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S. J. Dick (NASA)
When in 1874 and 1882 Venus passed in front of the face of the Sun, ``every country which had a reputation to keep or to gain for scientific zeal was forward to cooperate in the great cosmopolitan enterprise of the transit," in the words of 19th century historian Agnes Clerke. The United States was no exception. With a 177,000 dollar Congressional appropriation for the 1874 event, and 78,000 dollars for 1882, the Americans sent out eight well-equipped expeditions for each transit. Under the U.S. Transit of Venus Commission, the responsibility fell to the U. S. Naval Observatory. Relying heavily on photographic methods, the Americans returned 350 plates in 1874, and 1380 measurable plates in 1882. Simon Newcomb grew skeptical of the results, but William Harkness produced a final value, after adjustments with other constants, of 8.809 arcseconds, with a probable error of .0059 arcseconds, yielding an Earth-Sun distance of 92,797,00 miles, with a probable error of 59,700 miles. How important were the transit of Venus observations? In the end it was Newcomb who had the final say, for it was his system of astronomical constants that was adopted internationally at a Paris conference in 1896. In determining a final value for the solar parallax from all methods, Newcomb gave all photographic observations of the 1874 and 1882 transit a weight of 2, compared to a weight of 40 for Pulkovo Observatory's determination of solar parallax from the constant of aberration.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.