A marine chronometer is a precise timekeeping device used at sea to determine longitude. It was essential for navigation, especially during the Age of Exploration, as sailors needed accurate time to calculate their position. By comparing the local time, determined by the position of the sun, with the time on the chronometer, navigators could find their east-west location. Invented in the 18th century, the marine chronometer was a significant advancement in maritime technology. John Harrison, a British clockmaker, developed the first practical marine chronometer, known as H4, which greatly improved navigation accuracy. This innovation helped reduce shipwrecks and made long-distance sea travel safer.