Ole Roemer

A key figure in the history of optics.

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Thomas Bartholin
Thomas Bartholin (1616-1680)

Ole Christensen Rømer (or Roemer) is a key figure in the history of optics and astronomy. He lived between 1644-1710, making him a contemporary of other great names such as Liebniz, Huygens and Newton, all three of whom collaborated with his studies at one time or other.

Early influences and experiences

Rømer studied first in Copenhagen under Thomas Bartholin (his future father-in-law), the Professor of Mathematics and Anatomy and of interest to historians of optics for his three-volume work on De Luce Animalium (1647), hence the presence in our museum collection of this engraving by Jonas Suydehoef of the portrait of Bartholin by Karel van Mander, court painter to the Danish king.

Tycho Brahe
Tycho Brahe

Rømer began working on the notes of the Danish astronomer Tycho Brahe with a view to publication, however from 1672 until 1681 he was away from Denmark, having followed his friend Jean Picard to Paris and joined the Scientific Academy there.

Statue of Roemer in Denmark

Statue of Rømer

To the right is a photograph of the statue of a youthful looking Rømer at Vridsløsemagle near Taastrup. The statue was placed here relatively recently, adjacent to the site of Rømer's observatory, the location of which was only rediscovered in 1978.

1676 - Whilst in France, Rømer discovered and measured the velocity of light

Back in Denmark

Rømer succeeded to the post in charge of the Round Tower (Rundtarnen) observatory in central Copenhagen which had stood since 1642, making it one of oldest state observatories in Europe. He rebuilt the observatory accommodation at the top of the tower and ordered new observational instruments.

He sought to prove the Copernican theory (that the earth moves round the sun) in conscious opposition to his predecessor Tycho's view that the sun in fact revolved round the Earth (even if all the other planets still orbited the sun!). He never did prove this theory but did develop two important instruments in the process of trying.

In 1691 he built his Transit Instrument in his professor's university lodgings. This was a telescope mounted on a horizontal axis with a microscope mounted on its axis. In conjunction with a pendulum clock and a very large wall-mounted scale he could measure the height of any star as it passed over the meridian.

Reconstruction of Roemer's observatory
Reconstruction of
the 'Tusculum'

In 1704 close to the Pilenborg villa on university land near Vridsløsemagle he built his Observatorium Tusculanum ('Tusculum' being a nickname for this summer residence named after Cicero's villa). This was a clay-built half-timbered structure not unlike a house to look at house with a tiled roof and chimney. Inside the floor had been excavated to one foot below ground level to help secure the observational instruments against wind and vibration. Within this pit the instruments were mounted on 3 solid posts. The instruments comprised a Meridian circle and an Equinox Instrument, plus 4 pendulum clocks.

The Meridian Circle differed from its predecessor, the Transit Instrument, in that the microscope was stationary and a circular scale followed the movements of the telescope. N.B. circles (unlike sectors) give correct relative measurements whether or not the scale expands or contracts with temperature change!

Using this groundbreaking instrument Rømer tried, largely without success, to measure the parallax of the stars i.e. the movements that they appear to make in a year (though, in fact, it is the earth that is moving around the sun). He hoped to use these measurements as evidence of the earth's solar orbit but they were just too small to gather accurately.

Memorial plaque on the Rømer statue
Memorial plaque on
the Rømer statue


Rømer's pupils attempted to carry on his work after his death in 1710, but without any more success. By 1716-17 the instruments were reportedly already in a state of ruin and some of the iron components had apparently been stolen. The remnants were removed to the Roundtower and the Observatorium Tusculanum probably demolished.

The Transit Instrument and most of Rømer's experimental notes were destroyed in the Copenhagen fire of 1728

It seems that as early as 1765 the site of the observatory could no longer be identified. In the twentieth century aerial photography was used in an attempt to pinpoint the likely site, which was littered with ancient burial mounds.

The site of the observatory, with re-excavated pit and reconstructed support posts in May 2004

The site of the observatory in 2004
showing the re-excavated pit and
reconstructed support posts

Only in 1978 did old nineteenth century map analysis identify the site by reference to recorded sight-lines (mentioned by his eighteenth century pupils). Following archaeological examination an unfinished lens was found amongst the soil. The base of the instrument support posts was located and these have since been reconstructed. The Kroppedal Museum was founded a short walk away in 1979 and is now known as the Ole Rømer Museum, featuring displays on the archaeological investigation and a series of historic telescopes.

Roemer museum Interior of Roemer Museum 2004

Further study:

Visit the Ole Rømer Museum, Kroppedaals Alle 3, 2630 Taastrup