As everybody (now) knows, that first ray of sunlight you see in the morning took several minutes to travel from the sun to your window. Back in the day, although some speculated that light had a finite speed, nobody was really sure. But thanks to the chief of police in Copenhagan, now we not only know that light does have a speed, but we have a pretty good idea what it is. Perhaps a bit more illumination is called for here.
On the 25th of September, 1644, Christen Pedersen and his wife welcomed a new baby into their family: Ole Christensen Pedersen. They lived in the town of Arhus in what is now Denmark, but at the time was part of a political entity that predated the European Union by centuries: Demark-Norway. Around the time little Ole was born, Christen, who was a merchant and sea captain, was getting frustrated at being confused with various other people in Arhus also named Christen Pedersen. Since he’d been born on an island called Rømø, he decided to call himself Christen Rømer instead — and the name stuck, not only for him, but for his son too, who’s still known as Ole Rømer. It didn’t stick perfectly though, because you’ll find historical references to him under (at least) the names “Roemer,” “Römer,” and “Romer.”
Ole graduated from the University of Copenhagen and got a job designing fountains at Versaille for Louis XIV of France. When he was in his 30s he returned to the University and became a professor of astronomy, as well as the (or at least “a”) royal mathematician. His specialty was in measurements, particularly in establishing standards, including Denmark’s first national system of weights and measures. He also created the Rømer scale, which became the basis for the Fahrenheit temperature scale. Then in 1705 he was appointed the chief of police in Copenhagen. It may have been a ceremonial appointment, since he was over 60 at the time, but he jumped right into the job. He thought morale was low on the force, and took the (somewhat questionable) measure of firing everybody. Presumably the new hires were either more enthusiastic or too nervous to admit that they weren’t. He also introduced oil-fired street lights to the city, which was one of the first such systems in Europe.
But let’s backtrack a few years to Rømer’s astronomical work when he was still in Paris. He worked with Giovanni Cassini (namesake of the Cassini space probe that visited Saturn) to observe Jupiter’s moon Io being eclipsed. He noticed that as Earth and Jupiter got closer, the times between eclipses got slightly shorter. He and Cassini reasoned that this could be because light had a finite speed — but Cassini abandoned the idea. Rømer stuck with it, presented eight years of observational data to the French Academy of Sciences, and the data provided the first calculation of the actual speed of light. As far as anybody knows, it was the first time a universal constant was measured on Earth.
Ole Rømer is pretty well remembered — there’s a lunar crater named after him, the Rømer satellite project was begun (but never actually launched), There’s an Ole Rømer observatory in Denmark, and if you read some back issues of The Flash comic book, The Flash used to measure his super-speed in “Roemers.” But even as the chief of police, Rømer never issued any speeding tickets.
