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University of Copenhagen history: Ole Rømer discovers that light ... hesitates

7th december 1676 — A young Danish researcher gets an article published which revolutionises science. He has not written the text – but the discovery is his: Ole Rømer has found out that light has a certain speed.


th december 1676.

In the French periodical Journal des Scavans – one of the oldest scientific journals in the world – an article is published about a revolutionary discovery. Ole Rømer’s discovery that light has a speed. That “light hesitates”, as Rømer himself put it. The discovery made him world-famous and has fundamentally altered our understanding of the world around us.

As Morten Fink-Jensen, associate professor in history at the University of Copenhagen explained to the University Post: “He has a very central role in the Danish history of science, and is one of the major figures in the University of Copenhagen’s history. Someone that you today can really be impressed with, and learn from.”

Most people have probably heard of Ole Rømer and his discovery of the speed of light.

He is perhaps less known for his work as police chief and mayor of Copenhagen, master of the fountains for both the French and Danish King, founder of Denmark’s common units of measurement, the surveying of the Danish highways and the introduction of the Gregorian calendar.

The trip to Paris

Ole Rømer was born in 1644, grows up in Aarhus, and as an 18-year-old, becomes a student at the University of Copenhagen in 1662.

Unfortunately, there is a lot we don’t know about Ole Rømer, his scientific work and his private thoughts. The vast majority of his scientific records and measuring instruments were lost – along with almost the entire University of Copenhagen – in the great fire of 1728. He kept most of his measuring instruments in the Round Tower observatory, and as professor at the University of Copenhagen he resided in the professor homes in Store Kannikestræde. The fire consumed everything in both places.

As a student, the talent of Rømer is discovered by the famous professor Rasmus Bartholin. Ole Rømer is trained by the professor, lives in his home, and later contracts his first marriage with one of his daughters, Anna.

Rømer’s discovery brings resolution to 2,000 years of discussion, and is one of the largest steps towards the understanding of light that we have today.
Johan Gotthardt Olsen, biologist and host of a DR documentary on Ole Rømer

As Bartholin’s assistant ,Ole Rømer gets the prestigious assignment of rewriting Tycho Brahe’s astronomical observations with a view to publishing. They are also interested in this publication abroad, and in 1671 the French astronomer Jean Picard comes to Copenhagen. The reason is, that in order to be able to use Tycho Brahe’s observations in the astronomy of the day, they needed to know the exact geographical location of Tycho Brahe’s observatory on the island of Hven. Also the exact distance between Hven and Paris, where they were interested in using Tycho Brahe’s observations.

So Ole Rømer is given the task, in cooperation with Picard, to find out exactly where Tycho Brahe’s Observatory on Hven is located geographically in relation to Paris. He must have done his work well, as Picard subsequently applies for the Danish King’s permission to take Rømer back to Paris.

In 1672 Rømer travels to France. This is decisive.

The discovery

In Paris, Ole Rømer deals with something which many astronomers had no explanation for.

Johan Gotthardt Olsen – biologist, academic employee at the University of Copenhagen and host of a DR documentary on Ole Rømer explains:

“If you need to measure the precise longitudinal difference between two points, it is essential to know the exact time difference. This is possible, if a known, recurring, heavenly phenomenon can be detected in both locations, and the time of the phenomenon’s occurrence is also listed in both places. They knew of such a phenomenon, namely the eclipse and appearance of Jupiter’s largest moon, Io.”

But Io confused the researchers of the time. Like all moons it orbits its planet. But it seems to do it irregularly, and it was impossible to predict when it appeared. Here Ole Rømer made the bold hypothesis, which may explain why it sometimes takes a longer time, sometimes a shorter time, for the light of the moon to reach the Earth.

“Rømer relates the disagreements with the Earth’s position in relation to the Sun and Jupiter, and when the different measurements are taken, and reaches this conclusion: If the light travelling through space has a finite time, it takes longer to reach Earth from Jupiter when the two planets are far apart, and this can explain the discrepancy. This is a huge accomplishment,” says Johan Gotthardt Olsen.

So light requires time to reach the Earth – light hesitates, as Rømer puts it.

He argued his discovery in an oral presentation in front of the French Academy of Sciences. But Rømer did not publish the article himself, which was published on 7th December 1676. It was written by an anonymous scribe who attended the presentation and who published it in the Journal des Scavans. One year later it was translated into English. Ole Rømer had in this way no part in the actual publishing of his discovery.

Breakthrough in the history of ideas

One of the reasons why Ole Rømer was – and still is – world-famous for his discovery that light has a speed, is that for a long time beforehand, scientists had tried to find out what light actually is.

“For more than 2,000 years before Rømer made his discovery, philosophers have wondered what light was. Aristotle’s idea of light was that the transparent media (for example, the air) potentially enables seeing, but that the light from the Sun activates the transparent media’s ability to communicate images to our eyes. Just one of a handful of nutty ideas from the Greeks. But it should be remembered that they were groping in the dark (pun intended) and made virtually no experimental observations,” says Johan Gotthardt Olsen.

This is one of the quite fundamental, philosophical, questions that Ole Rømer’s discovery helped to clear up.

“Rømer’s discovery brings resolution to 2,000 years of discussion, and is one of the largest steps towards the understanding of light that we have today.”

The discovery that light has a certain speed, has revolutionised science and has had a major impact on our knowledge of the universe. According to Johan Gotthardt Olsen, it is what Ole Rømer’s work has later been applied to, that has made it so crucial:

“Perhaps the greatest inference from Rømer’s work will be drawn by a man walking around Bern, Switzerland in the beginning of the last century with his baby in a pram. During his time, they had made a lot of measurements of the speed of light and found out that it is always the same in a vacuum. From this fact, the young man Albert Einstein deducts his theory of special relativity. Rømer’s fellow countryman, Niels Bohr, adds to his achievement with his quantum mechanical explanation for the distribution of colours in spectral lines, namely the model of the atom.”

The king called Rømer home

Rømer’s article – which was written by the anonymous note taker – makes him famous throughout Europe. So famous, that the King of France employs Ole Rømer as a private tutor to his heir, and hires him to produce the fountains in the King’s castles. But the Danish King would also like to have a share of the fame:

“So our King called him home. The French should damn well not capitalise on this Danish genius. So Rømer comes home and becomes chief of police, surveyor, mayor, supreme court judge, councillor of state, rector of the university, and a whole lot more. He introduces a common system for weights and dimensions and the Gregorian calendar to Denmark, which we still use today, street lighting and a bunch of other stuff. He didn’t have much time to do further research,” says Johan Olsen.

When Ole Rømer came home to Denmark, he becomes professor at the University of Copenhagen, where he is made rector twice. But as professor, the Danish King assigns to Ole Rømer a wide range of tasks, which do not appear to belong to such a position:

“It actually means that he formally continues to be a professor, but that he actually has no time to be do anything. He is released from his obligations, so his academic activities was somewhat limited. Today, you would probably say that he had been bought off,” says Morten Fink-Jensen.

“The rare genius,” which Picard calls Rømer, “the lord of astronomy,” as he was also known, did not have much time to flourish scientifically. The absolutist king exploited instead his practical skills to reform large parts of society:

“He contributes to government reforms – everything that has to do with the overall infrastructure of society – on a solid scientific basis. He thereby contributes to the state becoming more efficient, but also that the state has legitimacy. That it has credibility. And that the state is based on science,” explains Morten Fink-Jensen.

Ole Rømer was an ideal applied scientist: I almost said ‘from research to invoice’. Applied science. To carry out engineering tasks to delight the absolute monarch.
Morten Fink-Jensen, university historian, UCPH

One of Ole Rømer’s reforms is to introduce a common measurement and weight unit for the whole Kingdom of Denmark:

“At the time, there were a lot of different local measurement units and weight units in Denmark – a pound on the island of Zealand was not the same as on Funen. A barrel was different in northern Jutland from southern Jutland. This led to incredible administrative problems.”

All these practical tasks meant that Ole Rømer had no time to do research – and this is something that Morten Fink-Jensen regrets, and he draws a parallel to the present day:

“Perhaps we could have had even more long-term benefits from Ole Rømer, if he had had more time to look after his research. And this is also a current issue: It may be that he came up with something, the significance of which was only really known 100 years later. And the government could not use it for anything right now.”

Morten Fink-Jensen describes Ole Rømer as an “ideal applied scientist: I almost said ‘from research to invoice’. Applied science. To carry out engineering tasks to delight the absolute monarch.

In this way, Ole Rømer had the same conditions as many researchers today, who also spend time on a number of things that have value here and now, but that might not lead to larger, long-term benefits.

But on one point, Ole Rømer was very different from the researchers of today.

No publications

It is annoying to posterity, that Ole Rømer hardly published anything. The article that was published 342 years ago, was not even written by himself.

And perhaps it is fortunate that the writer was anonymous, as his rendering of Ole Rømer’s revolutionary discovery is, according to several researchers, not the most fortunate. As the astrophysicist Jan Teuber writes in his book ‘Ole Rømer – scientist and servant to society’: “His article must be completely unique in the history of science. Not only in the relationship between the extent of it (approximately a page and a half in this book) and scientific impact, but also by the number of errors, distortions and messy turns of phrase that managed to squeeze itself into such a modest space.”

In fact, Ole Rømer’s lack of publications is not just a shame for posterity. Also during his own time, several of the sharpest minds asked the Danish genius to publish his observations. Among them was one of the greatest mathematicians and philosophers, G. W. Leibniz, who in several letters almost pleaded with Rømer to publish something. But even such a mind could not persuade the genius Ole Rømer.

“I believe he was a perfectionist,” Morten Fink-Jensen. “And if you are so careful and have to produce a research publication, where you have to be certain of your calculations, it takes time. And he might have found it difficult to find the time, because there has always been other practical tasks in front of him.”

What can we learn from Rømer today?

But even though Ole Rømer perhaps spent much of his time on practical tasks. It is, according to Morten Fink-Jensen, also one of the reasons why we can learn so much from him today:

“Something you can learn from him today is that he managed to convert knowledge into practical matters. In the wider, science-theoretical perspective, we owe it to Ole Rømer the insight in working in a manner which combines the theoretical perspective with practical aspects. He used, for example, an enormous amount of energy producing reliable and precise measuring instruments.”

Ole Rømer invented the meridian telescope, which was the most precise observation instrument at the time. But his measuring instruments, which were made of metal, expanded with higher temperatures, and that made his observations imprecise. He had to correct his observations for temperatures – but a temperature scale was yet to be invented.

So Rømer invented one.