Proceedings of the International Astronomical Union

Transits of Venus: New Views of the Solar System and Galaxy
Contributed Papers

The American transit of Venus expeditions of 1874 and 1882

Steven J. Dick a1
a1 NASA HQ, Code IQ, 300 E. St. SW, Washington, D.C., USA

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When in 1874 and 1882 Venus passed in front of the face of the Sun, most countries with a scientific reputation to keep or to gain made plans to observe the great event. The United States was no exception. The purpose was primarily to measure the solar parallax, and thereby determine the astronomical unit, the distance between the Earth and the Sun. With a $177000 Congressional appropriation for the 1874 event, and $78000 for 1882, the Americans sent out eight well-equipped expeditions for each transit. Under the U.S. Transit of Venus Commission, the responsibility fell to the U. S. Naval Observatory (Dick 2003). Relying heavily on photographic methods, the Americans returned 350 plates in 1874, and 1380 measurable plates in 1882. Simon Newcomb grew skeptical of the results, but in 1894 William Harkness produced a final value of the solar parallax, after adjustments with other constants, of 8.$\rlap^{\prime\prime}$809, with a probable error of 0.\rlap$^{\prime\prime}$0059, yielding an Earth-Sun distance of 92797000 miles, with a probable error of 59700 miles. This was a significant improvement over previous estimates. 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. Ironically, just at this time other methods were proving more accurate than Venus transits. 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. Thus the Venus transit observations played little role in the official value used for the astronomical unit in the 20th century.


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