He timed a moon of Jupiter, found it ten minutes late, and proved that light takes time to travel.
For two thousand years the question looked settled: light was instantaneous. Aristotle thought so, Kepler thought so, Descartes staked his physics on it. Then, in the autumn of 1676, a 32-year-old Dane at the Paris Observatory made a public prediction. The next eclipse of Io, Jupiter's innermost moon, due on 9 November, would come about ten minutes later than the tables said. It did. The moon itself was not running late. Its light was still crossing the extra distance to Earth.
Ole Rømer had made the first measurement showing that light has a finite speed. He went home to Copenhagen and spent the rest of his life making Denmark run on time: he fixed its weights, its miles and its calendar, lit its streets and policed its capital. Then, eighteen years after his death, a fire destroyed nearly everything he had written down.
Aarhus to Paris and back
The lifeline
1644
Born 25 September in Aarhus, Jutland, the son of a merchant and skipper
1662
Enters the University of Copenhagen and studies under Rasmus Bartholin
1672
Goes to Paris with Jean Picard and works as Cassini's assistant at the Observatory
1676
Predicts Io's delayed eclipse and shows that light travels at a finite speed
1705
Appointed mayor and police director of Copenhagen by the king
1710
Dies in Copenhagen on 19 September, aged 65
By the numbers
The measure of him
10 min
How late he predicted Io would emerge on 9 November 1676. It was.
22 min
His estimate of the time light takes to cross the full width of Earth's orbit
~140
Eclipses of Io he and Picard observed on Tycho's island of Hven in 1671
11 days
Removed from Denmark's calendar in 1700, when he brought in the Gregorian reform
How a late moon revealed the speed of light
The measurement
Io circles Jupiter about every 42½ hours, so its eclipses ought to run like a clock. Rømer saw that they ran early while Earth was approaching Jupiter and late while Earth was moving away. The difference was the time light needs to cover the changing distance. Today's figure for crossing the orbit is about 16½ minutes, so his estimate had the right idea and the right scale.
What he made, found and fixed
The work
1672
A working micrometer. In Paris he designed the first mechanically satisfactory micrometer for reading the fine scales of astronomical instruments.
1676
The speed of light. His account appeared in the Journal des sçavans on 7 December 1676. Using his figures, Christiaan Huygens later worked out about 212,000 km per second, roughly 70% of the true value.
1683
Denmark's first national weights and measures, introduced on 1 May as royal mathematician. He set the Danish mile at 24,000 feet, and his road survey left milestones that are still called Ole Rømer stones.
1690
The meridian circle, a telescope fixed to swing only along the meridian, the ancestor of the transit instruments that observatories relied on for the next two centuries.
1700
The Gregorian calendar for Denmark–Norway. He persuaded the king where Tycho Brahe had failed about a century earlier.
c. 1702
A temperature scale, devised while he was laid up with a broken leg. Daniel Fahrenheit visited him in 1708 and based his own famous scale on a modified version of it.
Doubted, burned, then vindicated
Recognition
In his lifetime
The idea that light takes time to travel was widely resisted at the Paris Observatory and beyond
He never published a full account himself; the 1676 report ran unsigned
His science was soon buried under civic duties: weights, roads, lamps, police
In the Copenhagen fire of 1728 nearly all his observations and instruments were lost
Today
James Bradley's discovery of stellar aberration in 1727 won the finite speed of light general acceptance
Fahrenheit's scale, which grew out of his, is still used every day across the United States
The Ole Rømer Medal is awarded every year by the Danish Natural Science Research Council
A lunar crater, an observatory in Aarhus and a museum at Kroppedal carry his name
The city he ran
Copenhagen
He came home in 1681 after nearly nine years in Paris. He observed from the top of the Round Tower and from his house in Kannikestræde, married Anne Marie Bartholin, his old teacher's daughter, and took on one civic job after another. As mayor and police director he set building rules, laid sewers, designed the city's first oil street lamps and tackled its poverty. A statue at his observatory site reads: "He showed the Danish People the Application of Natural Science."
1681
Becomes professor of astronomy at the University of Copenhagen
1704
Opens his own Observatorium Tusculanum at Vridsløsemagle, west of the city
1728
The great fire burns his papers; only a pupil's published accounts preserve his methods
"Philosophers have been labouring for many years to decide by some Experience, whether the action of Light be conveyed in an instance to distant places, or whether it requireth time."
— Rømer's demonstration, as printed in English Philosophical Transactions, 1677
