Thomas Earnshaw Box Chronometer

This instrument is a specialized timekeeper for finding longitude at sea. Thomas Earnshaw made this chronometer in England about 1798. It became part of the James Arthur Collection at New York University, and the university donated a portion of the collection, including this chronometer, to the Smithsonian in 1984.
To find longitude at sea, a chronometer was set to the time of a place of known longitude, like Greenwich, England. That time, carried to a remote location, could be compared to local time. Because one hour of difference in time equals 15 degrees difference in longitude, the difference in time between the chronometer and local time would yield local longitude. The instruments require careful handling to keep precise time. Although the original box for this instrument has not survived, most chronometers are fitted in a wooden box in a gimbal to remain level and compensate for the movement of a ship at sea.
Thomas Earnshaw (1749-1829) was a pioneer in chronometer development. He is credited with introducing to chronometer design two important features that became standard parts of the timekeeper in the 19th century—the detached detent escapement and, independently of his rival John Arnold, the bimetallic compensation balance. His simplifications permitted others to undertake batch production of chronometers, and his work received an award of £2500 from Britain’s Longitude Board in 1805.
Mechanism details:
Escapement: Earnshaw spring detent
Duration: 1 day
Power source: Spring drive with chain and fuse
Balance spring: helical, blued steel
Balance: Earnshaw, two-arm
Inscription: “Thos. Earnshaw / Inv. Et Fecit No. 451” on back plate
Barrel bridge possibly a replacement
Dial details:
Engraved and silvered brass
Indicates hours, minutes, seconds
Inscription: “Thos. Earnshaw / INVT. ET FACIT /No. 451” on dial
Blued steel lunette hands (minute hand is a replacement)
Brass bowl; screwed and milled bezel; convex and chamfered crystal
No box
No winding key
1. Mercer, Tony. Chronometer Makers of the World. Essex: NAG Press and Tony Mercer, 1991.
2. Thompson, David. Clocks. London: British Museum Press, 2005.
3. Whitney, Marvin E. The Ship’s Chronometer. Cincinnati: American Watchmakers Institute Press, 1985.
Object Name
box chronometer without case
date made
ca 1798
Earnshaw, Thomas
Physical Description
brass (overall material)
steel (overall material)
glass (overall material)
overall: 2 7/8 in x 3 1/2 in; 7.3025 cm x 8.89 cm
place made
United Kingdom: England, London
ID Number
accession number
catalog number
Time and Navigation
Measuring & Mapping
See more items in
Work and Industry: Mechanisms
Time and Navigation
Time and Navigation, National Air and Space Museum
Data Source
National Museum of American History, Kenneth E. Behring Center

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