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An octant measures angles by bringing two images together—that of the sun, for instance, and the horizon—and was used primarily to determine latitude at sea. The form was described by John Hadley in London in 1731 and still in use in the early twentieth century.
This instrument made during the American Revolution, and there are faint markings on the scale that may read "equality" and "justice." It has a mahogany frame and index arm, and ivory inset. The boxwood scale is graduated every 20 minutes from -5° to +95° and read by ivory vernier to single minutes of arc. There is also a back sight and a back horizon mirror for measuring angles greater than 90°. The radius is 17.75 inches. The "B. CONDY PHILADELPHIA 1778" inscription refers to the maker, Benjamin Condy (d. 1798).
Currently not on view
Object Name
Date made
business owner
Condy, Benjamin
Condy, Benjamin
Place Made
United States: Pennsylvania, Philadelphia
Physical Description
wood (overall material)
brass (overall material)
glass (overall material)
average spatial: 7.9 cm x 42.1 cm x 51.8 cm; 3 1/8 in x 16 9/16 in x 20 3/8 in
overall: 4 1/2 in x 22 in x 19 1/2 in; 11.43 cm x 55.88 cm x 49.53 cm
ID Number
catalog number
accession number
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Medicine and Science: Physical Sciences
Measuring & Mapping
Data Source
National Museum of American History
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