Ole Rømer’s Discovery of the Speed Of Light

How his observations gave us the first measure of the speed of light
John Loeffler

There is something about the speed of light that will always hold our fascination. Maybe it’s that human beings don’t like the idea of a universal speed limit or that something as seemingly ephemeral as light can have qualities like speed.

Whatever it is, it’s something that even the least scientifically inclined to know something about and that’s largely due to the work of Ole Rømer, who made the first real estimate of the speed of light.

Who was Ole Rømer?

Olaus Roemer
Source: Wikimedia Commons

Ole Rømer was the son of a modest Danish merchant in Aarhus, Denmark. Born in 1644, Rømer was sent to the University of Copenhagen at the age of 18 and studied Mathematics and Astronomy under the guidance of Rasmus Bartholin.

Rømer would go on to tutor the eldest son of the Louis XIV, king of France, and became a fixture in French scientific society of the time, regularly presenting papers on a variety of topics.

His work with machinery and mechanics was widely respected by his peers across Europe, and he lived in the then newly constructed observatory in Paris, where he stayed until leaving France in 1681 to take up a professorship at the University of Copenhagen.

His discovery of the speed of light was purely coincidental to his other work, but it is one of his lasting legacies.

What Was Known About Light in Rømer’s Time?

Speed Of Light
Source: Summer Skyes 11 / Flickr

The exact nature of light prior to 1676 was a hotly debated topic, going all the way back, as all things scientific usually do, to the Ancient Greeks.

It was Aristotle and the philosopher and mathematician Heron who first proposed that light propagated instantaneously, making the speed of light infinite.

This began a scientific back and forth over the centuries with every famous scientific mind of both Western and Islamic societies venturing an opinion about the nature and speed of light.

Galileo was Rømer’s nearest contemporary to attempt to measure the speed of light, but was unsuccessful owing to the relatively short distance he was measuring with and the lack of a precise enough clock to measure by.

The final word on the speed of light by the time Rømer made his discovery belonged to Descartes, who argued convincingly for the instantaneous propagation of light, making the speed of light infinite and immeasurable.

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Rømer’s Discovery of the Speed of Light

Io Jupiter
Source: NASA | JPL | University of Arizona

Rømer’s discovery came as a result of his astronomical work, building off the earlier work of Galileo. Discovered in 1610, the Galilean moon of Jupiter, Io, had been extensively studied by astronomers, so much so that it’s orbital period around Jupiter was known to astronomers at the time.

Making a complete orbit of Jupiter in only 1.759 days, Rømer made meticulous recordings of Io’s orbit around Jupiter as part of his work. It was these records that held the key to Rømer’s discovery.

Every orbital period, Jupiter would eclipse Io for several minutes to an observer on Earth. As Rømer recorded the length of each eclipse, he noticed a peculiar pattern.

Whenever the Earth was moving towards Jupiter, the start time of Io's eclipse would come earlier and earlier until the nearest distance between the Earth and Jupiter, after which the start time of Io's eclipse would be later and later until the Earth was at its farthest from Jupiter, at which point the pattern would repeat.

Roemer Diagram
Source: Wikimedia Commons

Rømer knew that the position of the Earth had no influence on the length of Io’s orbital period, so something else had to account for the difference he was seeing the in the recorded start times of the eclipse.

It was then that Rømer realized that what he was seeing was the difference in the time it took light to travel between Io and Earth.

As Earth moved closer to Jupiter, light had less distance to travel and as Earth moved away from Jupiter, light had to travel farther. It took the extraordinary distance between Earth and Jupiter for such a discrepancy in the time it took light to travel to become observable.

Once it did, however, the speed of light could be easily calculated by taking the diameter of the Earth’s orbit and dividing it by the difference in time between the quickest and longest recorded eclipses, which Rømer calculated to be about 22 minutes.

This put the first approximation of the speed of light at around 131,000 miles per second.

Rømer’s Legacy

Roemer Telescope
Source: Wikimedia Commons

While Rømer’s discovery was off by about 55,000 miles a second this was an extraordinarily accurate approximation given the imperfect data about Earth’s orbit that was available at the time as well as the slight discrepancy in Rømer’s observations and the actual difference in Io’s eclipses.

The actual difference between the earliest and latest start times for Io's eclipse is about 17 minutes, not 22.

It would be up to others to further refine the exact measurement of the speed of light to what we know today, but Rømer’s discovery put an end to a two-thousand-year-old debate in a way that any scientist in the world could replicate for themselves.

Rømer’s discovery opened up new ways for measuring distances in the solar system. “For it is now certain,” wrote Isaac Newton in Principia, “from the phenomena of Jupiter’s satellites, confirmed by the observations of different astronomers, that light is propagated in [finite] succession and requires about seven or eight minutes to travel from the sun to the earth.”

As Europe was about the begin its Scientific Revolution in the 18th century, not a small part of that momentum is owed to Rømer’s work. By proving what was possible through application of science, Rømer helped prepare the way for the era of discovery that followed.

Rømer himself wasn’t finished, though. He went on to mentor and guide other young scientists who would become household names for centuries to come.

Rømer never stopped with work with machinery, even inventing the mercury thermometer and inspiring Fahrenheit to develop his system of temperature measurement.

Nothing will be as impressive as his discovery of the once immeasurable speed of light though. He wasn’t looking to make history, but Ole Rømer is proof that curiosity and coincidence are the essential fuel that drives scientific discovery.

Via: Isis Journal (1940) / JSTOR

Correction: An earlier version of this article incorrectly stated that the length of Io's eclipse appeared to lengthen and shorten depending on the Earth's relative position to Jupiter. Rømer actually observed that the times at which an eclipse would begin would come earlier or later, depending on the Earth's position. We apologize for the error.

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