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San Antonio hosts two observing expeditions for transit of Venus


On this day in 1882, two expeditions, one from the United States Naval Observatory and one from the Belgian Royal Obervatory, came to San Antonio to observe the transit of Venus. Such transits are very rare events that offer an opportunity of determining the distance of the earth from the sun by mounting expeditions to widely scattered sites over the earth, from which different tracks of the planet across the sun can be observed. On previous occasions the exact moments when the disk of Venus was just fully on the edge of the solar disk were unknown because of a distortion of the planet's image known as the "black drop effect." For the 1882 event many nations sent expeditions to a variety of sites; San Antonio was considered the best observing station in North America. The Belgian expedition produced many drawings of the black drop effect, both at San Antonio and at the companion station in Chile, but these led again to an uncertain estimate of the earth-sun distance. The American results were equally disappointing, not least because the funds allocated by Congress were cut off.

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