The story you’re about to read is of great importance to any science enthusiast and the history of general science. It is the story of an astronomer from Denmark, credited with the discovery of velocity’s finite nature and its first measurement. 

The man behind the idea – Ole Roemer

Ole Roemer was born in 1644 and died in 1710. Besides his work in astronomy, Roemer is also the inventor of the modern thermometer showing the temperature between two fixed points, those being the points at which water boils and freezes, respectively. He studied at the University of Copenhagen, where he familiarized himself with astronomy by studying Tycho Brahe’s work, to which he had access because of his mentor, Bartholin, who was assigned with preparing them for publication. 

Ole Roemer Speed Of Light
Ole Roemer

Giovanni Cassini was a pioneer in using the Galilean moons to establish longitude values. He was appointed as the director of the Royal Observatory in France, which opened in 1671. One of the first things he did was send Jean Picard, a French astronomer, and priest, to Brahe’s place of observation, at Uraniborg. Roemer started working there as Picard’s assistant, and Picard was so impressed by the skills that he arranged for Roemer to come to work at the Royal Observatory too. 

The eclipses of Io

Roemer was compiling the extensive set of data from the Uraniborg observatory, which were observations on the moon Io, the innermost moon of Jupiter, when he observed something strange. Some numbers didn’t seem to fit. He timed the eclipses of Io over a period of more years, and he noticed that the time interval between two successive eclipses becomes shorter as Earth moves towards Jupiter on its orbit and longer when moving away from it.

From the data, it came out that when Earth was nearest to Jupiter, in a configuration called opposition in astronomy, the eclipses of Io would happen approximately 9 minutes earlier than calculated from the average orbital period. Similarly, when Earth is in conjunction, that is, at the point farthest from the planet, the eclipses occur about 9 minutes later than predicted. 

How Did One Man Calculate The Speed Of Light Using A Moon of Jupiter? 2
Image Credits: ABCC

The difference couldn’t be explained with any of the known facts, and Roemer didn’t seem to work the problem out. It took him a brilliant insight to figure where the problem comes from. Of course, now, it doesn’t seem so brilliant any more, and it is easy not to acknowledge the difficulty of the problem nowadays. Still, back in those days, the whole perception of the Universe was a lot different.

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The generally accepted idea about light was that it has an infinite velocity, an idea sustained vigorously by the philosopher Descartes himself. He said that if the speed of light were to be finite, the Sun, Earth, and Moon would be really out of alignment during a lunar eclipse. Since such a misalignment has not been observed, he concluded that the velocity of light has to be infinite.

And so, you can imagine that a conclusion such as the one Roemer was about to make would cause great debate. Still, Roemer was smart enough to see no other solution to his problem than to consider the velocity of light to be finite. It takes longer for light to reach the Earth when it is farthest from Jupiter and a shorter time to reach Earth when they are closest. 

How Did One Man Calculate The Speed Of Light Using A Moon of Jupiter? 3

Much of the problem was already solved by now. Some sources say that Roemer also calculated the value of the velocity of light and some credit Christiaan Huygens for the first calculation. It is true that Huygens was an early supporter of Roemer’s idea and that Roemer communicated his observations and conclusions to him. However, the calculation was not that hard to do, as it only implied some basic geometry and arithmetic. Huygens’ words are:

“If one considers the vast size of the diameter KL, which according to me is some 24 thousand diameters of the Earth, one will acknowledge the extreme velocity of Light. For, supposing that KL is no more than 22 thousand of these diameters, it appears that being traversed in 22 minutes makes the speed a thousand diameters in one minute, that is 16-2/3 diameters in one second or in one beat of the pulse, which makes more than 11 hundred times a hundred thousand toises;”

A toise being a unit used for length in pre-revolutionary France, equal to approximately 2 meters. 

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Interesting enough, Roemer’s superior, Cassini, was skeptical about the conclusion and wasn’t really convinced by Roemer’s argument. Picard also shared a similar idea to that of Cassini. However, the discovery was a lot better received in England, and Newton and Edmund Halley, for example, were known to be supporters of the ‘theory.’ 

I hope you enjoyed today’s short story, short but inestimably valuable to science’s history and how we perceive the world today. Roemer’s intuition and courage stand as a great inspiration to all of us, even nowadays, more than 300 years later. 

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25 thoughts on “How Did One Man Calculate The Speed Of Light Using A Moon of Jupiter?”

  1. Carlos Fernández

    Is there any spatial object or a probe name after Roemer? His calculation is of inmense importance un astronomy, something to do with light should be name after him

    1. There’s a crater on the Moon named after him, and a failed Danish satelite project, not to mention, a museum, an observatory and a couple of streets around Denmark.

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  11. “If the speed of light were to be infinite the sun,moon and earth are really out of alignment during Lunar eclipse”
    I can’t understand this.what is the relationship between speed of light and alignment of sun,moon and earth or lunar eclipse?? Can anyone explain me about this??

  12. It would be rimiss of me to note your failure to mention Jean Luck’s assistant – Leonard Nimoy, but that would be illogical captain.

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