The invisible line that could make you rich

The Greenwich Meridian is the place to be for New Year 1999

The citizens of Blackheath - long written off as south London's poor relation to Hampstead - have spotted the ultimate in oneupmanship. They may not have a garden suburb, or Keats House, or Kenwood and its concert bowl, but they have something special to make New Year's Eve 1999 go with a swing.

Blackheath has realised that it stands on the line from which all time and space are measured. It may be an imaginary line, except for bits of brass here and there, but it passes through the village's fine late- Georgian and Regency villas, standing along the open heath.

For months the Blackheath Society has been receiving calls from residents keen to know if their homes are on the Prime Meridian. As it bears their borough's name, the people of Greenwich might be thought to be equally concerned, but they have been preoccupied with the Docklands Light Railway extension and the troubled Millennium Exhibition by the Thames.

"It is standard cocktail talk that if you live on the meridian you can let your house to Japanese or Americans for pounds 1,000 a day," said Neil Rhind, chairman of the Blackheath Society.

The snag in dealing with the residents' enquiries was tracing the imaginary line. The society's obvious first port of call was the Royal Observatory in Greenwich Park, just across the heath. The meridian at the observatory passes through the centre of a telescope known as a transit circle, built in 1848 by the seventh Astronomer Royal, Sir George Airy. Until 1884, each country kept its own meridian. Then, at a conference in Washington, Sir George's line was agreed as the world's Prime Meridian.

But the home of time - now part of the National Maritime Museum - has no maps detailed enough to show whether the line passes through one Blackheath house or its neighbour. Mr Rhind was referred to the Ordnance Survey only to find that its maps did not show Longitude Zero either. He was "flabbergasted".

In fact, anyone with a pencil and ruler can work it out by linking up the marks for 0 degrees longitude shown on the top and bottom of the relevant maps.

Technically this is not the Airy line but the "Bradley line" determined by an earlier Astronomer Royal. But they are only 19ft apart and a pencil line on a 1:25,000 scale map - the largest scale on general sale - will cover both meridians. To be sure of the line through a terrace of houses, more detailed maps are available, but at a hefty price.

From the North Pole, the Greenwich Meridian crosses ice and water until it "enters" Britain just east of Hull. It heads south over the Humber and passes just east of Cleethorpes before continuing down through Louth, west of Cambridge, east of Enfield and then through Walthamstow and Leyton before crossing the Thames. After Blackheath it crosses Hither Green railway junction, goes through Oxted in Surrey and reaches the Channel at Peacehaven, Sussex.

Stung by Mr Rhind's protests, the Ordnance Survey says it is considering the "possibilities for appropriate publications" to assist people about the route of the line. So is the Blackheath Society.

John Sabido, a barrister who lives between Blackheath and Lewisham, has no need of a map. His house was given a plaque on the 100th anniversary of the adoption of the Airy line. "I think it's going to be quite crowded round here," he said, a trifle apprehensively. Just a bit. The latest estimate is that 10-13 million people will visit the Greenwich area during 1999.