Like spacetime itself, Stephen Hawking seemed to appear from nowhere and expand rapidly to fill the publishing universe. Within a year, he was almost certainly the most famous scientist since Albert Einstein and he had established a reputation everywhere in the English-speaking world as the last word on black holes and the making of the universe.

Since black holes had, in 1988, only a hypothetical existence, and since the making of the universe was, and still is, a subject of pure speculation, it was an extraordinary achievement. By 1992, A Brief History of Time had sold six million copies and a paperback edition was still only a twinkle in his publisher's eye. The book sat in the bestseller lists for 237 weeks and was translated into 40 languages. By 1998, roughly one copy existed for every 750 people on the planet.

Hawking had become a global cultural icon, turning up in an episode of Star Trek and several episodes of The Simpsons. This is not bad for somebody whose lasting contribution to science could be a thing called Hawking radiation and whose lasting contribution to publishing is known as the Hawking effect. The first involves a piece of arcane quantum mechanical reasoning that can never be put to the test, because there is no way you could ever get a black hole into a laboratory. The second is a widespread but as yet not convincingly confirmed proposition that if one science book on an inexplicable topic can become a worldwide bestseller, then surely others can.

Some of its success can be put down to the author's public persona. But part of the success must also lie in the timing. Hawking was born in 1942, and he came of age with the idea of the Big Bang. He entered the postgraduate world at the point where physicists began to agree that, yes, the universe must have had a beginning, and that space and time must have appeared from nothing in a burst of unimaginable energy and expanded to enclose a universe that contains, at the latest guess, 200bn galaxies, each home to 200bn stars and stretches for 13.7bn light years.

The next obvious question was: how did it happen? And why did it happen in a way that produced a sentient life form capable of asking such a cosmic question, and then giving it a witty title such as A Brief History of Time?

So is it the greatest science book ever? The answer is "no". Science moves fast and all science books are out of date by the time they are published. Hawking's book dealt with themes that bothered the cosmic physics community 20 years ago. Now cosmic scientists wrestle with different questions. Einstein once said that the most incomprehensible thing about the universe was that it was comprehensible. Steven Weinberg, the author of a brilliant but much earlier history of time called The First Three Minutes, put the same thought another way: the more the universe seemed comprehensible, he said, the more it also seemed pointless. Hawking's gift to the reading world was to spell out the same big and hugely puzzling questions, in words that ended with a note of hope. That still doesn't explain its monstrous success.

"How could a book like this take its place alongside the Bible and Shakespeare as books that every literate home wants to have on its shelves, whether or not it is read?" asks Graham Farmelo, author of a forthcoming biography of Paul Dirac, the second greatest physicist of the 20th century. "Somehow it does not quite wash to attribute most of his sales to a sympathy vote for his disability, even if many are astounded by his tremendous courage in overcoming it." He thinks Hawking has done science great service by unapologetically setting out to describe the excitement of understanding nature at its finest level. "Just as important he - more than anyone else - made it legitimate for really first-rate scientists to spend time communicating with the public."

Paul Davies, a theoretical physicist with a string of popular books - among them About Time, and God and the New Physics - thinks the greatest service of Hawking to science has been to make the public acknowledge that scientists have as much authority to address the great questions of existence as poets, novelists and playwrights. He thinks the problem with Hawking's book is that it was actually too brief: it delivered too many ideas in too few pages.

"I am sure that even Stephen would not call it the greatest book ever written," he says. "Stephen managed to reach parts of the reading public that no other scientist or science writer ever approached, Einstein included. There must be many people who have read - or attempted to read - only one science book, and that is A Brief History of Time".