Longitude recounts the story of one of science's greatest leaps forward: the development of a clock accurate enough to determine a ship's longitude.  The stimulus was a £20,000 prize offered by Act of Parliament in 1714.  It took clockmaker John Harrison forty years of genius, hard work and political battles to win the prize.

Dava Sobel begins by explaining the great need for a reliable longitude method.  Sailors have always been able to determine their latitude by the length of the day, but working out longitude was a much greater challenge.  Multitudinous shipwrecks resulted from confusion over longitude, including that of a Royal Naval fleet in 1707 which led to the establishment of the Longitude Prize.

The most likely candidates for the prize were astronomical: comparing the position of the stars relative to the moon to predictions made in almanacs.  This complex method had various drawbacks, not least the absence of the moon for several nights each month.  Nevertheless, the astronomical method had powerful supporters who were less than impressed by the suggestion that the problem could be solved with a humble clock.

 The theory is simple: if you know the local time as well as the time at a reference longitude (e.g. London), then it is a trivial matter of arithmetic to work out the longitudinal difference.  The challenge is building a clock that can hold the reference time with sufficient accuracy throughout a sea voyage of many months.  The longitude problem boiled down to the precision of a clock mechanism.

John Harrison was the Yorkshireman who, in pursuit of the prize, dedicated his life to developing ever more accurate clocks.  His first three (H1, H2, H3) were large brass ensembles that introduced a number of important innovations, including the bimetallic strip and caged roller bearings.  However they failed to impress the astronomy-biased Board of Longitude, despite successful sea tests - and, more importantly, failed to satisfy their maker.  H4, a tiny pocket watch of elegant design, was the real breakthrough, paving the way for marine chronometers on which every seaman would come to depend.

Nevertheless, prizing the prize money out the Board proved as hard as creating the legendary H4, partly due to the powerful opposition of Nevil Maskelyne, Astronomer Royal and a rival for the prize.  Ultimately, King George III had to intervene on Harrison's behalf to procure his rightful winnings.

Longitude details all the twists and turns of the invention of the first marine chronometer, as well as recounting its further development by other clockmakers.  And it remembers with fitting praise the man who did so much for navigators the world over.