Fleuriais, Georges-Ernest, 1840-1895.

Transits of Venus are uncommon events, occurring only four times every 243 years, however in the eighteenth and nineteenth centuries, these rare events were of considerable practical importance to astronomers. Precise measurements of the timing and position of the planet as it passed across the disk of the sun offered the best means available to resolve one of the classic problems in observational astronomy: the determination of the distance from the earth to the sun,

The transit in 1769 was a formative event in the history of the American Philosophical Society, which helped to organize observations in North America and to coordinate the analysis and distribution of the data. Securing funding for equipment from the provincial government on the pretext that their observations would aid in improving navigation, the APS sponsored groups of observers at Cape Henlopen, N.J., the State House Yard in Philadelphia, and most famously at David Rittenhouse's estate in Norriton, Pa., making the Transit of 1769 one of the first examples of a truly cooperative, American scientific enterprise. The precision of the measurements made by these teams turned out in the long run to have been less important than the vehicle for disseminating them. Meeting a long-cherished goal of Benjamin Franklin, the Society published the results of the Transit observations in the first volume of its Transactions in 1771, making it the first scientific journal published in North America and the surest sign to Europeans of institutional maturity in the colonies.

The next Transits of Venus, in 1874 and 1882, became the focus of a truly international effort to observe the transit from dozens of points in order to reduce observational error. The French Académie des Sciences organized a total of ten expeditions, including parties that set up in Haiti, Martinique, Mexico, Florida, Chile, and Cape Horn. The expedition to Santa Cruz on the Patagonian (Argentine) coast was led by the naval officer Georges-Ernest Fleuriais (1840-1895), director of the Cartography Department of the French Navy. Aboard the ship Volage, Fleuriais sailed to Argentina and made observations of the transit just before Venus passed its ascending node on December 6, 1882.

Although several sites were hampered by poor weather conditions, these expeditions generated valuable data that confirmed and refined the data gathered by the better known expeditions of 1874. In both years, photography was used as a key instrument for recording and measuring the transit, with the end result that the solar parallax was estimated at 8.847" plus or minus 0.012", corresponding to a distance of the Sun of 92,385,000 miles.

From the guide to the Passage de Vénus, Mission de Santa Cruz (Patagonie), Photograph Album, 1903-1962, (American Philosophical Society)