This is a fascinating story with intrigue, science, history, swashbuckling sea farers,  that is told in a popular, vernacular style that is easily understood.  Sobel builds her story carefully and cleverly, introducing the hero, John Harrison (1693-1776) half way through her account of how ships in the 16, 17, and 18^th Centuries were constantly slamming into unanticipated coasts and reefs because they had no idea how far east or west they were.  Latitude was easily calculated given the horizon and the sun, but longitude was anyone’s ‘dead reckoning’ until Parliament formed the Board of Longitude to determine who should win a 20,000 pound prize for figuring out how to measure longitude accurately, within half a degree.  We meet Isaac Newton, Galileo, Cassini, Flamsteed (the first Royal Astronomer), Halley (of comet fame, and the second), and Captains Cook and Bligh, as John Harrison, a simple woodworker with no formal education in science or horology (clock making) invents H1 through H4 and finally the pocket watch sized H-5(1770), wins the prize, and revolutionizes navigation through his uses of wood, brass, and innovative methods to compensate for temperature, moisture, and the ocean’s movements.  Sobel reasonably claims that Harrisons’ chronometer designs enabled Britain to rule the seas and establish their Empire.  Not bad for a modest clockmaker.  Fun story and informative.