Description

Geographical position 

Ole Rømer’s Observatorium Tusculanum (1704)
Kroppedal Museum, Vridsløsemagle, Kroppedals Allé 3, 2630 Taastrup near Copenhagen, Denmark

See also:
Rundetårn (Round Tower), Copenhagen, Denmark (1637)
Østervold Observatory, Copenhagen  University, Denmark (1861)
New observatory: Brorfelde Observatoriet, Copenhagen University (1953).

Please do not confuse Ole Rømer’s Observatorium Tusculanum with this Ole Rømer Observatory:
Ole Rømer Observatory, Aarhus University, Denmark (1911)

Location 

Lat. 55° 41′ 06″ N, long. 12° 18′ 42″ E, elevation 32m above mean sea level.

 

IAU observatory code 

no IAU code

[not to be mistaken for IAU 155 Ole Rømer Observatory, Aarhus, Midtjylland, Denmark]

Description of (scientific/cultural/natural) heritage 

Ole Christensen Rømer (1644--1710) is best known is his proof (1676) based on the observation of the Jupiter moons that the speed of light is finite -- and not infinitely large.

But here we discuss his invention of the meridian circle, called the Rota Meridiana, in 1704 at this site Tusculanum, which is a highlight in instrument making.

 

Introduction of the Meridian Circle

 

 

But Rømer is also a talented instrument maker -- after his return to Copenhagen, in his official residence he installed in 1691 a transit instrument in a window facing south (Fig. 1).
This instrument worked very well, but because it only covered a small section (69°) of the meridian, the process of determining its orientation was laborious and involved other instruments. Rømer realized that a single instrument covering the full meridian would have a significant advantage, because stars could be observed to the north below the pole and the same stars again twelve hours later above the pole. In this way important constraints on the mounting errors of the axis could be established and easily monitored. This led him to design and build what would become the world’s first meridian circle.

He could not mount such an instrument neither in his residence nor at the Astronomical Tower in Copenhagen, but in the summer of 1704, he finally managed to build a small observatory near the summer residence of his father-in-law in Vridsløsemagle, a village 17 km west of Copenhagen. The place was by posterity called the Observatorium Tusculanum with an erudite reference to Cicero’s residence outside Rome.

The main instruments, both of his recent invention, were a meridian circle and a transit instrument in the prime vertical.

The left panel of the illustration (Fig. 2a) was produced by Rømer’s assistant, L. Skive, in 1704, and intended for a formal publication that never appeared. The meridian circle (to the left) with a telescope (AD), a divided circle (B) and circle reading microscopes (F and G).
To the right of the meridian circle, is the transit instrument in the prime vertical, designed for accurate determination of the declination of the Sun near the equinoxes.

The footprint of the building is outlined in the top right of Fig. 2a.
The building was a simple, half-timbered house with mud-built walls. It was small, some 5.5m square, but sufficient for the purpose. Rømer wrote to Gottfried Wilhelm Leibniz (1646--1716), 15th Dec 1700, who had asked for advice for building a new observatory:

"I differ a good deal from the opinion of those who until now have established observatories for show rather than for use and have suited the instruments to the building rather than the building to the instruments."

In 1704, Rømer put this philosophy into practice. The downside of his idea was of course that after the death of Rømer (1710) and his successor, L. Skive (1711), the place was abandoned, and the building soon deteriorated.

Fig. 2b shows the meridian circle in a less idealized sketch by one of Rømer’s students. Here a lowered floor level below the instrument is shown, allowing sufficient room for the observer.

 

The meridian mark

Towards south, at some distance, Rømer had a pole erected with an engraved cross. It served as a reference point, a meridian mark, for monitoring the stability of the instrument axis and as a help when carrying out adjustments.

 

The Triduum observations

The meridian circle at Tusculum was used diligently from 1705 to 1711, but nearly the whole body of observations was lost in the Fire of Copenhagen in 1728 together with the instrument itself. Surviving the fire was a set of observations, the Triduum observations, from three days and nights 20-23 October 1706. This was a carefully planned and very successful observing run encompassing all transits of the Sun, Moon and the five planets as well as 88 stars, a total of 253 observations. The meridian circle had been accurately adjusted before the run and a special effort was made in additional calibrations. As a bonus, an eclipse of the Moon was also observed using the same clock.   

Because of the untimely death of Rømer’s successor during the plague of 1711, the observations were not published until 1735 (P. Horrebow: Basis Astronomi├ª).  

Rømer’s meridian observations, collected during three days and nights in October 1706 (the "Triduum"), were by far the most accurate observations from such am early epoch.

Tobias Mayer (1723--1762) therefore used them together with his own observations from 1756 in his study of stellar proper motions, a study, which Friedrich Wilhelm Herschel (1738--1822) used when discovering the apex-motion of the Sun.

Meridian observations came to play a major role in the studies of stellar proper motions in the second half of the 18th century.
It would take time before meridian circles became common. Developments in the 18th century were slow and it took time to realize that the accuracy or traditional instruments like the mural quadrant was insufficient. The breakthrough therefore did not come until the early 19th century.

 

History 

Ole Christensen Rømer (1644--1710) studied astronomy in Copenhagen, 1662--1671. Jean Picard (1620--1682) invited him to Paris, where he worked from 1672 to 1681 at the new observatory of the Académie Royale des Sciences (micrometer 1572, mechanical models for planetary orbits, including a Jovilabium, 1677, a Lunarium 1680).

Ole Christensen Rømer (1644--1710) is known for his spectacular discovery in Paris, in 1676, that the speed of light is finite, which he measured with astonishing precision -- by observing the Jupiter moons: he saw that light arrives from Jupiter with a delay depending on the distance to Jupiter.
Already Galilei had proposed a method, based on the times of the eclipses of the four moons of Jupiter, using the Jovian system as a cosmic clock in order to solve the  longitude problem.

As a professor of mathematics in Copenhagen from 1681 he unified weights and measures.

His most important instrumental achievement was the development of the transit instrument (Fig. 5) and the meridian circle (1704), an innovative accurate measuring instrument for star positions. Rømer wanted to measure the parallax precisely in order to prove Copernicus’ (1473--1543) heliocentric world view.

Rømer was also a very careful observer, who discussed instrumental errors long before this was normal like in the 19th century. Especially, Rømer derived a measurement error formula for his meridian instrument in his Adversaria (1705). Today this measurement error formula is attributed to Tobias Mayer (1723--1762), who found it 51 years later, i.e. in 1756, without knowing Rømer’s derivation.

 

Further development of the Meridian Circle

Since Tycho and especially in the 17th and 18th century, large mural quadrants were used, made by the London instruments like William Cary (1759--1825), John Cary (1791--1816), John Bird (1709--1776) or Jesse Ramsden (1735--1800).

Rømer’s meridian circle (1704) offered the possibility to measure accurate star positions.
By observing the angle of a celestial body above the horizon, one gets its altitude; the altitude is largest when the celestial body crosses the meridian line - and this is connected to the time.

 

The next step in the development was the repeating circle, invented by Tobias Mayer (1723--1762) -- starting his work in 1750, inspired by the transit instrument (1690) and the alt-azimuth (1704) of Ole Rømer. Mayer published his invention as Nova methodus perficiendi instrumenta geometrica et novum instrumentum goniometricum in "Commentaria Societatis Goettingensis" II (Göttingen 1752); later published by Johann Friedrich Benzenberg (1777--1846) in 1812. Mayer’s repetition circle could be used for surveying, but especially for the longitude problem for navigation. Inspired by Tobias Mayer the instrument makers Borda (1775) and Lenoir (1805) developed repeating circles.

200 years after Rømer -- Johann Georg Repsold (1770--1830)  opened a workshop in Hamburg in 1799, and developed the first modern meridian circle (1802), cf. Repsold’s Stintfang Observatory on Bastion Albertus in Hamburg and Göttingen Observatory.

A little bit later, in 1806, Edward Troughton (1753--1835, F.R.S. 1810) created the large Groombridge transit circle, which Stephen Groombridge (1755--1832) used for his star catalogue..

And in the 19th century, following Johann Georg Repsold and a little bit later Reichenbach, the meridian circle and the transit instrument became a standard equipment of observatories.

State of preservation 

 

Rømer established his countryside Observatorium Tusculanum (in Taastrup), 17 km west of Copenhagen in 1704, and it was used after the death of Rømer (1710) and the death of L. Skive (1711) until 1716. Most instruments were lost in the Fire of Copenhagen (1728). Some smaller instruments were moved to the Rundetårn in Copenhagen, others can be seen in the Kroppedal Museum near Tusculanum (opened in 1979).
 

There is not much left from the observatory, but the pillars of the meridian circle, the three wooden posts carrying the two instruments found: These remnants of Rømer’s observatory were demonstrated in the archaeological excavation in 1978, and the outline of the building and its internal structure (the walls and the fireplace) are now marked with stone.

 

 

Comparison with related/similar sites 

The remaining three pillars of Observatorium Tusculanum can be compared with the meridian pillars of the Seeberg Observatory in Gotha.

Improved meridian circles became main instruments in observatories around the globe in the 19th century -- starting with Johann Georg Repsold’s meridian circle in Hamburg (1802) -- and also a large part of the 20th century.

 

Threats or potential threats 

No threats. The meridian pillars are protected in the Kroppedal Museum Taastrup.

Present use 

The Kroppedal Museum Taastrup was established nearby, originally called the Ole Rømer Museum, where artefacts from the excavation are exhibited together with collections of historical astronomical instruments and objects related to the life and work of Ole Rømer. The museum, later renamed Kroppedal Museum, is now also a local museum for the district.

The area surrounding the observatory was farmland until about 1970 when it was included in the newly established Vestskoven (The West Forest) for the benefit of the increasing population of the suburbs of Copenhagen.

No attempt has been made so far to demonstrate the location of the meridian mark. From descriptions by Horrebow and Ramus, and from the sightlines included in the Triduum observations, we believe it must have situated some 135-175m south of the meridian circle and there are good chances it could be found.

As can be seen in the map (Figure ...) the old observatory was located near the top of a hill with free views towards the four cardinal directions as required by the two main instruments. It is now in the outskirts of the new forest at a distance of some 600 m from the village of Vridsløsemagle and from the Kroppedal Museum.
https://cloud2.schultz.ch/index.php/apps/files/?dir=/&fileid=79119#



Following the archaeological investigations, the outline of the building, its internal walls, and the chimney are now marked with stone, three wooden instrument piers have been erected, and also the lowered floor level below the meridian circle recreated, cf. Figure 11b. A statue of Ole Rømer (Figure 11a) was removed from the Technological University in Copenhagen in 1988 and is now standing close to the site.

As the new forest keeps growing, the Tusculan site is now in a small clearing in the woods only open towards north. The clearings towards west and east are almost closed and the statue of Rømer will soon be hidden. Towards south, the view of the sky is obstructed by tall trees.

The Tusculan site was modest and short-lived, yet it played a significant role in the history of astronomy. It already has a visitors’ centre in the form of the Kroppedal Museum, but we believe it deserves clear sights to the cardinal directions. This would improve its potential for outreach and education. It is our expectation, that a recognition of Observatorium Tusculanum by the IAU as Outstanding Astronomical Heritage will encourage relevant authorities to protect and preserve this site and its surroundings.

 

Astronomical relevance today 

no astronomical relevance, but for history of astronomy an important step in the development of the meridian circle.

References

Bibliography (books and published articles) 

• Benzenberg, Johann Friedrich (ed.): Erstlinge von Tobias Mayer. Düsseldorf 1812.
   
• Herbst, Klaus-Dieter: Die Entwicklung des Meridiankreises 1700--1850. [The development of the meridian circle 1700--1850] Genesis eines astronomischen Hauptinstrumentes unter Berücksichtigung des Wechselverhältnisses zwischen Astronomie, Astro-Technik und Technik. Bassum, Stuttgart: GNT 1996.
   
• Horrebow, Peder: Basis astronomiae sive astronomiae pars mechanica [...]. Havniae [Copenhagen]: apud D viduam beati Hieron. Christiani Paulli 1735.
   
• Nielsen, Axel V.: Ole Rømer and his Meridian Circle. In: Vistas in Astronomy 10 (1968), p. 105-112.
   
• Wolfschmidt, Gudrun: Mondtopographie und Längengrad. In: Tobias-Mayer-Symposium anläßlich des 250. Todestages von Tobias Mayer. Hg. von Erhard Anthes und Armin Hüttermann. Leipzig: AVA - Akademische Verlagsanstalt (Acta Historica Astronomiae, Band 48) 2013, S. 161-210.

  
  
  

• Contributions to the text (including images) made by:
  
  
  
  • Claus Fabricius, Institut de Ciències del Cosmos, Universitat de Barcelona, Spain, member of Commission A1
    
  • Erik Høg, University of Copenhagen, Denmark, member of Commission A1, C3, F2
    
  • Bertil F. Dorch, University of Southern Denmark, Odense, Denmark, member of Commission C4
    
  
  

Links to external sites 

 

• Biography of Ole Rømer (1644--1710).
  In: The Abraham Zelmanov Journal 1 (2008), p. I-VI.
  
  
• Dystrup, Allan: Cloudy Nights, Classic Astronomy History.
  
  
• Ole Roemer -- A key figure in the history of optics.
  
  
• Kroppedal Museum Taastrup.
  
  
• Kroppedal Museum (linkedin.com)