Jaw Micrometer

Jaw Micrometer

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Hoping to improve our understanding of the distance between the Earth and the Sun, the United States sponsored eight parties to observe the 1874 transit of Venus across the face of the sun, and equipped each with an identical set of apparatus made by Alvan Clark & Sons. For photographing the sun, each party had a horizontal telescope—known as a photoheliograph—with a lens of 5-inches aperture and nearly 40 feet focal length. To measure the exact distance between the lens and the photographic plate, there was a jaw micrometer. William Harkness, an astronomer affiliated with the U.S. Naval Observatory and the U.S. Hydrographic Office, designed the photoheliograph and the jaw micrometer.
The word “Kerguelan” marked in pencil on the wooden box indicates that this micrometer was used at the observing station on a Kerguelan island in the southern Indian Ocean.
Ref: Simon Newcomb, ed., Observations of the Transit of Venus, December 8-9, 1874 (Washington, D.C., 1880), pp. 30 and 72.
Currently not on view
Object Name
date made
Alvan Clark & Sons
place made
United States: Massachusetts, Cambridge
overall in case: 2 1/8 in x 6 5/16 in x 6 1/4 in; 5.3975 cm x 16.03375 cm x 15.875 cm
ID Number
accession number
catalog number
Credit Line
U.S. Naval Observatory
Science & Scientific Instruments
See more items in
Medicine and Science: Physical Sciences
Data Source
National Museum of American History
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