Ole Rømer

There are few sources on Århus native Ole Christensen Rømer until his entry in 1662 at the University of Copenhagen, at which his mentor had been Rasmus Bartholin, who had published his discovery of the double refraction of a light ray by Iceland spar (calcite) in 1668 while Rømer was living in his home.

Rømer had been afforded every opportunity to learn mathematics and astronomy using Tycho Brahe's astronomical observations, as Bartholin had been given the task of preparing them for publication.

The determination of longitude is a significant practical problem in cartography and navigation.

Rømer had joined the observatory of Uraniborg on the island of Hven, near Copenhagen, in 1671.

Over a period of several months, Jean Picard and Rømer had observed about one hundred and forty eclipses of Jupiter's moon Io, while in Paris Giovanni Domenico Cassini had observed the same eclipses.

By comparing the times of the eclipses, the difference in longitude of Paris to Uraniborg could be calculated.

Cassini had observed the moons of Jupiter between 1666 and 1668, and discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed.

Rømer had gone to Paris in 1672 and continued observing the satellites of Jupiter as Cassini's assistant.

Adding his own observations to Cassini's, Rømer had observed that times between eclipses (particularly those of Io) got shorter as Earth approached Jupiter, and longer as Earth moved farther away.

Cassini on August 22, 1676, had made an announcement to the Academy of Sciences: “This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.” Although Cassini seems to have abandoned this reasoning, Rømer had adopted and set about buttressing it in an irrefutable manner, using a selected number of observations performed since 1671 by Picard and himself.

Rømer had presented his results to the French Academy of Sciences, and it is summarized soon after by an anonymous reporter in a short paper, Démonstration touchant le mouvement de la lumiére trouvé par M. Roemer de l'Académie des sciences, published December 7, 1676, in the Journal des sçavans.

The reporter, failing to understand Rømer’s presentation, obfuscated his's reasoning in the process, and Rømer himself is never to publish is results, but he has made the first quantitative measurements of the speed of light.

Ole Christian Roemer, a Danish astronomer having studyied Jupiter’s moons from 1671 to 1677, observes that light moves at a finite speed.

Christian V of Denmark had introduced the land register of 1688, an attempt to work out the land value of the united monarchy in order to create a more just taxation.

Science has witnessed a golden age in Denmark during his reign due to the work of the astronomer Ole Rømer, in spite of the king’s personal lack of scientific knowledge and interest.

Like his late contemporary Charles XI of Sweden, who had never been outside Sweden, Christian V speaks German and Danish only and is therefore often considered poorly educated due to his inability to communicate with visiting foreign diplomats.

Contemporary sources also often consider Christian V dependent on his councilors , a notion the Danish monarch has done nothing to dispel.

In his memoirs, he listes "hunting, love-making, war and maritime affairs" as his main interests in life. (Nielsen, Kay Søren (1999). Christian V – Konge og sportsmand. The Royal Danish Arsenal Museum, Net Publications, 1999.)

He dies on August 25 of complications caused by a hunting accident and is interred in Roskilde Cathedral.

His oldest son succeeds him as Frederick IV.