Danish astronomer and mathematician. His primary discovery was the finite velocity of light. Observations of Jupiter's satellites had disagreed with Cassini's tables, and Römer demonstrated in 1675 that the error was due to the time taken by light to cross the changing distance between Jupiter and the Earth. He announced that light took 11 minutes to travel the distance between the Earth and the Sun.
Römer also developed the transit instrument, which was to become the fundamental instrument of positional astronomy.
Römer was born in Orhus, Jutland, and went to the University of Copenhagen in 1662, where he studied mathematics and astronomy.
In 1671 Jean Picard, who had been sent by the French Academy to verify the exact position of Tycho Brahe's observatory, wasimpressed by Ræmer's work and invited him back to Paris with him. In Paris, Ræmer was made a member of the Academy and tutor to the
crown prince. He conducted observations, designed and improved scientific instruments, and submitted various papers to the Academy. He returned to Denmark in 1681 to take up the dual post of Astronomer Royal to Christian V and director of the Royal Observatory in Copenhagen. He was mayor of Copenhagen and prefect of the police, and was also a senator and head of the state council.
It was through the precision of both his observations and hiscalculations that Ræmer not only demonstrated that light travels at a finite speed but also put a rate to it. Noticing that the length of time between eclipses of the satellite Io by Jupiter was not constant, he realized that it depended on the varying distance between the Earth and Jupiter. He was able to announce in September 1679 that the eclipse of Io by Jupiter predicted for 9 November would occur ten minutes later than expected. Römer's prediction was borne out; his interpretation of the delay provoked a sensation. He said that the delay was caused by the time it took for the light to traverse the extra distance across the Earth's orbit.
Römer was born 25 September 1644 in Orphus to a merchant and skipper Christen Pedersen and Anna Olufsdatter Storm, daughter of an alderman. Christen Pedersen had taken to using the name Römer, which means that he was from Römö, to disambiguate himself from a couple of other people named Christen Pedersen. There are few sources on Ole Römer until his immatriculation in 1662 at the University of Copenhagen, at which his mentor was Rasmus Bartholin who published his discovery of the double refraction of a lightray by Iceland spar (calcite) in 1668 while Rèmer was living
in his home. Rèmer 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.
Römer was employed by the French government: Louis XIV made him teacher for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles.
In 1681, Römer 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, bothat the University Observatory at Rundet¥rn 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 Römer's observations.
In Römer's position as royal mathematician, he introduced the first national system for weights and measures in Denmark in May 1, 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, practicalities making it too inaccurate at the time. Notable
is also his definition of the new Danish mile. It was 24,000
Danish feet, which corresponds to 4 minutes of arc latitude, thus making navigation easier.
In 1700, Römer managed to get the king to introduce the Gregorian calendar in Denmark-Norway - something Tycho Brahe had argued for in vain a hundred years earlier. Römer also developed one of the first temperature scales.
Fahrenheit visited him in 1708 and improved on the Römer scale, the result being the familiar Fahrenheit temperature scale still in use today in a few countries.
Römer also established several navigation schools in many Danish cities. In 1705, Römer 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. This was the start of a social reform.
In Copenhagen, Römer 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.
Römer and 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, in essence using the Jovian system as a cosmic clock; 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, Römer joined the observatory of Uraniborg on the island of Hven, near Copenhagen, in 1671. Over a period of several months, Jean Picard and Römer observed about 140 eclipses of Jupiter's moon Io, while in Paris Giovanni Domenico Cassini observed the same eclipses. By comparing the times of the eclipses, the difference in longitude of Paris to Uranienborg 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 Römer went to Paris and continued observing the satellites of Jupiter as Cassini's assistant. Römer 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 published a short paper in August 1675 where he states:
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.
Römer compared the duration of Io's orbits as Earth moved towards Jupiter and as Earth moved away from Jupiter. Oddly, Cassini seems to have abandoned this reasoning, which
Römer adopted and set about buttressing in an irrefutable manner, using a selected number of observations performed byPicard and himself between 1671 and 1677. Rèmer presented his results to the French Academy of Sciences, and it was summarised soon after by an anonymous reporter in a short paper, Demonstration touchant le mouvement de la lumi¨re trouve par M. Roemer de l'Academie des sciences, published 7 December 1676 in the Journal des s§avans. Unfortunately the paper bears the stamp of the reporter failing to understand Rèmer's presentation, and as the reporter resorted to cryptic phrasings to hide his lack of understanding, he obfuscated Rèmer's reasoning in the process. However only interpretation of the presented numbers makes sense: As forty orbits of Io - each of 42.5 hours - observed as the Earth moves towards Jupiter are in total 22 minutes shorter than forty orbits of Io observed as the Earth moves away from Jupiter, and Rèmer concluded from this that light will travel the distance, which the Earth travels during eighty orbits of Io, in 22 minutes. This makes it possible to calculate the strict result of Rèmer's observations: The ratio between the speed of light of the speed with which Earth orbits the sun, which becomes 80ú42.5 hours/22 minutes 9,300. In comparison the modern value is circa 299,792 km s - 1/29.8 km/s. Römer neither calculated this ratio, nor did he give a value for the speed of light. However, many others calculated a speed from his data, the first being Christiaan Huygens; after corresponding with Rèmer and eliciting more data, Huygens deduced that light travelled 16+2/3 Earth diameters per second, misinterpreting Römer's value of 22 minutes as the time in which light traverses the diameter of the Earth's orbit.
Rèmer's view that the velocity of light was finitewas not fully accepted until measurements of the so-called aberration of light were made by James Bradley in 1727. In 1809, again making use of observations of Io, but this time with the benefit of more than a century of increasingly precise observations, the astronomer Jean Baptiste Joseph Delambre reported the time for light to travel from the Sun to the Earth as 8 minutes and 12 seconds. Depending on the value assumed for the astronomical unit, this yields the speed of light as just a little more than 300,000 kilometres per second.
A plaque at the Observatory of Paris, where the Danish astronomer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet.