The subtitle here tells the reader exactly what the book is about; what it doesn't say is how much fun it is to read. The Greek astronomers could measure latitude as early as the third century b.c., but more than 2,000 years passed before the development of a reliable method for measuring longitude. Former New York Times reporter Sobel (coauthor, Arthritis: What Works, 1989, etc.) sets the stage by recounting the difficulties early navigators had in determining their exact longitude. After the loss of many ships and human lives as a result of navigational errors, in 1714 Parliament offered a rich prize for a practical way to measure longitude at sea. British astronomers saw a solution in the stars, by making sufficiently accurate measurements of lunar positions and comparing them to positions calculated for a known reference point. But the calculations could take hours and were tricky even in the best of circumstances; one future astronomer royal, under pressure, botched a measurement of the longitude of Barbados. Enter John Harrison, an apparently self-taught English clockmaker. Over a period of 40 years, he developed four increasingly precise chronometers capable of holding accurate time over a long, rough sea voyage. Comparing the chronometer's time to local sun time, a navigator could measure longitude with high precision in short order. Despite fierce opposition from astronomers (who scorned a "mere mechanic"), Harrison's clocks were enthusiastically endorsed by every mariner who put them to the test (including such luminaries as Cook and Bligh). With the support of King George III, the clockmaker eventually prevailed and won the prize. Sobel tells his story (and the larger history of the search for longitude) clearly, entertainingly, and with a fine sense of the era in which it took place. Breezily written and full of fascinating characters and facts, here's a science book as enjoyable as any novel.