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.
Subjects: Maritime History.