A high degree of success for the story of longitude

Marianne Macdonald on the bestselling novel about a carpenter and a scientific conundrum

An extraordinary account of a carpenter who taught himself horology to solve one of science's most urgent problems - how to calculate longitude - has become an unexpected publishing success.

Dava Sobel, 49, a former science reporter for the New York Times, has been astonished by sales of her book. It has been in the top 10 of the American bestseller list for more than 20 weeks and is now top of the British list.

Simply entitled Longitude, it is the story of a Yorkshire carpenter, John Harrison, who invented a clock that allowed sailors to chart their geographical position on the high seas. In doing so, he solved a centuries- old conundrum which had caused the death of thousands of travellers and the loss of fortunes. Such was its importance that in 1714 the British government offered a pounds 20,000 prize - worth several million pounds today - to whoever could solve the problem.

In simple terms, Harrison's clock allowed sailors to measure their position by allowing them to tell the time accurately on board for the first time. The Earth takes 24 hours to complete a revolution of 360 degrees, so each hour of time difference marks 15 degrees of longitude. His clock allowed sailors to compare the time at sea (easily done at noon) with the time in London, or another place of fixed longitude.

Ms Sobel became interested in the subject when she met Will Andrews, curator of scientific instruments at Harvard and formerly in charge of the three Harrison clocks, H1, H2 and H3, in the Maritime Museum in Greenwich.

"He was organising a symposium on marine instruments and invited me along," she said. "I was intrigued by his passionate interest in the subject so I went along and I found the Harrison story unbelievable."

By then a freelance writer, Ms Sobel wrote an article, but nobody wanted to publish it until it was at last taken by Harvard magazine.

The publisher George Walker read it, was fascinated and then asked Ms Sobel to write a book on the subject.

She admits: "It is a title to strike dread in the heart." Yet the subject gripped her - the battle of a self-taught genius against the scientific Establishment which refused to recognise that Harrison's invention could work or to give him the prize.

Speaking on Start the Week yesterday, Ms Sobel said his invention was vital. "It was so important because sailors were dying by the thousands and because cargoes worth the wealth of nations were being lost.

"Latitude one can tell by determining the height of the sun above the horizon, but once out of sight of land ... your longitude is anybody's guess. It's quite sobering to realise that all the great voyages of exploration were undertaken against this obstacle."

Harrison decided to solve the longitude question, she told the Radio 4 programme.

"He had taught himself clock-making and he knew of this problem because he lived near a major port. One of the major difficulties with having a clock work on board ship ... is dealing with the fact that a ship sails through vastly different temperatures and anything made of metal will expand or contract.

"It was really his genius in combining metals which overcame that problem. It had never been done before."

5 Longitude; Fourth Estate; pounds 12.99.