Now voyager

Dava Sobel sits eating a heroic bowl of porridge in the restaurant of her Kensington hotel. In the background, Celine Dion grinds her way through the Titanic theme on what seems to be an endless loop. As ambient noise for a meeting with the woman who found buried treasure in the search for a means to avoid the mishaps that befall ships lost at sea, the soundtrack surely falls into the you- couldn't-make-it-up category. It also acts as a reminder that, before long, James Cameron's whale of a blockbuster will have an unlikely rival in the nautical-adventure stakes.

Funded by Granada and scripted by former Python Terry Jones, the film of Sobel's improbably bestselling drama of 18th-century marine horology is ticking along nicely. To play the villain of the piece, chortles the author into her breakfast, the producers have approached John Malkovich. At which point she remembers how, back in East Hampton on Long Island, she used to mull over this esoteric little book project with her two older brothers. "We used to have fun. We used to say: `Longitude - The Movie.' Ha! Ha! Ha! And now it's happening."

If Granada chooses, the film could pack in sufficient shipwrecks and mutinies to make Captain Bligh blanch. Yet it also has to span the four decades during which John Harrison - a gruff Yorkshire-born clockmaker - perfected a set of life-saving timepieces that more than merited the pounds 20,000 reward (pounds 1m- plus today) offered by a 1714 Act of Parliament to the inventor of an accurate method of calculating longitude at sea. Harrison, whose dogged genius belatedly won several hundred thousand fans when Sobel's book appeared, never got the feet-dragging Board of Longitude to admit that he had met all its demands. Instead, with prodding from a kindly George III (scope for a reprise from Nigel Hawthorne?), he extracted the cash in grudging dribs and drabs from a snobbish scientific elite.

The intricacies of this tale almost match the convoluted innards of a Harrison chronometer. Yet its teller trusts the Granada team not to warp or smash it on its passage to the screen. "I think they've done brilliantly with it. I was very pleasantly surprised... People ask me, `Didn't you want to write the screenplay yourself?' Lord, no! I'll go the movies along with everyone else and enjoy it."

The Longitude empire will stretch even farther in the coming months. A BBC documentary is also on its way, and a handsome illustrated version with captions composed by Will Andrewes - the English-born keeper of scientific instruments at Harvard University who first inspired Sobel to wrest Harrison's memory out of the clutches of antiquarians. Andrewes convened the Longitude Symposium at Harvard in November 1993, which Sobel attended and then wrote up in the Harvard Magazine: "It was the excellence of the symposium that really excited me about the topic."

George Gibson of Walker Books in New York first spotted the recondite subject's promise as a book; Christopher Potter at Fourth Estate followed suit across the Atlantic, after a baker's dozen of patriotic rebuffs. Sobel recalls that "Among the 13 rejection letters from British publishers were several who asked, why would we want to get our history from the colonies?" The rest, however, is a cherishable slice of publishing history.

Sobel is devouring porridge behind the Albert Hall because Fourth Estate has finally released a paperback of Longitude. It follows a 22-month top-10 residency which led to British hardback sales alone in excess of 400,000. Needless to say, the new edition has jumped into the paperback charts at number one.

In the evening, she faces an expert lecture audience at the Royal Geographical Society - the sort of ordeal that calls for solid sustenance. Yet, the night before this test of any writer's mettle, she went out dancing (her great extracurricular passion) with her husband. He is a dance teacher, who specialises in plotting the sensuous co-ordinates of the Argentinian tango.

In dance, as in good prose or good horology, precision and aesthetics have to interlock. And Dava Sobel has sashayed between the cultures of science and writing with a rare fleetness of foot. She was born, 50 years ago, in New York. Her mother trained as a chemist; her father was a GP. They kept a sloop anchored off the Bronx - a district better known beyond the Five Boroughs for its mobsters than its mariners. This craft led them into scrapes afloat during family holidays that cured little Dava (rhymes with saver) of the urge to set sail on her own.

Instead, the Bronx High School of Science beckoned. A notice that now sounds antediluvian announced the academy's intention to limit girl pupils to a 20 per cent quota. "It would have been common sense," Sobel says. "This was the 1950s. They expected the girls to marry and have children. Why give this education to someone who would waste it? So that was the rule: four boys to every girl."

At college, she majored not in a science discipline but in theatre history. Then a job as a science writer attached to Cornell University - the home of such astronomical luminaries as Carl Sagan - hastened her hift from limelight to starlight. At one of Sagan's public lectures, "I can remember sitting there and thinking, this is the most phenomenal thing I have ever heard. All I want to do now is to learn about this."

And learn she did, as a science reporter for the New York Times, a magazine feature writer and the author of popular books with an astronomical bent. One explored the possibility of intelligent life elsewhere in the universe. So does she think that anyone is out there? "Yes, but inconceivably far away. I don't believe this planet has been contacted or visited." We drift, alone, across the starry oceans.

Thinking of the barriers that impeded intelligent life in the Bronx, has her gender slowed her progress through the Boy's Own galaxy of physical science? She reports that it didn't even register with the old salts who lapped up Longitude. "The fact that my name is ambiguous helped. I get many letters from people who assume I'm a man, even though I said on page one that I was not."

Sometimes, being a woman may have assisted in grilling macho scientists "known among writers to be a tough interview". In general, though, "I don't really feel that I have been discriminated against, or gotten special advantages. I guess I'm in that time-frame where it's been a bit unusual, but it hasn't been a big deal."

She did still have to bear the usual double load of family and career. (Her children, Zoe and Isaac, are now teenagers bored by history lessons from teachers who lack their mother's narrative gifts.) Then her first marriage ended, just about the time that she began to wrestle with the problems of direction-finding. In Longitude, we meet Commander Rupert Gould, a naval officer who in the 1920s set to work on restoring Harrison's neglected masterpieces in the aftermath of his divorce. "By putting them to rights, he nursed himself back to health and peace of mind."

The gorgeous contraptions that Gould found so therapeutic had one adverse effect, I suggest. Like all knowledge-machines (cars, computers, or the tape recorder that stopped me ever learning decent shorthand), they can breed dependency. If you ever wished to make a case for that world-class fusspot Nevil Maskelyne - the nit-picking Astronomer Royal who thwarted Harrison, and would certainly have worn an anorak had they existed in 1760 - it would turn on the fact that Maskelyne insisted sailors learn a lot of real science to take their bearings, rather than rely on what even Sobel calls "a magic-box solution".

Even today, she reports, the satellite pulses that guide hi-tech seafarers can lead them astray. "It's only this year that celestial navigation has been dropped from US naval training, and there's lots of discussion about that. Because if you depend on the satellite, you could be going 180 degrees in the wrong direction. If you don't know how to go out there and check with the sextant, you could be in terrible trouble."

Her next book will return to the greatest watcher of the skies of all. In 1613, Galileo Galilei placed his eldest daughter, Virginia in a convent of Poor Clares, a Franciscan order committed to the contemplative life. ("They don't nurse. They don't teach. They pray.") For the rest of her short life, Virginia, corresponded with her father as he made the series of shattering breakthroughs in mechanics and astronomy that climaxed with his trial for heresy at the Inquisition's hands in 1633.

In contrast to the "remarkably focused, incredibly hard-working" - and, you suspect, rather gauche - John Harrison, Galileo and his daughter fill a much broader canvas of emotions and ideas. He wrote poetry and satire as well as scientific treatises; although cloistered, she was evidently a pretty sharp chip off the paternal block. To start with, Sobel had to translate her letters (his are lost), now in the archives in Florence. "And she was a wonderful writer: very clever, very funny. She's definitely his kid."

Yet he shut this 13-year-old girl away from the world for ever. Here Sobel warns against back-projecting contemporary views. "When I first saw this story, it looked like a feminist story. But I no longer think it is. Because if you look at this period, he did the right thing for her. I don't see it as a case of his destroying her life, or anything like that."

Longitude read like a jewelled miniature; a verbal version of the famous watch ("H.4") that crowned Harrison's 40 years of toil. The Galileo book sounds much more like a broad-brush Renaissance panorama, crammed with incident and spectacle. "It's her letters, his science, the Medici dynasty, the bubonic plague, the Roman Inquisition, and the whole Catholic religion. Because he was a good Catholic, all the way through."

Even though he cropped up in the textbooks, Harrison stayed strangely invisible until this meticulous but visionary writer came along to rescue him from beneath the sands of time. In Galileo's case, her task is to separate man from myth: "As with all myths, the associations are erroneous." To prepare, Sobel spent time in Rome among the places where he lived and suffered ("It's really tough", she giggles, "but someone's got to do it").

She also took an adult-education class to brush up her Italian, a language she had studied at college "for no reason at all" except that she loved it. "Italian grammar is very regular, but it's odd," she concludes, "because the pronouns attach to the verb. So if you're not up on your grammar, you never know who is doing what to whom."

Passion and precision (with a sulphurous whiff of power); aesthetic beauty and empirical truth: Sobel's dances to the music of time fuse qualities that our culture prefers to keep apart. Speaking of Longitude, she argues that the book attracted readers as a pleasing artefact, not just a repository of arcane lore. "It had a nice feel in the hands; the weight of it, the paper, everything about it was appealing, and that helped it."

Meanwhile, odd strands of adventitious poetry wrap themselves like seaweed round the fact that fills her work. After imagining the plight of desperate sailors marooned in the trackless ocean, she has gone on to evoke the shuttered world of a young woman whose horizons ended with the convent walls. A vast change of tack, of course, except that Virginia Galilei had to choose her nun's name when she took her final vows. So she became Sister Maria Celeste. No, you couldn't make it up