Ole Rømer

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

Rømer and the speed of light

The determination of longitude is a significant practical problem in cartography and navigation.

Related Topics:
Longitude - Cartography - 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.

Related Topics:
Galileo - Clock - 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.

Related Topics:
1616 - 1617

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

Related Topics:
Uranienborg - Hven - 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.

Related Topics:
Jean Picard - Io - Giovanni Domenico Cassini

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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:

Related Topics:
1666 - 1668 - 1672 - Paris - 1675

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: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.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:(translation)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: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.

Related Topics:
Académie royale des sciences - December 7 - 1676 - November 9

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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, 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 kilometres per second.

Related Topics:
Aberration of light - James Bradley - 1727 - 1809 - Delambre - Speed of light - Kilometre - Second

~ ~ ~ ~ ~ ~ ~ ~ ~ ~