Danish scientist, universally credited as the discoverer of the speed of light. Born 1644, died 1710.

As a young man of 18 summers, Ole Rømer arrived at Copenhagen from provincial Århus, matriculating at the University of Copenhagen, where he studied astronomy under the tutelage of his later father-in-law, Rasmus Bartholin. The young Rømer worked on a "reduction" (proofreading) of Tycho Brahe's unpublished observations for Bartholin.

Jean Picard, a French astronomer, visited Copenhagen in 1671, his purpose being the determination of the exact geographical location of Uraniborg, the better to make sense of Tycho's observations. Rømer accompanied Picard to the island of Ven to measure the difference in longitude between the Observatory at Paris, Rundetårn Observatory in Copenhagen, and Uraniborg.

Rømer's work was so precise and meticulous that Picard became convinced of Rømer's special talents. He petitioned the king of Denmark to allow Rømer to accompany him to Paris. Permission was granted, and in 1672, Rømer and Picard went to Paris, with Tycho's observations. Rømer became a member of the Académie des sciences and was given lodgings at the newly-built observatory.

It was in Paris, in 1675/1676, that he made the discovery for which he is so justly famous. After precise studies of the period of the innermost moon of Jupiter, he realised that light has a finite velocity - or, as Rømer put it, "light hesitates". He calculated the time it takes for light to travel from the Sun to the Earth at between 8 and 11 minutes (the actual time being 8 minutes and 20 seconds).

In France, Rømer also displayed a gift for practical, applied science. He worked on calculations in connection with water pipes to royal palaces and constructed pump stations. As mathematics tutor to the Dauphin, Rømer invented a series of orreries, to illustrate the motions of celestial bodies. Copies were made and sent abroad as gifts from the King of France to such luminaries as the Shah of Persia, the King of Siam and the Emperor of China. King Christian V of Denmark ordered two orreries, which are now in the museum at Rosenborg Castle.

In 1681, Rømer was recalled to Denmark. He became professor of astronomy at the University of Copenhagen, and director of Rundetårn Observatory. In his domicile in Store Kannikestræde in Copenhagen, he set up a private observatory, in 1691. Another private observatory, near Vridsløsemagle, was set up in 1704.

During these years, Rømer worked intesively on a stellar catalogue, paying particular attention to the parallax motion of stars - his purpose being to provide a decisive proof the the Earth revolves around the Sun. To correctly measure the passage of time for his observations, Rømer needed to correct his clocks for temperature variance, so he needed precise thermometers. As a result of his research into this subject, he realised that both the melting point and boiling point of water are fixed temperatures, which can be used to calibrate a thermometer - this later inspired G.D. Fahrenheit to do so.

Rømer was an obedient servant of the absolutist monarchy of Denmark, and his contributions to the bureaucracy were extensive. He served on numerous royal commissions; oversaw the conversion of Denmark to the Gregorian calendar (in 1700); introduced standard measures all over the country (in the 1680s); was police chief, fire chief and mayor of Copenhagen; and served on the Danish supreme court. He was rector of the University of Copenhagen twice, and the university's chief librarian.

Despite his many achievements, his list of publications is remarkably slender. Many of his observations were never published, and most were lost in the dreadful Fire of Copenhagen in 1728. However, his working papers (Adversaria, published 1910) and a three-day series of observations made in 1706 (Triduum) have been preserved.

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