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Niels Bohr Institute > Who, What, When > > Ole Rømer > Ole Rømer in Copenhagen

07 February 2012

Ole Rømer in Copenhagen

The astronomer Ole Rømer
In January 1681, after almost nine years in Paris, Ole Rømer finally returned to Denmark, where he was appointed professor of astronomy at the University of Copenhagen. The same year he married Anne Marie Bartholin, Rasmus Bartholin’s daughter.

Ole Rømer observes in his kitchen in Kannikestræde with the passage instrument.

Ole Rømer was an active observer both at the university observatory at the top of the Round Tower and in his own home in Kannikestræde. He used instruments he had invented himself or had improved on. Already in Paris in 1672 he had designed the first mechanically satisfactory micrometer for reading the scales on the instruments. He introduced the telescope with crosshairs and a micrometer on the focal plane and the use of the pendulum clock. In doing so Ole Rømer invented two types of instruments, the passage instrument and the meridian circle, which meant a great breakthrough in the accuracy of positional astronomy.

Unfortunately, virtually all of Ole Rømer’s observations and instruments were lost in the fire in Copenhagen in 1728, and since he only published very few articles, much of his accumulated knowledge has been lost.

The King’s mathematician
On his return to Denmark Ole Rømer was also appointed as the Royal Mathematician, which obliged him to many administrative and practical tasks. In this position he introduced the first national system of weights and measures. By virtue of his astronomical background, he sought to define measurement and weight units that were based astronomical constants. This was reflected in his definition of a Danish mile, which was set at 24,000 feet, equivalent to 4 geographic arc minutes.

In the year 1700 Ole Rømer got the King to introduce the Gregorian calendar in Denmark-Norway – something Tycho Brahe had tried to do in vain 100 years earlier.

Ole Rømer also developed one of the first temperature scales. Fahrenheit visited him in 1708 and drafted the Rømer scale, which resulted in the well-known Fahrenheit scale, which is still used in some places today.

Ole Rømer’s definition of the Danish mile was developed in connection with his work surveying the Danish roads. As part of the project the first Danish milestones were set up. A number of these still exist today and they are called Ole Rømer stones.

Ole Rømer as a civil servant
In 1705 the King appointed Ole Rømer as the mayor and police director in Copenhagen. In these positions he set up building regulations and established sewers. He designed the first streetlights with oil lamps and laid the foundations for later social reforms by focusing on they city’s beggars and prostitutes.

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