US NAVAL OBSERVATORY / SCIENCE PHOTO LIBRARY US NAVAL OBSERVATORY / SCIENCE PHOTO LIBRARY
Transit of Venus observers. Group portrait of astronomers gathered to observe the 1874 Transit of Venus on 9 December 1874. Such transits (where Venus passes across the Sun as seen from the Earth) are rare, occurring in pairs over 100 years apart. Several expeditions were organised to different locations to observe the 1874 transit. The combined results helped established the distance of Venus and the Sun from the Earth, and hence the size of the Solar System. These observers are at the old observatory of the US Naval Observatory, Washington DC, USA, with the 26-inch refractor building in the background. Admiral Charles Henry Davis (1807-1877) is behind the coelostat at left (hat in hand), with Canadian-US astronomer Simon Newcomb (1835-1909) seated at left. Between Davis and Newcomb is US astronomer Henry Draper (1837-1882).
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