Uranienborg (Swedish: Uraniborg) was a Danish astronomical observatory operated by Tycho Brahe; built circa 1576-1580 on Hven, an island in the Øresund between Zealand and Scania, which at that time was part of Denmark. Shortly after its construction the observatory was expanded with an underground facility, Stjerneborg, on an adjacent site.
Tycho abandoned Uranienborg and Stjerneborg in 1597, after he fell out of favor with the Danish king, and left the country. The observatory was destroyed in 1601, after Tycho's death. Hven was later lost to Sweden. The Round Tower in Copenhagen was inaugurated in 1642 as a replacement. The grounds are currently being restored.
The building was dedicated to Urania, the Muse of Astronomy and named Uranienborg, "The Castle of Urania." It was the first custom-built observatory, and the last to be built without a telescope as its primary instrument. The cornerstone was laid on August 8, 1576.
The main building of Uraniborg was square, about 15 meters on a side, and built mostly of red brick. Two semi-circular towers, one each on the north and south sides of the main building, giving the building a somewhat rectangular shape overall. The main floor consisted of four rooms, one of which was occupied by Tycho and his family, the other three for visiting astronomers. The northern tower housed the kitchens, and the southern a library. The second floor was divided into three rooms, two of equal size and one larger. The larger room was reserved for visiting royalty. On this level the towers housed the primary astronomical instruments, accessed from outside the building or from doors on this floor. Outrider towers, supported on pillars, housed additional instruments slightly further from the building, giving them a wider angle of view. On the third floor was a "loft", subdivided into eight smaller rooms for students. Only the roofs of the towers reached this level, although a single additional tower extended above the loft in the middle of the building, similar to a widow's walk, accessed via a spiral staircase from the 3rd floor. Uraniborg also featured a large basement; it housed an alchemical laboratory in one end, and storage for food, salt and fuel at the other.
The observatory had a large mural quadrant affixed to a north-south wall, used to measure the altitude of stars as they passed the meridian. This, along with many other instruments of the observatory, was depicted and described in detail in Brahe's 1598 book Astronomiae instauratae mechanica.
A large wall, 75 meters on a side and 5.5 meters high was planned to surround Uraniborg, but never built, instead a high earth mound was constructed and lasted until today being the only remnant of the observatory still in place. Uraniborg was located in the very middle, with an extensive set of intricate gardens between the mound walls and the building. In addition to being decorative, the gardens also supplied herbs for the Tycho's medicinal chemistry experiments. The gardens are currently being re-created, using seeds found on-site or identified in Tycho's writings.
Uraniborg was an extremely expensive project. It is estimated that it cost about 1% of the entire state budget during construction.
Shortly after construction it became clear that the tower-mounted instruments were too easily moved by wind, and Tycho set about constructing a more suitable observation site. The result was Stjerneborg ("castle of the stars"), a smaller site built entirely at ground level and dedicated purely to observations (there was no "house"). The basic layout was similar to Uraniborg, with a wall of similar shape surrounding the site, although the enclosed area was much smaller. The instruments were all placed underground, covered by opening shutters or a rotating dome in buildings built over the instrument pits.
Upon losing financial support from the new king, Christian IV of Denmark, Tycho abandoned Hven in 1597 and both Uraniborg and Stjerneborg were destroyed shortly after Tycho's death. Stjerneborg was the subject of archaeological excavations during the 1950s, resulting in the restoration of the observatory. Stjerneborg now houses a multimedia show.
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- Astronomiae instauratae mechanica at the Royal Library, describing Brahe's instruments, with English translation