A narrative account of John Harrison's successful attempt in the 18th century to solve the problem of calculating longitude at sea by his invention of the marine chronometer. The book details Harrison's early life as a carpenter and self-taught clockmaker, the differing approaches taken by Harrison and competitors to tackle the issue of calculating longitude, and particularly the Longitude Prize, a monetary prize offered by British Parliament as a reward for advances toward this end. Illustrations throughout by Duésan Petricic.