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It’s doubtful that Longitude, Dava Sobel’s best-selling chronicle of 18th-century navigational discovery, would have been so popular had it been sold as the search for the perfect clock. But in fact the two quests were equivalent. The Earth spins 360 degrees every 24 hours, so elapsed time and angular displacement are directly proportional. On sea or land, noon falls when the sun is directly overhead. If you have a reliable timepiece on board your vessel, the time it reads at local noon will reveal your position. While his competitors in the race for the longitude prize looked to the heavens for an answer, John Harrison took the trophy by focusing on the gears and windings of a series of horological marvels. Anyone who prefers, as I do, tinkerers to scientists—the thoroughly useful phonograph to the pie-in-eye-in-the-sky Hubble space telescope—will surely cheer. Sobel reads at 8 p.m. at the Museum of American History’s Carmichael Auditorium, 14th & Constitution Ave. NW. $13. For reservations call (202) 357-3030. (GD)