Ole Rømer: Meaning (information, definition, explanation, facts)

Ole Christensen Rømer (September 25, 1644 – September 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

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, Rømer 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 at the great fire of Copenhagen in 1728.

In the year 1700, he managed to get the government 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.

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 (1616-17) 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. In 1672 Rømer went to Paris and continued observing the satellites of Jupiter as Cassini's assistant.

Rømer observed that times between eclipses got shorter as Earth approached Jupiter, and longer as Earth moved farther away. He hypothesized that this variation was due to the time it took for light to travel the lesser or greater distance, and 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.

This discovery was published in a short paper, "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des Sciences", in Journal des scavans, 7 December 1676. 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.

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 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 kilometers 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])