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George Biddell Airy (1801-1892), English astronomer and geophysicist, 1858. Airy was Astronomer Royal and director of the Royal Greenwich Observatory from 1835 to 1881. He initiated the efficient and accurate observation of stellar positions, published works on celestial mechanics and made discoveries in theoretical and practical optics. He reorganised the Greenwich Observatory and discovered errors in planetary theory in relation to the motion of the Earth and Venus. In geophysics Airy is best known for his calculation of the mean density of the Earth. He worked it out by using pendulums to ascertain the difference in the strength of gravity between locations at the bottom of a mine shaft of known depth and on the surface. (Photo by Ann Ronan Pictures/Print Collector/Getty Images)