Chronometer

A highly accurate timekeeper used in traditional celestial navigation to determine longitude.

A marine chronometer keeps the time at a known reference meridian while observations establish local time aboard the vessel. The difference between those times can be converted into longitude because the Earth rotates through 15 degrees in one hour.

Traditional marine chronometers were engineered to maintain a stable rate despite changes in temperature and the motion of a ship, and were commonly mounted in gimballed boxes. John Harrison’s eighteenth-century work was central to making accurate timekeeping at sea practical. Electronic systems have replaced chronometers for routine position fixing on most vessels, but the instrument remains important to the history and practice of celestial navigation.