Longitude tells the life-and-death story behind the grid work of lines appearing on every world map and globe. In the early days of sail, mariners had no accurate means of determining their position at sea. Indeed, the entire Age of Exploration was carried out without anyone?s ever knowing where he was. Since ships lost track of their longitude as soon as they lost sight of land, innumerable tragedies befell navies and traders ? until John Harrison, a self-educated clockmaker, solved the problem of longitude in the mid-eighteenth century and laid claim to the great Longitude Prize offered by Parliament.