ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Airy's transit circle. Astronomers at the Royal Observatory, Greenwich, London, England, using Airy's transit circle. A transit circle is used to observe the transit of stars across a fixed line. This one, famous for its use in setting Greenwich Mean Time (GMT), was proposed by the British astronomer George Biddell Airy (1801-1892). The first observations were taken on 4 January 1851. Following an agreement in Washington DC in 1884 to create an international time zone system, it was decided that the location of Airy's transit circle would define the zero degree meridian, the Prime Meridian. The observatory is now a museum, where the instrument is on display in its original location. Artwork published in the 1891 edition of The Midnight Sky by Edwin Dunkin.
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