Legend has it that ancient mariners lived in terror of sea monsters and other perils of their own imaginings. A more legitimate fear was the knowledge that, once out of sight of land, they had no way of knowing precisely where they were. With the sun and stars as guides, skilled navigators could determine their position north or south of the equator, but locating themselves east and west—calculating the longitude—was largely a matter of guesswork. In 1714, Britain's Parliament offered a cash award to anyone who could solve the problem.
This is an absorbing account of both the formidable technical obstacles involved and the brilliant exertions of clockmaker John Harrison, whose marine chronometer won him the prize. In an elegant, highly readable overview of a daunting conundrum, Sobel, a former science reporter for The New York Times, reveals not only the mechanical-difficulties Harrison had to overcome in devising a clock far more accurate than any the world had ever known but also the hazards thrown in his way by England's royal astronomer, who, not surprisingly, favored a heavenly solution to the longitude problem. (Walker, $19)