chronometer

chronometer , instrument for keeping highly accurate time, used especially in navigation. Before the advent of radio time signals it was the only device that provided the time accurately enough for a ship at sea to determine its longitude. A mechanical chronometer is a spring-driven escapement timekeeper, like a watch, but its parts are more massively built. Changes in the tension of the spring caused by variations in temperature are compensated for by devices built into it. Modern chronometers are electronic, using the vibrations of a quartz crystal to regulate the rate at which a time-indicating display moves.

chronometer, a timepiece with a nearly constant rate, intended primarily for the determination of longitude by comparing local time, computed by celestial or satellite navigation, with time at the prime meridian measured by a clock. In practice the chronometer would normally have been used to rate the error of the hack watch which could be taken on deck so that the chronometer itself need not be shifted unnecessarily. The chronometer is normally set to Greenwich Mean Time (Universal Time) and rated by radio time signals. Three chronometers used to be carried so as to detect the errant instrument if one of the three became unreliable. It was not until the 19th century that the mechanical construction of marine timepieces reached a sufficiently high standard, and improvements in manufacture resulted in a reduction of cost, that the chronometer became generally available at sea. See also board of longitude.

Mike Richey