Top Eleven: Ole Roemer

The second in the Top Eleven is the first quantitative measurement of the speed of light, by Ole Christensen Roemer (whose last name ought to contain an o-with-a-slash-through-it, that I’ve rendered as an “oe”).

Who: Ole Roemer (1644-1710), a Danish astronomer.

When: The crucial observations were made around 1675.

What: Roemer made careful observations of the orbit of Jupiter’s moon Io (which circles the planet once every two days or so), and noted that the time between eclipses of Io (times when it disappeared behind Jupiter) got shorter as the Earth moved closer to Jupiter, and got longer as the Earth moved farther away. The shift was pretty small (tens of minutes), but large enough to be accurately measured.

Roemer correctly deduced that the difference in times was a consequence of the finite speed of light. As the Earth and Jupiter get closer together, it takes less time for light to travel from Jupiter to Earth, and thus the time bewteen observations of eclipses gets shorter. As the Earth and Jupiter move farther apart, the travel time gets longer, and the time between observations increases.

Why It’s Important: Just the idea that the speed of light is finite, rather than infinite is an important development in physics (and was moderately controversial for a good many years). It’s also the first reasonably precise measurement of a constant of nature– he was hampered by the lack of a good measurement of the distance from the Earth to the Sun, but his method gives a good value for the speed of light.

Reasons to Vote for Him:: First measurement of an important physical constant.

Reasons to Vote Against Him: Didn’t actually get a single number (though Huygens took his measurements and got a pretty good value for c). Has a name with non-standard characters in it.


  1. #1 Craig Pennington
    January 25, 2006

    Ole Christensen Rřmer. C’mon, non-standard characters? It’s not like the name is in Chinese. Google search “ole jupiter site:dk” (sans quotes) and copy-n-paste. (I’ve done some web writing on Gödel, which is what put me on to that trick, but because the umlaut is more common, you don’t really need to limit the country domain to get it early in the results.)

  2. #2 Mark Paris
    January 25, 2006

    Maybe it’s too early, but I had trouble figuring out what Reomer was measuring. Is the time difference you mention the time between an expected eclipse and the time that it was actually observed?

    I followed the link to the Wikipedia article. Roemer’s and Galileo’s work (not to mention many others) should be a lesson to people who wonder how all those primitives in centuries or millenia past could possibly do the things they did without our modern technology, or at least the help of aliens. Those guys were smart.

  3. #3 Kristjan Wager
    January 25, 2006

    If you want to write ø, you can write ø

  4. #4 Chad Orzel
    January 25, 2006

    Craig Pennington: Ole Christensen Rřmer. C’mon, non-standard characters?

    I’m trying to be fair and balanced here, but it’s hard to come up with reasons to vote against some of these guys…

    Mark Paris: Maybe it’s too early, but I had trouble figuring out what Reomer was measuring. Is the time difference you mention the time between an expected eclipse and the time that it was actually observed?

    That’s how I understand it, yes. You can measure the period between eclipses, and use that to predict when the next one should ocur, and you find that the prediction is systematically off by a few minutes one way or another.

  5. #5 Peter Lund
    January 29, 2006

    Reason to vote for him: taught Fahrenheit how to make thermometers.

    Reason to vote against him: partially responsible for one of the English-speaking world’s silly measurement units.

  6. #6 John Hardy
    February 1, 2006

    Another reason to vote for him:
    he annoyed the hell out of Cassini.

  7. #7 peter kasten
    June 16, 2006

    the statement about roemer contains some historical mistakes.Roemers discovery was in 1676.He observed only eclipses of the moon Io and calculated the delaytime of light of 22 minutes for the earth-orbit-diameter.He didn´t calculated the speed of light,only when Christiaan Huygens wrote his work Traite de la lumiere 1678,published 1690.

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