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ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Sir David Gill (1843-1914), British astronomer. Gill was born in Aberdeen, Scotland, but spent much of his career in South Africa. He used the parallax of Mars to determine the distance from Earth to the Sun. A parallax is a change in the angle between two stationary objects that occurs when the observer changes his viewing position. Gill also measured distances to the stars using this technique and perfected the use of the heliometer, an instrument used to measure the diameter of the Sun. He was a pioneer in the use of astrophotography, and one of the early advocators of the Carte du Ciel ('Map of the Sky', or Astrographic Chart) project that emerged in 1887.
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