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19th century Mars astronomy. Telescope dome and hut, part of the 1877 expedition to Ascension Island to observe the opposition of the planet Mars. Mars is in opposition when it is on the opposite side of the Earth to the Sun. The 6-month expedition was led by the Scottish astronomer David Gill (1843-1914), and the results enabled an accurate and improved calculation of the Sun-Earth distance. This observing station, known as 'Garrison', was the first one assembled by the expedition. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROYAL ASTRONOMICAL SOCIETY)