The Rundetårn is a 17th-century tower
A tower is a tall structure, usually taller than it is wide, often by a significant margin. Towers are distinguished from masts by their lack of guy-wires....

 located in central Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...

, Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

. One of the many architectural projects of Christian IV
Christian IV of Denmark
Christian IV was the king of Denmark-Norway from 1588 until his death. With a reign of more than 59 years, he is the longest-reigning monarch of Denmark, and he is frequently remembered as one of the most popular, ambitious and proactive Danish kings, having initiated many reforms and projects...

, it was built as an astronomical observatory. It is most noted for its 7.5-turn helical
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

 corridor leading to the top, and for the expansive views it affords over Copenhagen.

The tower is part of the Trinitatis Complex which also provided the scholars of the time with a university chapel, the Trinitatis Church, and an academic library
Academic library
An academic library is a library that is attached to academic institutions above the secondary level, serving the teaching and research needs of students and staff...

 which was the first purpose-built facilities of the Copenhagen University Library
Copenhagen University Library
Copenhagen University Library in Copenhagen, Denmark, is the main research library of the University of Copenhagen. Founded in 1582, it is the oldest library in Denmark....

 which had been founded in 1482.

Today the Round Tower serves as an observation tower for expansive views of Copenhagen, a public astronomical observatory and a historical monument. In the same time the Library Hall, located above the church and only accessible along the tower's ramp, is an active cultural venue with both exhibitions and a busy concert schedule.



Astronomy had grown in importance in 17th-century Europe. Countries had begun competing with each other in establishing colonies, creating a need for accurate navigation across the oceans. Many national observatories were therefore established, the first in 1632 at Leiden in the Dutch Republic
Dutch Republic
The Dutch Republic — officially known as the Republic of the Seven United Netherlands , the Republic of the United Netherlands, or the Republic of the Seven United Provinces — was a republic in Europe existing from 1581 to 1795, preceding the Batavian Republic and ultimately...

. Only five years later the Round Tower Observatory, first referred to as STELLÆBURGI REGII HAUNIENSIS, would follow.

Planning and preparations

After Tycho Brahe
Tycho Brahe
Tycho Brahe , born Tyge Ottesen Brahe, was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...

 had fallen out of favour and left Denmark, Christian Longomontanus had become Christian IV's new astronomer and the first professor of astronomy at the University of Copenhagen
University of Copenhagen
The University of Copenhagen is the oldest and largest university and research institution in Denmark. Founded in 1479, it has more than 37,000 students, the majority of whom are female , and more than 7,000 employees. The university has several campuses located in and around Copenhagen, with the...

. In 1625 he suggested the king to build an astronomical tower as a replacement for Brahe's Stjerneborg
Stjerneborg was Tycho Brahe's underground observatory next to his palace-observatory Uraniborg, located on the island of Hven in Oresund....

 which had been demolished after his death in 1601.

Longomontanus' initial proposal was to erect the new observatory on the top of the hill Solbjerget, now known as Valby Bakke. But since there were also plans for the construction of a new students' church and a library for the university, the idea of merging the three buildings into one grand complex emerged.

Already in 1622, Christian IV had bought the land where it was ultimately decided to build the Trinitatis Complex. His original plans for the site are not known but as it was conveniently located next to the Regensen dormatories
Regensen is a dormitory for students at the University of Copenhagen and Technical University of Denmark. It is situated in the heart of the old city and is a neighbour to Rundetårn. It was inaugurated 1 July 1623, but it burned down along with the rest city in the Great Fire of Copenhagen in 1728...

 and the university, it was chosen for his new prestigious project.

Although there is no clear proof, it is generally accepted that Hans van Steenwinckel the Younger
Hans van Steenwinckel the Younger
Hans van Steenwinckel the Younger was a Flemish-Danish architect who specialised in the Dutch Renaissance style, typical of prestigious Danish buildings from the first half of the 17th century...

 was charged with the commission to design the new edifice though he did not live to see the tower completed.
From 24 November 1636, stones were brought to the site for the foundation, first from the city's ramparts and later from the area around Roskilde
Roskilde is the main city in Roskilde Municipality, Denmark on the island of Zealand. It is an ancient city, dating from the Viking Age and is a member of the Most Ancient European Towns Network....

. Bricks were ordered from the Netherlands since local manufacturers could not meet the high quality standards requested. In February 1637, a contract was signed with a Henrik van Dingklage in Emden
Emden is a city and seaport in the northwest of Germany, on the river Ems. It is the main city of the region of East Frisia; in 2006, the city had a total population of 51,692.-History:...

 for the supply of bricks for the construction. The first three ship loads were to be delivered in May, the next three loads the following month and the remainder on demand.

The Trinitatis Complex was set for construction in a crowded neighbourhood of narrow streets and alleyways. The area first had to be cleared. On 18 April 1637, 200 men, soldiers and personnel from Bremerholm began to demolish the half-timbered houses
Timber framing
Timber framing , or half-timbering, also called in North America "post-and-beam" construction, is the method of creating structures using heavy squared off and carefully fitted and joined timbers with joints secured by large wooden pegs . It is commonplace in large barns...

 occupying the site.

Construction phase

The foundation stone was laid on 7 July 1637. When Hans van Steenwinckel died on 6 August 1639, Leonhard Blasius was brought to Denmark from the Netherlands as new Royal Building Master. Unlike his predecessor, he would become a mere transitional figure in Danish architecture, dying just four years after his arrival in the country without leaving any notable buildings of his own design. On several occasions construction work came to a standstill due to shortage of funds. Churches in Denmark and Norway were therefore ordered to contribute a share of their earnings during the construction years. In 1642, the tower was finally completed, though the church was completed only in 1657 and the library in 1657.

Time as an observatory

Christian Longomontanus became the first director of the observatory. In the Great Fire of 1728
Copenhagen Fire of 1728
The Copenhagen Fire of 1728 was the largest fire in the history of Copenhagen, Denmark. It began on the evening of October 20, 1728, and continued to burn until the morning of October 23. It destroyed approximately 28% of the city , left 20% of the population homeless, and the reconstruction lasted...

 the Trinitatis Complex was severely damaged but was rebuilt.

Demise and later years

During the early 19th century, the Round Tower became outdated as an astronomical observatory. Instruments were growing still larger while the tower could not be expanded and, at the same time, light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...

 from the surrounding city and vibrations caused by the ever increasing traffic in the streets below had made the observations inaccurate. The University therefore decided to build Østervold Observatory
Østervold Observatory
Østervold Observatory is a former astronomical observatory in Copenhagen, Denmark owned and operated by the University of Copenhagen . It opened in 1861 as a replacement for the University's old observatory at Rundetårn....

 on the old bastioned fortifications of the city, which had become outdated and were being decommissioned. The new observatory was inaugurated in 1861 to the design of Christian Hansen
Christian Hansen (architect)
Hans Christian Hansen was a Historicist Danish architect who worked 18 years in Greece where he was active in the transformation of Athens from a small town to the country's capital and an international metropolis...


Notable ascents

  • In 1726, The Czar Peter the Great ascended the corridor on horseback while visiting Copenhagen.
  • In 1902, a Beaufort
    Beaufort (automobiles)
    Beaufort was a German manufacturer of automobiles solely for the British market. It existed from 1902 - 1919 and was created with English capital....

     car was the first motorised vehicle to ascend this Round Tower.
  • A medal in the Round Tower's collection of medals indicates that the first bicycle race held in the tower took place as early as 1888, possibly in connection with The Nordic exhibition of Industry, Agriculture, and Art.
  • In 1911, the newspaper Socialdemokraten arranged a bicycle race down the Round Tower.
  • In 1971, Ole Ritter
    Ole Ritter
    Ole Ritter is a former Danish racing cyclist.-Amateur years:As an amateur he rode for ABC Denmark....

     won a bicycle race against Leif Mortensen
    Leif Mortensen
    Leif Mortensen is a Danish former professional road bicycle racer.- Palmarès :196719681969...

     up the Round Tower in a time of 55.3 seconds.
  • In 1993, Henrik Djernis
    Henrik Djernis
    Henrik Djernis is a Danish cyclist....

     won a bicycle race against Jens Veggerby in a time of 50.05 seconds.
  • In 1989, Thomas Olsen went up and down the Round Tower on a unicycle
    A unicycle is a human-powered, single-track vehicle with one wheel. Unicycles resemble bicycles, but are less complex.-History:One theory of the advent of the unicycle stems from the popularity of the penny-farthing during the late 19th century...

     in 1 minute and 48.7 seconds, which is a world record.


The Round Tower is a cylindrical
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...

 tower built in masonry of alternating yellow and red bricks, the colours of the Oldenburg
House of Oldenburg
The House of Oldenburg is a North German dynasty and one of Europe's most influential Royal Houses with branches that rule or have ruled in Denmark, Russia, Greece, Norway, Schleswig, Holstein, Oldenburg and Sweden...

s. The bricks used were manufactured in the Netherlands and are of a hard-burned, slender type known as muffer or mopper. On the rear side, it is attached to the Trinitatis Church, but it has never served as a church tower
Steeple (architecture)
A steeple, in architecture, is a tall tower on a building, often topped by a spire. Steeples are very common on Christian churches and cathedrals and the use of the term generally connotes a religious structure...


Steenwinckel—whose name is otherwise synonymous with Dutch Renaissance architecture in Denmark—with the Trinitatis Complex has left his signature style. Unlike his other buildings with their lavish opnamentations and extravagant spires, the complex is built to a focused and restrained design. Hans van Steenwinckel must have been up on the situation in Holland, cogniziant that the style which he had once learned from Hendrick de Keyser
Hendrick de Keyser
Hendrick de Keyser was a Dutch sculptor and architect born in Utrecht, Netherlands, who was instrumental in establishing a late Renaissance form of Mannerism in Amsterdam...

 had been altogether abandoned.

The architects now setting the agenda in the Netherlands, masters such as Jacob van Kampen (Amsterdam City Hall), Pieter Post
Pieter Post
Pieter Jansz Post was a Dutch Golden Age architect, painter and printmaker.-Biography:...

The Royal Picture Gallery Mauritshuis is an art museum in The Hague, the Netherlands. Previously the residence of count John Maurice of Nassau, it now has a large art collection, including paintings by Dutch painters such as Johannes Vermeer, Rembrandt van Rijn, Jan Steen, Paulus Potter and Frans...

 in the Hague) and Philip Vingboons, now favoured a style characterized by sobriety and restraint. It is now known as Dutch Baroque or sometimes Ditch Classicism. Its proponents often relied on the theoretical works such as those of Palladio and of Jacopo Barozzi da Vignola. Steenwinckel may have paid a visit to his native Netherlands prior to his change in style but it will have been too early for him to have seen any of the period's buildings realized.

Spiral ramp

Instead of stairs, a 7.5-turn spiral ramp forms the only access way to the towertop observatory as well as the Library Hall and the Bell-Ringer
A bell-ringer is a person who rings, tolls or peals a large bell, usually a church bell, by tugging on a long rope. Since the invention of the carillon, the need for bell ringers has declined...

's Loft, both located above the church. The ramp turns 7.5 times around the hollow masonry core of the tower before reaching the observation deck and observatory at the top, on the way also affording access to the Library Hall as well as the Ringer's Loft. This design was chosen to allow a horse and carriage to reach the library, moving books in and out of the library as well as transporting heavy and sensitive instruments to the observatory.

The winding corridor has a length of 210 m, climbing 3.74 m per turn. Along the outer wall the corridor has a length of 257.5 m and a grade of 10%, while along the wall of the inner core the corridor is only 85.5 m long but has a grade of 33%.

Observation deck

The observation deck is located 34.8 m above street level. Along the edge of the platform runs a wrought-iron lattice
Latticework is a framework consisting of a criss-crossed pattern of strips of building material, typically wood or metal. The design is created by crossing the strips to form a network...

 made in 1643 by Kaspar Fincke, Court Artist in metalwork. In the latticework, Christian IV's monogram
A monogram is a motif made by overlapping or combining two or more letters or other graphemes to form one symbol. Monograms are often made by combining the initials of an individual or a company, used as recognizable symbols or logos. A series of uncombined initials is properly referred to as a...

 and the letters RFP are seen, the letters representing the King's motto: Regna Firmat Pietas - Piety strengthens the Realms.


The observatory is a small dome
A dome is a structural element of architecture that resembles the hollow upper half of a sphere. Dome structures made of various materials have a long architectural lineage extending into prehistory....

d building, built on the roof of the tower. Built in 1929, the current observatory is 7 m high and has a diameter of 6 m. Access is by a narrow winding stone staircase from the observation deck.

Rebus inscription

On the upper part of the façade of the tower, there is a gilded rebus
A rebus is an allusional device that uses pictures to represent words or parts of words. It was a favourite form of heraldic expression used in the Middle Ages to denote surnames, for example in its basic form 3 salmon fish to denote the name "Salmon"...

 inscription. Christian IV's draft of it, written in his own hand writing, is kept at the Danish National Archives
Danish National Archives
Danish National Archives is the National archive of Denmark. The primary purpose is to collect, preserve and archive historically valuable records from central authorities, such as ministries, agencies and national organisations and make them available to the public...

. The rebus includes the four Hebrew consonants of the Tetragrammaton
The term Tetragrammaton refers to the name of the God of Israel YHWH used in the Hebrew Bible.-Hebrew Bible:...

. The rebus can be interpreted in the following way: Lead God, the right teaching and justice into the heart of the crowned King Christian IV, 1642.

The Round Tower today

Today the Round Tower serves as an observation tower, a public astronomical observatory, an exhibition and concert venue and a historical monument.

Public observatory

In 1860 the University of Copenhagen decommissoned the Round Tower as a university observatory but in 1928 it was reconstructed as an observatory with access for amateur astronomers and the general public. It is open from mid-October to mid-March.

Exhibitions & concerts

Since 1987, the Library Hall has served as an exhibition space, featuring various exhibitions of art, culture, history and science. At the same time, it is used as a concert venue, every year hosting around a hundred concerts.


The observation deck affords extensive views over the rooftops of the old part of Copenhagen with its many spires. On clear days, both the Øresund Bridge and Sweden can be seen in the distance.

Rundetårn Unicycle Race

Every year in spring, a unicycle race is held in the Round Tower. The contestants have to go up and down the tower. The world record, set in 1988, is 1 minute and 48.7 seconds.

Cultural references

  • In Hans Christian Andersen
    Hans Christian Andersen
    Hans Christian Andersen was a Danish author, fairy tale writer, and poet noted for his children's stories. These include "The Steadfast Tin Soldier," "The Snow Queen," "The Little Mermaid," "Thumbelina," "The Little Match Girl," and "The Ugly Duckling."...

    's fairy tale The Tinder Box
    The Tinder Box
    "The Tinderbox" is a literary fairy tale by Hans Christian Andersen about a soldier who acquires a magic tinderbox capable of summoning three powerful dogs to do his bidding. When the soldier has one of the dogs transport a sleeping princess to his room, he is sentenced to death but cunningly...

    , the largest of the three dogs is said to have eyes as large as the Round Tower at Copenhagen.
  • In another Hans Christian Andersen fairy tale, The Elder-Tree Mother, an old married couple remembers how they used to go "up the Round Tower, and looked down on Copenhagen, and far, far away over the water; then we went to Friedericksberg
    Frederiksberg Park
    Frederiksberg Park is one of the largest and most attractive greenspaces in Copenhagen, Denmark. Together with the adjacent Søndermarken it forms a green area of 64 hectares at the western edge of Inner Copenhagen...

    , where the King and the Queen were sailing about in their splendid barges!".
  • A 1:3 scale replica of the tower is build in the Danish city Solvang, CA
    California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...


  • The asteroid
    Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

     5505 Rundetårn
    5505 Rundetårn
    5505 Rundetårn is a main belt asteroid with an orbital period of 1777.6666407 days . The asteroid was discovered on November 6, 1986....

     commemorates the tower.
  • In Denmark
    Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

    , heights of buildings are often compared to the height of the Rundetårn.
  • A phrase in Danish
    Danish language
    Danish is a North Germanic language spoken by around six million people, principally in the country of Denmark. It is also spoken by 50,000 Germans of Danish ethnicity in the northern parts of Schleswig-Holstein, Germany, where it holds the status of minority language...

     is "Which is highest, the 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
    Apples and oranges
    A comparison of apples and oranges occurs when two items or groups of items are compared that cannot be validly compared.The idiom, comparing apples and oranges, refers to the apparent differences between items which are popularly thought to be incomparable or incommensurable, such as apples and...


External links

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