From Academic Kids

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Ole Rømer.

Ole Christensen Rømer (September 25 1644September 19 1710) was a Danish astronomer who made the first quantitative measurements of the speed of light (1676). Rømer was born in Aarhus and died in Copenhagen.


General biography

Missing image
the Rundetårn, or round tower, in Copenhagen, on top of which the university had its observatory from the mid 17th century until the mid 19th century, when it was moved to new premises. The current observatory there was built only in the 20th century to serve amateurs.

Rømer was employed by the French government: King Louis XIV made him teacher for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles.

In 1681, he returned to Denmark and was appointed professor of Astronomy at Copenhagen University. He was active also as an observer, both at the University Observatory at the Round Tower and in his home, using improved instruments of his own construction. Unfortunately, his observations have not survived: they were lost in the great fire of Copenhagen in 1728.

In his position as royal mathemathican, 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 24000 Danish feet, which corresponds to 4 minutes of arc latitude, thus making navigation easier.

In the year 1700, he managed to get the king to introduce the Gregorian calendar in Denmark-Norway – something which Tycho Brahe had argued for in vain a hundred years earlier.

He also developed one of the first temperature scales. Fahrenheit visited him in 1708 and improved on the Rømer scale, the result being the familiar Fahrenheit temperature scale still in use today in a few countries.

He also established several navigation schools in many Danish cities.

In 1705, Rømer was made the second Chief of the Copenhagen Police, a position he kept until his death in 1710. He fired the entire force as one of his first acts, being convinced that morale on the force was alarmingly low. He was the inventor of the first street lights (oil lamps) in Copenhagen, and worked hard to try and control the beggars, poor people, unemployed, and prostitutes of Copenhagen. This was the start of a social reform.

In Copenhagen he 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.


As well as inventing the first street lights in Copenhagen, Ole Rømer also invented the Meridian Circle, the Altazismuth and the Passage Instrument.

Rømer and the speed of light

The determination of longitude is a significant practical problem in cartography and navigation. King Philip III of Spain offered a prize for a method to determine the longitude of a ship out of sight of land. 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, Rømer joined the observatory of Uranienborg on the island of Hven, near Copenhagen, in 1671. Over a period of several months, Jean Picard and Rømer 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 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 Io's) got shorter as Earth approached Jupiter, and longer as Earth moved farther away. Cassini published in August of 1675 a short paper where he states:

Cette seconde inégalité paraît venir de ce que la lumière emploie quelques temps à venir du satellite jusqu'à nous, et qu'elle met environ dix à onze minutes à parcourir un espace égal au demi-diamètre de l'orbite terrestre.
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-diametre of the terrestrial orbit.

Oddly, Cassini seems to have abandoned this hypothesis, which Rømer adopted and set about buttressing in an irrefutable manner. He estimated that the time for light to travel the diameter of the Earth's orbit, a distance of two astronomical units, was 22 minutes. This is somewhat greater than the currently accepted value, which is about 16 minutes and 40 seconds.

His discovery was presented to the Académie royale des sciences and summarised soon after in a short paper, "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des sciences", in the Journal des scavans, December 7, 1676. In the paper, he stated « that for the distance of about 3000 leagues, such as is very near the bigness of the diameter of the Earth, light needs not one second of time ». He went so far as accurately predicting the delay that the November 9, 1676 eclipse of Io would have: 10 minutes. 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.

As Rømer had no accurate value for the astronomical unit, he gave no value for the speed in his paper beyond the aforementioned lower bound. However, many others calculated a speed from his data, the first being Christiaan Huygens; after corresponding with Rømer and eliciting more data, Huygens deduced that light travelled 16.6 Earth diameters per second. If Rømer had used his own estimates of the Earth to Sun distance at that time, he would have obtained a speed of about 135,000 km/s.

Rømer's view that the velocity of light was finite was 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, 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.


  • R.J. MacKay and R.W. Oldford. "Scientific Method, Statistical Method and the Speed of Light", Statistical Science 15(3):254–278, 2000. (mostly about A.A. Michelson, but considers forerunners including Rømer. Also available on line: [1] (

External links

da:Ole Rømer de:Ole Rømer es:Ole Roemer fr:Ole Christensen Rømer id:Ole Christensen Roemer is:Ole Rømer it:Olaus Roemer nl:Ole Rømer ja:オーレ・レーマー no:Ole Rømer pl:Ole Rømer pt:Ole Rømer sl:Ole Christensen Rømer sv:Ole Rømer


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