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A sailor can look at the sun and find how far north or south they are . But for centuries , no one could find how far east or west they were . This caused many shipwrecks . Ships hit rocks they could not see coming . In 1714 , the British Parliament offered £ 20,000 to anyone who solved this problem . That was an enormous sum of money . John Harrison grew up in Lincolnshire , England . He was a carpenter , not a scientist . He taught himself to build clocks . He worked on the problem for forty years . A ship carries a clock set to the time at home . The sailor checks the sun at noon . If the sun is late , the ship has travelled west . The gap tells you exactly how far . Harrison built four timekeepers . The fourth one sailed to Jamaica in 1761 . After weeks at sea , it was wrong by only five seconds . Five seconds meant an error of only about two kilometres . Harrison 's clock changed everything . Ships could now sail safely across open ocean . Every GPS satellite today solves the same problem Harrison solved with a clock in his hand .
