Ole Christensen Rømer (Danish: [ˈoːlə ˈʁœˀmɐ]; 25 September 1644 – 19 September 1710) was a Danish astronomer who, in 1676, first demonstrated that light travels at a finite speed. Rømer also invented the modern thermometer showing the temperature between two fixed points, namely the points at which water boils and freezes. Rømer made his discovery regarding the speed of light while working at the Royal Observatory in Paris and studying Jupiter's moon Io. He estimated that light takes about 11 minutes to travel from the Sun to Earth. Using today's knowledge of the Sun-Earth distance, this would amount to a speed of light of approximately 220,000 kilometers per second, compared to today's accepted value of just under 300,000 kilometers per second. In scientific literature, alternative spellings such as "Roemer", "Römer", or "Romer" are common.
Biography
Rømer was born on 25 September 1644 in Aarhus to merchant and skipper Christen Pedersen (died 1663), and Anna Olufsdatter Storm (c. 1610 – 1690), daughter of a well-to-do alderman. Since 1642, Christen Pedersen had taken to using the name Rømer, which means that he was from the Danish island of Rømø, to distinguish himself from a couple of other people named Christen Pedersen. There are few records of Ole Rømer before 1662, when he graduated from the old Aarhus Katedralskole (the Cathedral school of Aarhus), moved to Copenhagen and matriculated at the University of Copenhagen. His mentor at the university was Rasmus Bartholin, who published his discovery of the double refraction of a light ray by Iceland spar (a transparent form of the mineral calcite) in 1668, while Rømer was living in his home. Rømer was given 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. Rømer was employed by the French government: Louis XIV made him tutor for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles. In 1681, Rømer returned to Denmark and was appointed professor of astronomy at the University of Copenhagen, and the same year he married Anne Marie Bartholin, the daughter of Rasmus Bartholin. He was active also as an observer, both at the University Observatory at Rundetårn and in his home, using improved instruments of his own construction. Unfortunately, his observations have not survived: they were lost in the great Copenhagen Fire of 1728. However, a former assistant (and later an astronomer in his own right), Peder Horrebow, loyally described and wrote about Rømer's observations. In Rømer's position as royal mathematician, he introduced the first national system for weights and measures in Denmark on 1 May 1683. Initially based on the Rhine foot, a more accurate national standard was adopted in 1698. Later measurements of the standards fabricated for length and volume show an excellent degree of accuracy. His goal was to achieve a definition based on astronomical constants, using a pendulum. This would happen after his death as practicalities made it too inaccurate at the time. Notable is also his definition of the new Danish mile of 24,000 Danish feet (circa 7,532 m). In 1700, Rømer persuaded the king to introduce the Gregorian calendar in Denmark and Norway – something Tycho Brahe had argued for in vain a hundred years earlier.
Rømer developed a temperature scale while convalescing from a broken leg. After visiting with Rømer in 1708, Daniel Gabriel Fahrenheit began making his thermometers using a modified version of Rømer's scale that eventually evolved into the Fahrenheit scale still popular in the United States and a few other countries. Rømer also established navigation schools in several Danish cities. In 1705, Rømer was made the second Chief of the Copenhagen Police, a position he kept until his death in 1710. As one of his first acts, he fired the entire force, being convinced that the morale was alarmingly low. He was the inventor of the first street lights (oil lamps) in Copenhagen, and worked hard to try to control the beggars, poor people, unemployed, and prostitutes of Copenhagen. In Copenhagen, Rømer made rules for building new houses, got the city's water supply and sewers back in order, ensured that the city's fire department got new and better equipment, and was the moving force behind the planning and making of new pavement in the streets and on the city squares. Rømer died at the age of 65 in 1710. He was buried in Copenhagen Cathedral, which has since been rebuilt following its destruction in the Battle of Copenhagen (1807). There is a modern memorial.
Determination of the speed of light
The determination of longitude is a significant practical problem in cartography and navigation. Philip III of Spain offered a prize for a method to determine the longitude of a ship out of sight of land, and Galileo proposed a method of establishing the time of day, and thus longitude, based on the times of the eclipses of the moons of Jupiter; this method was not significantly improved until accurate mechanical clocks were developed in the eighteenth century. Galileo proposed this method to the Spanish crown (1616–1617) but it proved to be impractical, because of the inaccuracies of Galileo's timetables and the difficulty of observing the eclipses on a ship. However, with refinements, the method could be made to work on land. After studies in Copenhagen, Rømer joined Jean Picard in 1671 to observe about 140 eclipses of Jupiter's moon Io on the island of Hven at the former location of Tycho Brahe's observatory of Uraniborg, near Copenhagen, over a period of several months, while in Paris Giovanni Domenico Cassini observed the same eclipses. By comparing the times of the eclipses, the difference in longitude of Paris to Uraniborg was 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. In 1672 Rømer went to Paris and continued observing the satellites of Jupiter as Cassini's assistant. Rømer added his own observations to Cassini's and observed that times between eclipses (particularly those of Io) got shorter as Earth approached Jupiter, and longer as Earth moved farther away. Cassini made an announcement to the Academy of Sciences on 22 August 1676:
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