The book was Longitude, the story of English clockmaker John Harrison, who solved the problem of how to measure longitude accurately. The trickiest technological problem of the 18th century meant that sailors, unable accurately to determine their position, were often lost at sea when they lost sight of land.

In London in 1714, Parliament offered a reward to anyone who could successfully measure longitude; that propelled Harrison on his 40-year quest to build a clock that would keep time at sea and withstand the effects of pitching, rolling, humidity and temperature change. Today his invention is called the chronometer.

Lapped up by layman and scientist, Longitude seemed overnight to overhaul attitudes to non-fiction, creating a demand for accessible single volumes penetrating the mysteries of arcane, often scientific or quasi-scientific, subjects, all tempered with a large dollop of human interest.

Simon Winchester's The Map that Changed the World introduced geologist William Smith and his greatest achievement, the planet's first geological map, of England and Wales. In Guns, Germs and Steel, Jared Diamond explored the reasons for the longevity of Eurasian civilisations. David Bodanis dissected the meaning of history's most celebrated equation in E=mc2.

But popularising a publishing genre was hardly Sobel's mission statement when she began writing Longitude. 'I'm still proud of it and I'm grateful to it,' says Sobel, from Shanghai, of an award-laden work that received 29 hardcover printings and was translated into more than 20 languages. 'It brought me here. But when it first came out I was tremendously surprised by people's response to it. No one had responded positively to the subject, even for a magazine article. Every-one hated it; no one was interested.

'There was a longitude symposium at Harvard in 1993 and at the last minute Harvard Magazine said yes to an article. At the time I didn't know about John Harrison. Later, when people asked what I was doing and I replied that I was writing a book about longitude, they would look down or turn away. I really didn't expect anyone to find it interesting. I'd like to think it caught the public's imagination because it was so well written, but I have to say it was brilliantly published.

'It seemed like a folie a deux but my publisher and I loved the idea and poured ourselves into it. It was a rare opportunity in life: to work on something we really wanted to work on. We believed in it totally and decided to ride with it.

'I asked my son if he wanted to read it and he said no. From the book's success I then learned a powerful lesson. My son announced that he wanted to work in theatre; his sister was worried that he wouldn't make it. And I was not the one to tell him not to live his dream. He's now working on Broadway.'

Sobel likes 'writing books better than being a journalist' but still regards herself as a 'science writer'. Her articles in Harvard Magazine led to a job as a science reporter on The New York Times, after which she became a contributor to a shelf of magazines including Omni, Discover and The New Yorker.

What Sobel describes as her 'most unforgettable assignment' for The New York Times required her to live for 25 days behind boarded-up windows as a research subject in a 'chronophysiology' laboratory, an experiment that led to a series of articles. 'It was an all-consuming experience,' she says. 'It looked like I'd have a lot of time to fill so I took lots of books to read and things to do. But the business of being observed took over. I had a catheter in my arm that allowed the researchers to draw blood at ridiculously frequent intervals and I was always doing something.

'It was difficult and physically tiring, but interesting. It was also interesting to watch what happened to the observers, who didn't like being watched at all. There was tension between us.'

Despite packing her bags for the heavens with The Planets, one form of 'method reporting' in which Sobel will not be indulging is taking a US$200,000 commercial sub-orbital space flight. 'I don't have the money and I don't want to go as a tourist,' she says. 'I have a fantasy that somehow I'll be needed on a Nasa mission and if that comes true then I'll go.

'A small fortune came from Longitude, but I don't know anything about managing money; if I did I'd be in better shape. I gave a lot of it away, paid for the children through college, paid off the mortgage. Then it was gone.'

Nor does Sobel expect The Planets to eclipse Longitude in the overall takings charts. 'It may do really well, but I don't think it will see the same success,' she says. 'It's had a more complicated publishing history.'

Galileo's Daughter, based on the remaining 124 letters to Galileo from his eldest child, which were translated from Italian by Sobel, made another big impression on sales figures.

'That was different again. I expected the story to appeal to people and the book had a large readership, which was gratifying. It was number one in The New York Times paperback best-sellers list for five weeks, but the money doesn't last. That's not one of my strengths.'

Sobel has high hopes for her latest creative venture: a play. Titled And the Sun Stood Still, the work brings back centre stage the 15th- and 16th-century astronomer Nicolaus Copernicus, who posited the dangerously heretical theory of heliocentricity.

'I've just finished it for the fourth time. Now it sounds like a play,' says Sobel. 'I read a lot of plays and books about playwriting and I saw a lot of plays in order to write it. I showed the first drafts to a couple of directors and their comments helped me improve it a lot.

'It's a truism to say a play is different to a book but I didn't realise the depth of that truth until I started to write it. It took me more than two years and it was an extremely difficult process. I don't know what I'll do next, another play perhaps, but if there's a way to make my life any more difficult I'm sure to find it!

'This play is about a surprise visit paid to Copernicus by fellow astronomer [Georg Joachim] Rheticus, who persuaded him to publish De Revolutionibus. It's about what might have been said between them. Rheticus was a Lutheran from the University of Wittenberg; Copernicus was a Polish Catholic. This was the Reformation and it was a dangerous meeting.

'I enjoy the writing process, sitting and researching things like this. If I didn't I'd try to get a real job. Someone said to me that what I do must be like writing an endless term paper. It is, that's why I enjoy it.'

Sobel has a favourite planet: 'Earth, of course.' She follows tips for saving it ('I turn the heat down and the lights off, I don't drive if I don't have to') but Saturn has also captured her heart.

'The rings just look so beautiful through a telescope,' she says. And then comes a surprising admission for someone who was a science specialist at high school: 'But I'm not very good with equipment.'

She will, however, continue to spread the word about the heavens at symposia, seminars and talks to enthusiastic students such as those at the recent gathering in Shanghai who, she says, left her feeling emotional.

'I wasn't expecting to have such a crowd and I was surprised, bewildered and touched. Everything was said through an interpreter and they were extremely attentive and patient. They'd read my books and wanted to hear what I had to say,' adds Sobel humbly. 'And I didn't get the impression they were all astronomy majors.'

Having gazed at a few stars in her time, does Sobel believe there is anybody out there? Is there intelligent life elsewhere in the universe? 'The answer is ... maybe. I'm asked that question by different interest groups - people convinced they've been abducted, people who have seen UFOs - because people like to think we're not alone. There's no evidence; only probability.'

Sobel might be excused for concerning herself occasionally with more earthly difficulties. Being a star author brings its rewards, but may also come with some surprising gifts - and associated problems.

'A craftsman-builder called me and said, 'I've read your book and I've made you a clock.' He'd read Longitude, so he gave me this huge clock.

'It's stopped running.'

Writer's notes

Genre Popular science

Latest work And the Sun Stood Still

Next project Another play, perhaps

Age 60

Born The Bronx, New York

Family Married, two children

Lives East Hampton, New York

Other jobs Journalist

Other works include Longitude (1995), Galileo's Daughter (1999), The Planets (2005), Best American Science Writing (2004, ed.)

What the papers say

'She has an extraordinary gift of making difficult ideas clear.'

- The Daily Telegraph

'The Planets has something for everyone. Myth, poetry, science fiction, geology, mineralogy, cosmology and even etymology find their way into Sobel's almost fable-like narratives ...'

- The New York Times

'Sobel is a master storyteller ... What she has done, with her choice of excerpts and her strong sense

of story, is bring a great scientist to life.'

- The New York Times Book Review on Galileo's Daughter

Author's bookshelf

Ragtime by E.L. Doctorow

'I like the rhythm of it. The story has a beat and picks up everything in its path.'

The Accidental Indies by Robert Finley

'Imaginatively, poetically written.'

The White Album by Joan Didion

'Her prose is so sharp, so clear.'

The Planets: A Cosmic Pastoral by Diane Ackerman

'Diane and her poems have been an inspiration to me for years.'

The Book Nobody Read by Owen Gingerich

'It's about Copernicus but it's such an unusual story.'