Excerpted from Mapping Mars by Oliver Morton (Fourth Estate). Copyright Oliver Morton. All rights reserved. No part of this excerpt may be reproduced or reprinted without publisher's permission.

Mapping Mars by Oliver Morton

Marlow in Joseph Conrad, Heart of Darkness


Charles Dickens, Our Mutual Friend

Maps of the earth begin a short walk from the flat where I live. Go down the High Road, up Royal Hill towards the butcher's, left along Burney Street and then right on to Crooms Hill. At the corner, if you care for such things, you can see a blue plaque of the sort with which London marks houses where people who have made a significant contribution to human happiness once lived. In this case, it was the poet Cecil Day Lewis; as you climb the hill, you'll pass another one marking the home of Benjamin Waugh, founder of the National Society for the Prevention of Cruelty to Children. Near the top of the hill sits a grand (but plaqueless) bow-fronted white house, called simply the White House. Walk round the White House's walled garden, down a little alleyway and through a gate in the high brick wall on your right, and you emerge into Greenwich Park. To your right, the beautiful semicircle of the rose garden; to your left a steep path lined by trees. And as you walk out on to the grass, London spread at your feet. As views go, it's not particularly extensive - the horizon is nowhere more than twenty kilometres away and in many directions much closer - but it's vast in association. The once imperial cityscape is woven from threads that stretch throughout the world.

Across the river to the east sits the squat black-glass bulk of Reuters, information from around the globe splashing into its rooftop dishes. Upstream and on the near side sit the long, low workshops where for more than a century men have made undersea cables to tie the continents together. New skyscrapers devoted to global businesses sit in the redeveloped heart of the docks that used to handle the lion's share of the world's sea trade. Within the park itself there are plants from every continent except Antarctica. At its foot sits the old naval college, where generations of Britannia's officers, my late father included, learned to rule the waves.

Through it all the Thames runs softly, looping around the Isle of Dogs, a local feature leading, as Conrad says in Heart of Darkness, 'to the uttermost ends of the earth'. Little sails down this umbilicus of empire now - but above it the new trade routes of the sky are sketched out by aircraft arriving and departing from London's four airports, carving their way through the air we all breathe and the stratosphere we shelter under. To the west the Thames beneath them is still daytime blue; to the east it is already evening dark.

Dawn may feel like an intervention by the sun, rising above a stationary earth; sunset reveals the truth of the earth's turning, a slipping away into night. That turning defines two unique, unmoving points on the surface of the earth: the poles, the extremes of latitude. Add one more point - just one - and you have a co-ordinate system that can describe the whole world, a basis for all the maps and charts the sailors and pilots need, a way of deciding when days start and end. And that third point is right in front of you, the strongest of all Greenwich's links to the rest of the earth. In the middle of the park is the old Royal Observatory, a little gathering of domes perched clubbily on a ridge. Within the observatory sits a massive metal construction called a transit circle. The line passing through the poles and through that transit circle is the earth's prime meridian: 0 degrees, 0 minutes, 0 seconds. All earthly longitudes are measured with respect to that line through Greenwich Park.

The English have taken the Greenwich meridian as the starting point for longitudes since the observatory was founded in the seventeenth century. But it wasn't until the late nineteenth century - at a time when its home in Greenwich was under the stewardship of Sir George Airy, Astronomer Royal, the man who had that great transit circle built - that the Greenwich meridian was formally adopted by the rest of the world. With worldwide navigation a commonplace, and with telecommunications making almost instantaneous contact between continents a possibility, there was a need for a single set of co-ordinates to define the world's places and time zones. Over the years a variety of possible markers to define this prime meridian were suggested - islands, mountains, artefacts like the Great Pyramid or the Temple in Jerusalem. But a meridian defined by an observatory seemed best. In 1874, at a conference in Washington DC, and over spirited French opposition, Greenwich was chosen. Airy's transit circle came to define the world.

Airy was, by all accounts, an uninspiring but meticulous man. He recorded his every thought and expenditure from the day he went up to Cambridge University to more or less the day he died, throwing no note away, delighting in doing his own double-entry bookkeeping. He applied a similar thoroughness to his stewardship over the Royal Greenwich Observatory, bringing to its workings little interest in theory or discovery but a profound concern for order, which meant that the production of tables for the Admiralty (the core of the observatory's job) was accomplished with mechanical accuracy. He looked at the heavens and the earth with precision, not wonder, and though he had his fancies, they were fancies in a similar vein - ecstasies of exactitude such as calculating the date of the Roman invasion of Britain from Caesar's account of the timing of the tides, or meticulously celebrating the geographical accuracy of Sir Walter Scott's poem 'The Lady of the Lake'. This was a man whose love of a world where everything was in its place would lead him to devote his own time to sticking labels saying 'empty' on empty boxes rather than disturb the smooth efficiency of the observatory by taking an underling from his allotted labours to do so for him. After more than forty-five years of such service Airy eventually retired 200 yards across the park to the White House on Crooms Hill, where he died a decade later.

It's a little sad that the White House doesn't carry a blue circular plaque to commemorate Airy's part in the happiness brought to humanity by a single agreed-upon meridian, but surely there are monuments elsewhere. Maybe Ipswich has an Airy Street; he grew up there and remained fond of the place, arranging for his great transit circle to be made at an Ipswich workshop. There must be a bust of him in the Royal Astronomical Society. Or a portrait in some Cambridge common room. And even if there are none of these things, there is something far grander. Wherever else astronomers go when they die, those who have shown even the faintest interest in the place are welcomed on to the planet Mars, at least in name. By international agreement, craters on Mars are named after people who have studied the planet or evoked it in their creative work - which mostly makes Mars a mausoleum for astronomers, with a few science fiction writers thrown in for spice. In the decades since the craters of Mars were first discovered by space probes, hundreds of astronomers have been thus immortalised. But none of them has a crater more fitting than Airy's.

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