Measuring the time it takes for light to reach us once seemed as absurd as weighing a shadow. Long considered instantaneous, its spread had never been seriously questioned. It was not until the 17th century that a Danish astronomer, Ole Rømer, provided the first proof that light has a finite speed. This observation, the result of precise observations of the eclipses of the satellite Io around Jupiter, upset the structure of established knowledge.
At a time when astronomy was as much a scientific tool as a strategic issue for navigation or the measurement of time, this discovery introduced fundamental data into the understanding of the physical world. It opened the way to much later theories on space and time. Returning to Rømer’s journey means understanding how a detail observed in the sky reoriented an entire vision of nature.
An astronomer trained in the tradition of Tycho Brahe
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Ole Rømer grew up in the city of Aarhus, Denmark. The son of a trader, he became interested very early in the navigation instruments his father owned. At 18, he went to study at the University of Copenhagen, one of the oldest in the country. There, he trained in mathematics and sky observation, thanks to a renowned teacher, Rasmus Bartholin. He takes him under his wing and introduces him to the study of an immense scientific treasure: the surveys of the famous astronomer Tycho Brahe, who died several decades earlier. These precise observations of the movements of the planets must be transcribed to be usable. Rømer, still a student, is in charge.
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This rigorous work, which lasted several years, gave him solid experience in astronomical observation. In 1671, a French scientist, Jean Picard, came to Denmark to precisely locate the old Brahe observatory. Rømer collaborates with him, specifies Britannica. Impressed, Picard obtains permission from the Danish king to take the young Dane to Paris.
In Paris, Rømer joined the Royal Observatory and became an associate member of the Academy of Sciences in 1672. There he met Cassini, director of the institution, and became part of the European scientific elite. This stay marks a decisive turning point. He is no longer content with preserving the Danish astronomical heritage, he is preparing to transform it.
A unique demonstration of the finite speed of light
In 1676, while living at the Paris Observatory, Ole Rømer attempted to perfect the predictions of eclipses of Io, Jupiter’s satellite. The objective is practical: these eclipses serve as a temporal reference to calculate longitude, a method originally proposed by Galileo. But Rømer notices a persistent anomaly. Eclipses don’t always happen when expected. By studying these variations, he found that Io appears to appear with a delay of up to 11 minutes as Earth moves away from Jupiter. And with an equivalent advance when it gets closer.
Unlike his contemporaries, Rømer does not attribute these discrepancies to an irregularity in Io’s orbit. He makes a bold hypothesis. These shifts come from the time it takes for light to travel the distance between Jupiter and Earth. This proposition is directly opposed to the dominant doctrine, notably that of Descartes. The latter supported the instantaneous propagation of light. Rømer presented his findings to the Academy of Sciences in September 1676. The published article explained how light would take approximately 22 minutes to cross the diameter of the Earth’s orbit.
From this duration, the Dutch physicist Christiaan Huygens estimates the speed of light at around 220,000 km/s. This value, although underestimated (the current value is 299,792 km/s), remains incredibly close to reality for the time. Especially in the absence of precise data on interplanetary distances. Rømer’s experiment thus constitutes the first experimental proof that light has a finite speed. It is a major intellectual reversal, which shakes two millennia of philosophical beliefs.
After his stay in Paris, Ole Rømer returned to his country in 1681. He became a professor at the University of Copenhagen, but his role quickly went beyond teaching. The King of Denmark, Frederick IV, entrusted him with numerous responsibilities in the organization of the country.
Rømer was made responsible for mints, ports and road construction. He is also responsible for reforming the system of weights and measures. At the time, units varied from one region to another, which complicated trade. He therefore set up a national system, with precise and standardized measurements, such as the “Danish mile”, which is equivalent to approximately 7.5 kilometers. These reforms facilitate trade and strengthen the coherence of the country.
In the scientific field, Rømer continues his research. He improved observation instruments and designed more precise astronomical telescopes. He is also interested in temperature. Observing that heat can change the functioning of clocks, he imagined a temperature scale, dividing the difference between freezing water and boiling water into 60 units. This idea will be taken up by Fahrenheit to build the scale which today bears its name.
He also installed the first public lighting in Copenhagen with oil lamps. And he convinced the king to adopt the Gregorian calendar, still used today. Rømer is therefore a versatile scientist, who applies his knowledge to improve the practical functioning of society.
A partially lost, but lastingly influential work
While Rømer’s impact on science is indisputable, much of his direct work is now lost. In 1728, eighteen years after his death, a fire ravaged Copenhagen, destroying most of his instruments and manuscripts kept at the Round Tower and at his home. This disaster deprives posterity of a considerable number of observation data accumulated over several decades. Only a few notes, transmitted to his students, including Peder Horrebow, make it possible to reconstruct part of his work.
Despite this loss, Rømer had a lasting influence on the history of science. Isaac Newton explicitly cites his work in the
Principia Mathematica (1687), recognizing that “light takes about seven to eight minutes to reach us from the Sun.” This observation will prove fundamental in reflections on the nature of time and space. In the 20th century, Albert Einstein relied on the constancy of the speed of light to build the theory of special relativity.
More recently, historians of science emphasize the methodological importance of Rømer. His ability to combine empirical data and theoretical reasoning has served as a model for the modern scientific approach. He did not just observe, he interpreted with caution, confronting his models with physical realities. And he derived general laws from localized phenomena.
Its role in the transition between descriptive astronomy and mathematical physics is today recognized. His work marks the transition from a science dominated by speculation to a science based on measurement. If his name remains less known to the general public than that of Galileo or Kepler, his contributions are nevertheless of equal importance in the contemporary scientific structure.
