Marine Chronometer

A marine chronometer is a precision timepiece carried aboard ships to determine a vessel's longitude through celestial navigation, by comparing the time at a fixed reference meridian with local time observed at sea. The English carpenter and clockmaker John Harrison developed the first true marine chronometer after thirty-one years of experimentation, completing his fourth design, the H4, in 1761. Accurate knowledge of time over a long sea voyage was vital to safe navigation in an age with no electronic or communication aids, and the marine chronometer, among the most exact portable mechanical timepieces ever built, is credited with helping secure British naval and commercial dominance at sea.