The Earth-Sun distance values from the 18th-century transits of Venus did not achieve the accuracy many astronomers desired. But scientists hoped that improved instruments and techniques would yield significantly better results in the 19th century.

Many countries, spurred on by a combination of scientific inquiry, nationalism, and colonialism, mounted far-flung expeditions to observe the 1874 and 1882 transits. A new addition to the mix was the United States.

The U.S. Naval Observatory oversaw details for the eight official U.S. expedition teams in 1874, and it commissioned identical instruments for each of them. Expedition leaders felt that the key to success was the use of a new method for capturing precise details of the transit: photography.

Transit Expeditions in the 19th Century

Most of the 1874 transit-of-Venus expeditions found that their photographic methods did not give them images clear enough for accurate measurements. However, the Americans, who took more than 200 photographs of the transit, felt that their method was worth trying again in 1882.

The 1882 transit was visible in the United States, and the U.S. Naval Observatory's eight expeditions produced nearly 1,400 useful photographs. The astronomer William Harkness spent nearly 20 years analyzing the 1874 and 1882 data to determine the Earth-Sun distance. However, his calculation of 92,797,000 miles (plus or minus 59,700 miles) was still not accurate enough.

But the transit expeditions were not a total loss to the countries that sponsored them. They enhanced the international reputations of the newly established governments of Germany, Italy, and Mexico. The United States used its leading role in observing the transits as evidence that it was a major force in science and foreign affairs.

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