in favour of the principle of the universal day, beginning at the prime meridian.

And where would the fixed point be set? Where, exactly, was the prime meridian on which all this was to hinge? It was "the centre of the Transit Instrument at the Observatory at Greenwich". This is longitude 0, and on one side is the western hemisphere of the world; on the other is the eastern. If you go there, you can - and most people do - bestride the brass strip which marks the meridian and stand with one foot in both hemispheres, almost as if this were the only place in the world where such a thing is possible. But, of course, longitude 0 describes an imaginary line from Pole to Pole, so you could also stand on a spot just outside Le Mans, in northern France, or near the town of Caspe, in north-eastern Spain, or stand in the western Sahara. All these would mark the prime meridian just as well.

Except that Greenwich created it. And if accident and historical forces hadn't colluded, then longitude 0 could now be in Paris, or Madrid or Washington, or (as Willem Blaeu, the 17th-century cartographer decided) on the highest point of the island of Tenerife. But Greenwich is where it all starts.

This is an extract from `Greenwich: The Place Where Days Begin and End' by Charles Jennings (Little, Brown, price pounds 10, or telephone 0181 324 5515 to order a copy for pounds 8, incl p&p)