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Römer, Ole Christensen (or Roemer, Olaus)


(b. Aarhus, Denmark, 25 September 1644; d. Copenhagen, Denmark, 19 September 1710)


Römer came from a family of small merchants. In 1662 he was sent to the University of Copenhagen, where he studied with both Thomas Bartholin, professor of medicine, and his brother Erasmus Bartholin, a physician who was better known for his discovery of double refraction of light in Iceland spar. He lived in Erasmus Bartholin’s house and studied astronomy and mathematics under his direction; Bartholin was so impressed with Römer’s abilities that he entrusted him with the editing of the unpublished manuscripts of Tycho Brahe.

In 1671 Erasmus Bartholin was visited by Jean Picard, who had been sent by the Académie des Sciences to measure precisely the position of Tycho Brahe’s observatory, the Uraniborg, on the island of Hven. In September of that year Bartholin and Römer accompanied Picard to Hven, where, in order to redetermine the longitude of the observatory, they made observations of a series of eclipses of the first satellite of Jupiter, while G. D. Cassini carried out the same work in Paris. When Picard returned to Paris he took with him a notebook containing eight months’ observations, the original manuscripts of Tycho Brahe’s observational works, and Römer, whom he had persuaded to work there under the auspices of the Academy.

Upon his arrival in Paris, Römer was assigned lodgings in the new Royal Observatory building designed by Charles Perrault; he lived there for nine years. He was appointed by Louis XIV to be tutor to the dauphin and also made a number of astronomical observations, both in Paris and in other parts of France to which he was sent on behalf of the Academy. He constructed clocks and other devices, in which he displayed great mechanical skill and ingenuity, and invented a micrometer for differential measurement of position that was so superior to previous instruments that it was speedily adopted into general use.

Römer’s greatest work, however, grew out of the problem that he had initially considered with Picard, the times of the occultations of the satellites of Jupiter. These measurements were of considerable practical use, since it was recognized soon after the discovery of the Jovian satellites that their frequent occultations— particularly those of the first satellite, Io—by the planet itself represent well-defined moments of celestial time, which may be compared with time at the place of observation to establish geographical longitude. This knowledge was of particular use to mariners, and astronomers began to concern themselves with drawing up ephemerides predicting the times of eclipses at a fixed meridian, for example at Paris or Greenwich. Galileo had attempted to construct such an ephemeris, without notable success, and the task was assigned to the astronomers of the new Paris observatory by Colbert. G. D. Cassini and his nephew Maraldi discovered the first large inequality in the periodic times of the minima, that caused by the eccentricity of the orbit of Jupiter around the sun; their second discovery, announced by Cassini in August 1675, was more interesting, since the inequality seemed to depend on the position of the earth relative to Jupiter.

Cassini considered, but discarded, the idea that the fluctuation of periodic times might be caused by the finite speed of light; it remained to Römer to demonstrate that such was indeed the case. With rare exceptions, previous astronomers, both ancient and more recent—including Aristotle, Kepler, and Descartes— had held that light propagated itself instantaneously. Galileo, on the other hand, was not only convinced of its finite velocity, but also designed an experiment (although not an adequate one) by which the speed of light might be measured. These divergent views were discussed among the Paris academicians, and were well known to Römer.

In his observational work Römer noticed that the eclipses of Io occurred at longer intervals as the earth receded from Jupiter, but happened in closer sequence as the earth and that planet came closer together. Beginning from the point at which the earth and Jupiter were closest to each other, Römer tried to predict the time of occurrence of an eclipse of Io at a later date, when the earth and Jupiter had drawn further apart. In September 1676 he announced to the members of the Academy that the eclipse predicted for 9 November of that year would be ten minutes later than the calculations made from previous eclipses would indicate. Observations confirmed his hypothesis, and Römer correctly interpreted this phenomenon as being the result of the finite velocity of light. He was thereupon able to report to the Academy that the speed of light was such as to take twenty-two minutes for light to cross the full diameter of the annual orbit of the earth; in other terms, that the light from the sun would reach earth in eleven minutes (a time interval now measured to be about eight minutes and twenty seconds). The speed of light was thus established scientifically for the first time, with a value of about 140,000 miles per second—a reasonable first approximation to the currently accepted value of 186,282 miles per second.

Römer’s results were not immediately accepted by everyone. The Cassinis remained unconvinced for some time, and Descartes’s view concerning the instantaneous propagation of light retained some currency. It was only after James Bradley, in 1729, discovered the periodic annual displacements in the positions of all stars in respect to the ecliptic—their aberration—that Römer’s interpretation prevailed. The value for the time of the passage of light from the sun to the earth, deduced from Bradley’s aberration constant, was eight minutes and twelve seconds, a more accurate approximation than that originally obtained by Römer, whose result had incorporated additional perturbations of motion that he had not recognized.

In 1679 Römer undertook a scientific mission to England, where he met Newton, Flamsteed, and Halley. In 1681 he returned to Denmark to become professor of mathematics at the University of Copenhagen. He left France at a propitious time, since four years after his departure Louis XIV revoked the Edict of Nantes, and as a Protestant Römer would surely have been forced to leave the country, as was Christiaan Huygens. Christian V of Denmark appointed Römer his astronomer royal and director of the observatory and gave him a number of technical and advisory duties; at one time or another Römer served as master of the mint, harbor surveyor, inspector of naval architecture, ballistics expert, and head of a commission to inspect highways. He performed all these tasks with distinction, and in 1688 was made a member of the privy council. In 1693 he became first judiciary magistrate of Copenhagen and in 1694 he was made chief tax assessor, in which office he devised an efficient and equitable system of taxation.

Despite the press of other official duties, Römer did not neglect his work as astronomer royal and director of the Copenhagen observatory. He performed a large number of astronomical observations (as many, in fact, as had Tycho Brahe) and designed and constructed astronomical instruments, particularly transit circles. The Copenhagen observatory was one of the oldest in Europe; at the time that Römer became its director, it was housed in the “Round Tower” built by Christian IV for Longomontanus in 1637. Römer began his observations there but soon found the site unsatisfactory. He therefore converted his own house into an auxiliary observatory, and in 1704 created another observatory, the Tusculaneum, located between Copenhagen and Roskilde and equipped with excellent and innovative instruments. (Indeed, Römer would seem to have been the first astronomer to have attached a telescope to a transit circle.)

Concomitant to his work in astronomical instrumentation, Römer invented a new thermometer. He was the first to recognize that the scales of thermometers designed to give concordant results must be based upon two fixed points. For this purpose he chose the boiling point of water and the melting point of snow—the same points later used by Celsius. Fahrenheit met Römer in 1708 and (according to his letter to Boerhaave in 1729) adopted so many of Römer’s ideas and techniques that the Fahrenheit thermometer should really have been named the Römer thermometer.

In 1705 Römer became mayor of Copenhagen; he was soon thereafter named prefect of police as well. The same year Frederick IV, who had succeeded Christian V, made Römer a senator and in 1707 he named him head of the state council of the realm. In each of these capacities Römer served faithfully and well. He died at the age of sixty-five, survived by his second wife, whom he had married in 1698. His first wife, his mentor Erasmus Bartholin’s daughter Anna Maria, whom he married in 1681, died in 1694. He had no children.


I. Original Works. Most of Römer’s astronomical MSS were lost in the great fire that destroyed Copenhagen in 1728. The only surviving observations are the “Triduum,” published in J. G. Galle, O. Roemeri triduum observationum astronomicarum a. 1706 … institutarum (Berlin, 1845), which includes Römer’s notes on observations made 21–23 Oct. 1706; and a subsequently discovered work, the “Adversaria,” published as Ole Römers Adversaria, T. Eibe and K. Meyer, eds. (Copenhagen, 1910). Memoirs on astronomical observations, written with J. D. Cassini and Jean Picard, appeared in Mésavant1699, I (The Hague, 1731); see also “A More Particular Account of the Last Eclipse of the Moon,” in Philosophical Transactions of the Royai Society, 10 (1675), 275–260; “Observationes lunares,” ibid., 388–389; and “Démonstration touchant le mouvement de la lumière,” in Journal des savants (7 Dec. 1676), 233–236, translated into English in Philosophical Transactions of the Royal Society, 12 (1677), 893–894. Römer’s astronomical instruments were described in Machines et inventions approuvées par l’Académie des sciences, I (Paris, 1735), 81–89; and in Miscellanea Berolinensia, III (Berlin, 1727), 276–278.

II. Secondary Literature. The chief source of information concerning Römer’s methods and ideas was written by his disciple, P. Horrebow, Basis astronomiae sive astronomiae pars mechanica (Copenhagen, 1735). I. B. Cohen, “Roemer and the First Determination of the Velocity of Light,” in Isis, 31 (1940), 327–379, is a valuable study and includes a bibliography of works on Römer. See also G. van Biesbroeck and A. Tiberghien, “Études sur les notes astronomiques contenues dans les Adversaria d’Ole Rømer,” in Oversigt over det K. Danske Videnskabernes Selskabs Forhandlinger (1913), 213–324; M. C. Harding, Ole Rømer som ingenior (Copenhagen, 1918); M. Phil, Ole Rømers videnskabelige liv (Copenhagen, 1944); E. Strömgren, Ole Rømer som astronom (Copenhagen, 1944); and the notice by K. Meyer in Dansk biografisk leksikon, XX (Copenhagen, 1951), 329–400, with bibliography of secondary literature. See also William Derham, Artificial Clock-Maker; to Which Is Added a Supplemen. ContainingMonsieur Römer’s Satellite-Instrument, 2nd ed., enl. (London, 1700).

ZdenĚk Kopal

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