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In 1874 and again in 1882, the planet Venus passed between the Earth and the Sun, causing what was known as a Transit of Venus. In preparation for these important events, the U.S. government established a Transit of Venus Commission that would organize and equip eight teams of astronomers and send them to observation stations around the world. A key component of that equipment was a weight-driven heliostat designed to send an image of the sun into a horizontal telescope. This is the mirror of one of those heliostats. It is seven inches diameter, of unsilvered glass, and slightly thicker on one side than the other.
Ref: Simon Newcomb, Observations of the Transit of Venus, December 8-9, 1874 (Washington, D.C., 1883), p. 15.
Currently not on view
Object Name
date made
Alvan Clark & Sons
overall: 18 3/4 in x 21 1/2 in x 16 1/2 in; 47.625 cm x 54.61 cm x 41.91 cm
overall; heliostat: 1 3/4 in x 8 in; 4.445 cm x 20.32 cm
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Medicine and Science: Physical Sciences
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National Museum of American History
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