John Harrison invented the marine chronometer that solved the longitude problem at sea in the 18th century. Harrison was an English clockmaker who devoted much of his life to building a clock accurate enough to keep time during long ocean voyages.
A ship could determine latitude comparatively easily, but longitude required comparing local time with a reference time. Every hour of time difference represented 15 degrees of longitude. Harrison’s H4, completed in 1761, was a compact sea watch that stayed accurate despite motion, temperature changes, and humidity.
The 1714 Longitude Act offered a major prize for a practical solution. Harrison eventually received substantial recognition and financial rewards, although the process was politically difficult. His work did not create the only method of finding longitude: astronomical observations using lunar distances also worked. However, accurate chronometers made navigation far more convenient and reliable.
Harrison’s earlier H1, H2, and H3 timekeepers were large experimental clocks. H4 looked more like an oversized pocket watch and became the best-known member of the series.