JULIAN BAUM / SCIENCE PHOTO LIBRARY JULIAN BAUM / SCIENCE PHOTO LIBRARY
Refracting telescope. Artwork of the 23 centimetre (9-inch) Fraunhofer refractor telescope at Berlin Observatory, Germany, on 23 September 1846, the night that it was used to discover Neptune. German astronomer Johann Galle, aided by Louis D'Arrest, looked in a portion of the sky where the French astronomer Leverrier calculated a planet would be. This calculation was based on perturba- tions in the orbit of Uranus. Galle and D'Arrest compared the night sky to a new but unpublished star atlas, and within a few minutes found an unknown object just 1 degree from the predicted position. Observations the following night showed that it had moved, proving that it was a planet.
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