HARVARD COLLEGE OBSERVATORY / SCIENCE PHOTO LIBRARY HARVARD COLLEGE OBSERVATORY / SCIENCE PHOTO LIBRARY
Halley's Comet, photographic plate. Halley's Comet orbits the Sun every 76 years. This periodicity was discovered by Edmund Halley (1654-1742), after whom the comet is named. Comets are balls of ice and dust from the solar system. Heat from the Sun evaporates the ice, forming a tail. This tail, which because of solar wind always points away from the Sun, can be many millions of kilometres long and very bright. Photographed from Arequipa, Peru on 21st April 1910, with Harvard College Observatory's 8-inch Bache Doublet, Voigtlander telescope. The exposure was 30 minutes.
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