Ole Christensen Römer

1644-1710

Danish Astronomer and Physicist

Ole Römer is famous for demonstrating the finite velocity of light. He produced various scientific instruments including an improved micrometer, planetaria, the first transit circle, and an alcohol thermometer.

Römer was born on September 25, 1644, in Aarhus, Denmark. In 1662 he matriculated at the University of Copenhagen, where he studied astronomy and mathematics with Thomas (1616-1680) and Erasmus Bartholin (1625-1698). He lived at the home of Erasmus while a student, eventually becoming his personal assistant and marrying his daughter.

In 1671 Römer met Jean Picard (1620-1682) and accompanied him to the island of Hven to assist in redetermining the position of Tycho Brahe's (1546-1601) observatory Uraniborg. To this end they made measurements, in conjunction with those by Giovanni Domenico Cassini (1625-1725) in Paris, of a series of eclipses of Jupiter's moon Io. Römer returned to Paris with Picard and was elected to Académie membership in 1672. He was appointed tutor to the Crown Prince, responsible for making various astronomical observations, and constructed many precision instruments including an improved micrometer that was quickly adopted into wide use.

Römer's most important result was an out-growth of his further work on occultations of Jupiter's satellite Io. Cassini had already published ephemerides for the motions of Jupiter's satellites (1668) and in 1675 discovered an inequality responsible for periodic fluctuations in the timing of Jovian satellite eclipses. The effect seemingly depended on the position of Earth relative to Jupiter. Cassini entertained and then rejected the idea that these fluctuations were due to the finite velocity of light.

Römer pursued the issue more carefully and in September 1676 announced to the Académie that the eclipse of Io expected on November 9 would occur exactly ten minutes late. Skeptical, Académie members made careful observations. They reported the eclipse took place at 45 seconds after 5:35 A.M.—exactly 10 minutes late as predicted. Two weeks later before a baffled assembly of the Académie, Römer explained the delay was due to the finite velocity of light. Scrutiny of his observations and those of Cassini had revealed that the interval between successive occultations of Io diminished as Earth approached Jupiter and increased as Earth receded. Römer correctly surmised that this was because it took a shorter or longer time respectively for light to reach Earth. By comparing the predicted eclipse times of Io with those observed at various points in Earth's orbit, Römer estimated it took 22 minutes for light to cross Earth's orbit.

Using this estimate and his own estimate of Earth's orbital diameter, Christiaan Huygens (1629-1695) made the first determination of light's velocity. Though his value of 140,000 miles per second (225,000 kilometers per second) is about 25% too small, it represented a considerable achievement. Full acceptance of Römer's conclusion came only after James Bradley (1693-1762) announced his discovery of stellar aberration in 1729.

Römer returned to Denmark in 1681 to become professor of mathematics at the University of Copenhagen, Astronomer Royal to King Christian V, and Director of the Royal Observatory. In 1704 he built his own observatory, the Tusculaneum, which he equipped with quality instruments of his own design, including the first transit circle. Römer also invented a thermometer with a scale based on two fixed points that influenced Daniel Fahrenheit's (1686-1736) thermometric researches.

Römer held many civic and advisory positions including master of the mint, harbor surveyor, and inspector of naval architecture. He was Copenhagen's first judiciary magistrate (1693), chief tax assessor (1694), and then mayor (1705). He was also appointed senator and then named head of the state council of the realm (1707). He died in Copenhagen on September 19, 1710.

STEPHEN D. NORTON