US NAVAL OBSERVATORY / SCIENCE PHOTO LIBRARY US NAVAL OBSERVATORY / SCIENCE PHOTO LIBRARY
Hall's discovery of the moons of Mars, 1877. The original observing book notes describing the discovery of Mars' satellites by US astronomer Asaph Hall (1829-1907) using the 26-inch refracting telescope (the 'Great Equatorial') at the US Naval Observatory, Washington DC, USA. The discoveries were made on 12 and 18 August 1877, with the date of 11 August given here reflecting the contemporary convention that the observing day began at noon. Although not named as such until later, Deimos was discovered first, followed by Phobos.
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