Hamilton No. 1 Box Chronometer

This instrument is a specialized timekeeper for determining longitude at sea. It is serial no. 1 from a run of approximately 10,000 similar timekeepers made by Hamilton Watch Co, Lancaster, PA in 1942.
World War II created a dire chronometer shortage for the United States. Before the war, most chronometers for American military and civilian customers were imported. Only a few American firms—including William Bond & Son of Boston and the New York establishments of John Bliss Inc. and T.S. and J. D. Negus—finished chronometers from parts imported from European makers. Chronometer making was a craft, with only a few hundred produced in any given year. When the war started in 1941, European suppliers of parts and finished instruments halted exports to the United States.
Anticipating the arrival of war, the U.S. Naval Observatory had asked American domestic watch manufacturers in 1939 for their participation in mass-producing chronometers. Domestic watch manufacturers Hamilton and Elgin agreed to undertake the design and production, but only Hamilton’s product met Navy accuracy requirements. Hamilton delivered two prototypes to the Navy on 27 February 1942, which passed with an error rate of 1.55 seconds per day. The firm went on during the war to mass-produce 8900 more chronometers for the Navy, 1500 for merchant shipping and 500 for the Army. Between 1942 and 1944, the price dropped from $625 to $390 per timekeeper.
Hamilton’s design for its Model 21 chronometer did not copy traditional European standards. Instead the design introduced key changes to improve accuracy. The modifications included changes to the escapement and the chronometer’s oscillating unit—the balance and hairspring assembly.
To find longitude at sea, a chronometer would be set to the time of a place of known longitude, like Greenwich, England, the prime meridian. 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.
1. Dick, Steven J. Sky and Ocean Joined: The U. S. Naval Observatory 1830-2000. Cambridge: Cambridge University Press, 2003.
2. Whitney, Marvin. The Ship’s Chronometer. Cincinnati: American Watchmakers Institute Press, 1985.
Object Name
box chronometer
date made
Physical Description
brass (overall material)
steel (overall material)
glass (overall material)
wood (overall material)
overall - chronometer case: 7 1/2 in x 8 in x 7 3/4 in; 19.05 cm x 20.32 cm x 19.685 cm
overall - carrying case: 9 1/2 in x 12 in x 10 in; 24.13 cm x 30.48 cm x 25.4 cm
overall - from catalog card: 7 1/2 in x 7 1/2 in x 7 1/2 in; 19.05 cm x 19.05 cm x 19.05 cm
place made
United States: Pennsylvania, Lancaster
ID Number
catalog number
accession 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
Additional Media

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