Østervold Observatory  Østervold Observatory (or Copenhagen University Observatory; ) is a former astronomical observatory (IAU code 035) in Copenhagen, Denmark owned and operated by the University of Copenhagen (Københavns Universitet). It opened in 1861 as a replacement for the University's old observatory at Rundetårn. The first astronomical observatory operated by the University of Copenhagen was Rundetårn. It had been inaugurated in 1642 as a replacement for Tycho Brahe's Stjerneborg, but during the early 19th century had become outdated as astronomical instruments grew bigger and bigger while the tower could not be expanded. In the same time, light pollution from the surrounding city as well as vibrations caused by the still increasing traffic in the streets below had made the observations inaccurate. In 1861 the observatory was moved to Østervold, where a new observatory was constructed on the old bastioned fortifications of the city, which had become outdated and were being disabandonned. The well-known Russian-German astronomer, Heinrich Louis d'Arrest, who had been appointed professor in astronomy at the university in 1857, was chosen to plan the new observatory. He used the observatory for his study of nebulas, in 1867 publishing an astronomical catalogue of 1,942 nebulas with 4,900 positions. He also did significant work in spectroscopy, and discovered, for the first time, the differences in the evolutionary state and spatial distribution of stars. While d'Arrest used the main telescope, observer H. C. F. C. Schjellerup used the Meridian circle for creating one of the most comprehensive astronomical catalogues of the time of stars of the 8th-9th magnitude: 10,000 positions on 259 observation nights. The next director of the Østervold Observatory was Thorvald N. Thiele, a skilful mathematician who developed a new method of determining the orbit of Binary stars. He founded the tradition for numerical computational mathematics, which has lasted up to modern times. In 1907, Elis Strömgren was appointed professor and director of the observatory, and under his direction the computational work continued. During World War I, the Central Bureau for Astronomical Telegrams, founded by Astronomische Gesellschaft in 1882, was moved from Kiel, Germany, to the Østervold Observatory. It remained located in Copenhagen until 1965, when it was moved to the Smithsonian Astrophysical Observatory, Cambridge, United States.