Engraving of Airy's Altazimuth, a meridian instrument at the Royal Observatory, Greenwich (now part of Greater London). Designed by George Bidell Airy (1801-1892) the Astronomer Royal, in 1847, the instrument was made by Ramsome & Sims of Ipswich with optical parts by Troughton & Simms, London. The altazimuth was used to measure the position of the Moon as it moved acros the sky. Unlike transit instruments that can only make measurement in the north-south meridian, the altazimuth can point anywhere in the sky. From 'The Midnight Sky: Familiar Notes on the Stars and Planets' by Edwin Dunkin (1821-1898), a popular account of the stars and planets published in 1891. Dunkin was chief asistant at the Greenwich Observatory.
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