But the prime meridian, being a creation of geography and mathematics - with a gentle prod from politics - has a somewhat diffident, enigmatic character. Unlike the Empire State Building or the Eiffel Tower, it doesn't make you look up in wonder to marvel at man's ingenuity. In contrast to the Grand Canyon or the Great Wall of China, it cannot be viewed from outer space. Outside south-east London, there are fewer than 20 places on earth where it can be detected at all.
Being invisible in this material world has not made for an easy life. Down the centuries, eminent astronomers frequently disputed the meridian's true position. Map- makers disliked it on the grounds that its position in the eastern corner of the British Isles distorted their projections in the west. It was frequently moved.
But when Sir George Airy, the Astronomer Royal, pinned down a definitive location in 1851, some Francophile countries tried to have the prime meridian moved from London to Paris. In modern times, using satellites rather than sextants, American scientists have discovered that Airy's line (marked by a brass bar at the Royal Observatory in Greenwich) is as much as 336 feet to the west of where it should be: a point that is marked today, reputedly, by a tree and a wastepaper bin. Since I don't have a measuring tape to hand, however, I can't confirm which bin it is.
"It's the representation of the line that's most important," says Terrence Gibbons of the Millennium Experience, "not the precise degree of long- itude." But a local artist, Terry Watts, who's been bitten by the meridian bug, thinks an observatory, of all places, should be meticulous about this kind of thing: "They're prostituting scientific rigour for entertainment value," he proclaims. "If we're talking about Disneyland, fine. But we're not."
To cap it all, this very summer, less than six months before the meridian's finest hour, the custodians of EuroDisney were at it again. Apparently still hurting over losing the argument in 1884, when Greenwich won the right to house Longitude Zero, the French decided to reinstate their own "le meridien", lying just over two degrees east of London and running through their capital on a 600-mile journey from Normandy to Provence. Trees and olive groves are to mark the route.
So, wouldn't you know, the French line will be visible from outer space, though not until the trees grow. If the world adopted this new axis, we would be celebrating the new millennium nine minutes and 22 seconds earlier.
For all these reasons, I think, England has made little of its exalted position as the beginning and end of time. And you need a pair of stout walking boots, some skill with a compass, and no little sang-froid to follow the course of the meridian from its landfall 15 miles east of Hull to its departure point 10 miles east of Brighton.
If you're doing it the hard way, it's a trek - much of it a soul-destroying slog - of 254 miles. Apart from London, the only towns of note you pass are Cleethorpes, Louth and Boston in Lincolnshire, and East Grinstead and Lewes in Sussex. Barring your way, there's the straight-line, flat- earth, wide-field, grey-grim horizonless landscape of the Fens: the kind of country that turns man's thoughts to infinity - and worse.
But at numerous points along the route there is rich reward: an infinite variety of understated highs and lows to remind us that you don't have to travel to the back of beyond to discover things you never knew before.
The green folds and gentle vistas of the Wolds and the Chilterns; the chocolate-box village of Old Bolingbroke in Lincolnshire and the stunningly beautiful 15th-century steeple at Louth; the sudden, chalky ascents and breathless falls of the Surrey and Sussex Downs; the dramatic viaduct north of East Grinstead; an ancient church here, a monument dense in historical significance there; and perhaps most surprising of all, a vast regional park within a pounds 2 bus ride of London's East End.
This is the Lee Valley, where they're so proud of encompassing the line that it's represented, not once, but in five different places within a few hundred yards of each other. There are two sculptures, an avenue of trees, a line embedded in a pavement mosaic, and a Meridian Gateway depicting the heavens (only slightly disfigured by graffiti). Oh, what treasures to raise the meridian walker's spirits after all those punishing miles from nowhere to nowhere.
EARLY ONE morning in the autumn of 1997, David Pott had a dream, one so vivid that it woke him instantly and changed his life. David, 53, belongs to a religious community near the meridian in London. He's also a long- distance walker, who that same year had trekked the 680 miles between two of Britain's great religious centres, Iona and Canterbury.
His dream re-enacted the Biblical story of Moses setting up a pole in the wilderness for the children of Israel, and a bronze snake curls around it. But in the dream, the image was distorted. The pole became a line on a map and the snake turned into a path.
When he awoke, David immediately realised where his subconscious had been guiding him. He noted that the meridian passes through the affluent acres of England, France and Spain, and then traverses five countries in Africa. Three of them - Mali, Burkina Faso and Togo - are among the poorest in the world. The notion of linking the privileged north with the deprived south has since been taken up by a project called On the Line, which will connect diverse communities in the UK and Africa: fish-smokers in Grimsby and Ghana will come together; allotment holders in Glasgow will rub shoulders with market gardeners from Mali.
Developing that theme of reconciliation between people who wake up at about the same time every day but who live on opposite sides of the tracks, David Pott is campaigning to turn the English section of the meridian line into a permanent way; a national path of pilgrimage where walkers can link up with less fortunate people in far-off lands.
"I want to see the meridian put on a par with the equator", says Mr Pott, "where people attach significance to the act of crossing it."
To that end, he's spent much of the past year surveying every yard of the line between Humberside and Sussex, threading together in his notebook existing rights of way to create an unbroken 254-mile footpath. He has recorded all 112 locations where the route crosses the line (each will have its own wooden marker.) He has noted every point where a stile, footbridge, tunnel or maintenance work is required, and he has even devised a "scenic" 48-mile diversion between Holbeach (Lincs) and Somersham (Cambs), to spare mere mortals from the unremitting Fens.
So far, David Pott has attracted much interest from eminent individuals and societies, but nothing in the way of funds. He estimates the start- up costs to be pounds 40,000, and it may be several years before the Meridian Way takes its place among our great long-distance footpaths, although the first chestnut marker posts, donated last month, will be hammered in next year.
MOST OF the people I've encountered living on or near the meridian have only the vaguest perception of the fact, as if it was something they'd learnt in a geography class at school. Yet nearly every settlement along the line throws up a handful of those slightly oddball characters. That's why Swavesey, near Cambridge, will feature prominently in national television coverage on 31 December.
Swavesey (population 1,800) is the home of the indefatigable Bob Stone, a retired councillor who's organising an all-night knees-up in the market square, with sunshades over the tables in case of rain. "There's no such thing as bad weather," he tells me, "only bad clothes." Thousands are coming, including a coachload of Women's Institute members from London. And the television trucks will return next mid-summer for Bob's piece de resistance - a miniature dome, modelled on the real thing in Greenwich, made out of 30-foot long scaffolding poles, ropes and bunting.
On the night of 31 December, the line will literally come alight, when 20 beacons in towns and villages along the meridian will be triggered into life by a signal from East India Dock, directly across the Thames from the Dome itself. The Dock is another of the line's unsung landmarks: the starting point for many a sea voyage that enabled Britannia to rule the waves - and carry the vote at the Longitude Treaty. Here, a viewing platform is being constructed to enable 20,000 revellers to watch a son- et-lumiere show of stadium-rock
proportions, when four of the world's most powerful searchlights will shoot an intense white beam so far into the sky it might well be visible at Pole Hill, on the edge of Epping Forest.
Nowhere illustrates the meridian conundrum better than Pole Hill, where a modest obelisk lies in a clearing in the woods, commemorating the moment when the nations of the world synchronised their timepieces and momentarily silenced the French. But, where the meridian is concerned, it's never quite as simple as that. The pillar had been erected 60 years before the treaty, bang on what was then the meridian line. Along came Sir George Airy, who promptly moved the line, so 19 feet to the east of the obelisk, a small triangulation pillar was put up to mark that. One day, perhaps, the satellite people disputing his calculations will add a memorial of their own.
The brand new Millennium Dome and the grand old maritime and astronomical buildings of Greenwich itself are arranged with such geometric precision that the line, for the first and only time, almost becomes visible. The view from the top of Greenwich Park is among the most stirring in the kingdom, sufficient to revive the weariest of walkers. The effect is only slightly marred by theme park-style banners proclaiming "The Millennium Starts Here" outside the Royal Observatory. Inside, fantastical clocks, telescopes and other measuring devices demand measured contemplation.
But even here, you encounter a riddle. The first exhibit is an atomic clock counting down the milliseconds to midnight on 31 December. Yet it's fairly common knowledge that when they calculated the modern calendar, they forgot to include Year Zero between 1BC and 1AD, so the new millennium doesn't really arrive until midnight on 31 December next year. Nothing in the Observatory tells you this. I suppose it might damage the attendance figures. Once again, in the very cathedral of precision, accuracy has been sacrificed on the altar of hype.
I headed out of Greenwich to begin the southern section of David Potts's meridian trail: 70 miles to Peacehaven and the English Channel. But I felt disoriented. I may have known the time to within a millionth of a second, but I was distinctly confused on dates, and none the wiser about where I was. That mythical line really had slipped away into the mists of time.