Rundetårn (former spelling: Rundetaarn, lit. [The] Round Tower) is a 17th century tower located in Copenhagen, Denmark. The tower is part of the Trinitatis complex, which was built to provide the scholars of the time with an astronomical observatory, a student church and a university library. One of his most well-known buildings, Rundetårn was built upon request from King Christian IV and designed by architect Hans Steenwinckel the Younger. The foundation stone was laid on July 7, 1637, and the tower was finished in 1642. The Trinitatis church was not finished until 1656, and the library not until 1657. The observatory (which is the oldest functioning observatory in Europe) and the library were used by the University of Copenhagen until 1861. Astronomers having used the observatory include Ole Rømer (1644-1710) and Peder Horrebow (1679-1764). Back when it was first built, it was for the students who lived in little dormitories to study in the observatory and the library. Instead of stairs, a unique 7.5-turn helical corridor leads to the top of the tower, 34.8 m over street level, where the observatory and a minor planetarium is located. The corridor also provides access to the church and the library. This design was necessary to allow a horse and carriage to access the library, moving books in and out. In Hans Christian Andersen's fairy tale "The Tinder Box", the largest of the three dogs is said to have eyes as large as the Round Tower at Copenhagen (Rundetårn). The asteroid 5505 Rundetårn commemorates the tower. In Denmark, heights of buildings are often compared to the height of Rundetårn. A phrase in Danish is "Which is highest, Rundetårn or a crash of thunder?" (loud and high are the same word in Danish). It is often used in a discussion when the opponent tries to compare incomparable quantities - see also Apples and oranges. The Czar Peter the Great ascended the corridor on horseback while visiting Copenhagen, and in 1902, a Beaufort car was the first motorised vehicle to ascend this tower. The spiral walk is unique in European architecture. The 209m long spiral ramp winds 7.5 turns around the hollow core of the tower forming the only connection between the individual parts of the building complex.
The tower is the oldest functioning observatory in Europe. Until 1861 it was used by the University of Copenhagen.
