– Niels Bohr Institute - University of Copenhagen

21 August 2009

Ole Rømer. Portrait at the Institute of Astronomy, University of Copenhagen. Courtesy of the Ole Rømer Museum.

Store Kannikestræde 16 

This street is part of the University quarter. There is a memorial plaque for Ole Rømer on the front of Store Kannikestræde 16:


Here stood until 1728 the professorial residence where Ole Rømer, professor of astronomy, chief of police for Copenhagen, lived until his death, 19 September 1710).

Danish astronomer Ole Rømer (1644-1710) studied at the University of Copenhagen. He assisted Erasmus Bartholin (1625-1698), who is renowned for his study in 1669 of the passage of a beam of light through a crystal of Iceland spar, with the publication of Tycho Brahe's ‘‘observation plates'' (which were bought by Danish King Frederik III from Johannes Kepler's son).

From 1671 Rømer continued this work with Jean Picard (1620-1682) in Paris where, among other things, he designed the fountains at Versailles for Louis XIV. In 1676 he wrote what would be his only paper of any importance published during his lifetime - on his discovery of the ‘‘hesitation'' of light, that is, that light has a finite velocity. He returned to Copenhagen in 1681, where he became the king's mathematician, professor of astronomy at the university, and thereby director of the observatory at the Round Tower.

Rømer reordered the system of weights and measures in Denmark on the basis of a single unit, thus combining weight and length. Among his numerous public duties, he also was involved in providing street lighting for Copenhagen. He was responsible for introducing the Gregorian Calendar in Denmark in 1700.

Rømer was a great instrument maker, developing the transit instrument in 1689. His studies on temperature corrections led him to invent a thermometer that was based on the freezing and boiling points of water. Daniel Fahrenheit (1686-1736) learned about this idea from Rømer in 1708.

Rømer made his astronomical observations from his home in Kannikestræde and at a new observatory built to the west of Copenhagen, now the site of the Ole Rømer Museum. Most of Rømer's papers were destroyed in the fire of 1728.


Rømer’s Machina domestica, or house instrument,
was set up in Rømer’s home in Store
Kannikestræde. Source: Horrebow, ‘‘Basis
Astronomiae,'' 1734 - 1735.

Rømer’s Machina domestica 

The telescope is set in the plane of the meridian. Note that in the house opposite, Rømer has cut an opening in the roof to extend the circle of observation.

A counterweight Y is used to prevent the axle from sinking due to the weight of the telescope.

<< Click to enlarge the image >>

At D a lamp was placed that illuminated the threads in the telescope.
The declination of the star is read on the curve F through the microscope E.

The other celestial coordinate, the right ascension, was given by the time when the star crossed the meridian. As Rømer could not simultaneously read the clocks and observe the passage of the star across the meridian, he noted the time on the clock immediately before the star appeared in the field of the telescope.

On the wall there was a pendulum that ticked every second. Thus Rømer could hear how many seconds passed before the star crossed the meridian.

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