A chronometer is an extremely accurate clock that is used especially by sailors at sea. a very exact clock, used for scientific purposes. Mechanical timekeeping device of great accuracy, particularly one used for determining longitude (see latitude and longitude) at sea. Early weight-and pendulum-driven clocks were inaccurate because of friction and temperature changes and could not be used at sea because of the ship's motion. In 1735 John Harrison invented and constructed the first of four practical marine timekeepers. The modern marine chronometer is suspended to remain horizontal whatever the inclination of the ship and differs in parts of its mechanism from the ordinary watch. A chronometer may provide timekeeping accurate to within 0.1 second per day. See also Ferdinand Berthoud
A portable timekeeper, with a heavy compensation balance, and usually beating half seconds; intended to keep time with great accuracy for use an astronomical observations, in determining longitude, etc