Google Doodle honors first measurement of the speed of light

December 7, 2016 • 9:00 am

Somehow I missed this anniversary, and the Google Doodle shown below isn’t visible from the U.S. Here’s the skinny from CNet:

While my car has trouble going over 65 mph, the speed of light is much faster — approximately 186,282 miles per second. How do we know that? Well, we can all thank Danish astronomer Olaus Roemer.

To honor Roemer and the 340th anniversary of the determination of the speed of light, Google made a Doodle.

Roemer determined the speed of light in 1676 by observing the planet Jupiter eclipsing its moon Io 140 times. His measurements were taken from Copenhagen while his peer Giovanni Domenico Cassini took measurements of the same eclipses in Paris. Roemer compared the results to determine the speed of light.

In the Google Doodle, you see Roemer repeatedly and thoughtfully pacing back and forth after viewing his telescope. This simple illustration shows just how much detail and thoughtfulness Google applies to its Doodles.

340th_anniversary_of_the_determination_of_the_speed_of_light_5651280530767872_hp2x-0There’s more on this experiment on a Wikipedia page, which describes the method—a very clever one. It’s based on the assumptions that the period of revolution of Jupiter’s moon Io around its planet would be constant, but that as the Earth moved farther from Jupiter over a period of time, the time that Io appeared to us from behind its planet would be greater because light would take longer to travel to Earth. By measuring the time differential over many revolutions of Io, Rømer calculated that the speed of light was 220,000 kilometres per second. That’s pretty accurate: about 26% lower than the true value of 299,792 km/s. His observations were controversial, but the order-of-magnitude accuracy was supported by other astronomical observations, and then refined to the present value by experiments involving measurements solely on Earth:

More data, and why we’re celebrating on this date, comes from Wikipedia (my emphasis):

On 22 August 1676, Rømer made an announcement to the Royal Academy of Sciences in Paris that he would be changing the basis of calculation for his tables of eclipses of Io. He may also have stated the reason:[note 4]

This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.

Most importantly, Rømer announced the prediction that the emergence of Io on 16 November 1676 would be observed about ten minutes later than would have been calculated by the previous method. There is no record of any observation of an emergence of Io on 16 November, but an emergence was observed on 9 November. With this experimental evidence in hand, Rømer explained his new method of calculation to the Royal Academy of Sciences on 22 November.

The original record of the meeting of the Royal Academy of Sciences has been lost, but Rømer’s presentation was recorded as a news report in the Journal des sçavans on 7 December. This anonymous report was translated into English and published in Philosophical Transactions of the Royal Society in London on 25 July 1677.

26 thoughts on “Google Doodle honors first measurement of the speed of light

  1. The speed of light is so much associated with Einstein, I assumed the speed of light had only been measured more recently – such as in the late 19th century. It is surprising to realize how much good science had been done in the 17th century and earlier. Hooray for science.

    1. That the speed of light (not velocity!) was fixed and finite was established well before Einstein used it as one of his assumptions. Maxwell had calculated it as a function of the electric and magnetic properties of free space. The striking thing was that the result was independent of any frame of reference, and that it was therefore the same in any frame of reference was the basis of special relativity.

      1. Maxwell didn’t live long enough to see his Electromagnetic Field Theory confirmed.

        1865 Maxwell publishes “A Dynamical Theory of the Electromagnetic Field” The paper predicts the existence of electromagnetic waves and the speed at which they will propagate. EM waves had not been detected but Maxwell proposes (because the measured speed of light matched his theory) that light is an EM wave.

        1879 Maxwell dies aged 48.

        1887 Hertz publishes a paper confirming the existence of the EM waves Maxwell predicted

        1894 Hertz dies aged 36.

  2. This cute wee doodle showed up yesterday in NZ (we get the light before any other country! 🙂 ), and I was looking forward to reading about it here today. I’m surprised it didn’t make it onto US computers. I often wonder how they decide who sees these.

    1. “I often wonder how they decide who sees these.”

      By way of that contemporary marketing object of quasi-religious veneration, the algorithm?

  3. Today the metre and second are defined in such a way that the speed of light is also defined, so there is no longer any need to measure it!

    1. Well, sort of. The second is defined in terms of counting cycles of some hyperfine transition radiation from a particular atom – I forget which ; the speed of light in a vacuum is 299792458 m/s, and the metre is the derived unit taken from those two definitions – there is no definition of the metre other than that speed equals distance over time.
      The reason they did it that way (in 1983 or 84, as I recall) is that at the time they could measure periods of time to around one part in 10^11, but could only measure distances to around one part in 10^8.

  4. Cool. And I was just rereading Galileo: one of the first to try to measure the speed of light. I’ve always found it wonderful that Roemer was to use one of Galileo’s greatest discoveries to do his thing: the satelites of Jupiter.

    Does anyone know when they got their current names? (“Io” is on the doodle.) Galileo named them collectively after the Medici, if I recall. Wikipedia says Marius is responsible; is that correct? The translation of his book title I think is wrong: I was just reading the footnote in Drake’s stuff about “Belg” – it should be “Dutch” or “Netherlander”, not “Belgian”, at least according to him.

      1. An interesting tale: thank you!

        About the astrological thread: I am not sure that Galileo wasn’t doing it for the money or whatever. I seem to remember a passage in I *think* the Assayer where he makes fun of both astrologers and aurifactors.

  5. I remember in first year we tried to measure the speed of light in the physics lab at university. Even with the advantage of having nifty electronic gadgets we only got it within 20% or so (essentially we were using Fizeau’s method).

    It’s a remarkably hard thing to do, and Roemer was a genius for thinking of this way to do it and making the very difficult measurements.

    1. Don’t beat yourself up. Rømer’s figure, when extrapolated using modern knowledge of the dimensions of the solar system, was out by 26%.

      But, in fact, Rømer did not really know how big the solar system was. He concluded that light travels at the speed of just under three Earth orbit diameters per minute but he didn’t know how big the Earth’s orbit was.

      The most important part of his work is that he demonstrated that light had a speed and that it could be used to explain otherwise anomalous phenomena.

      1. “The most important part of his work is that he demonstrated that light had a speed”

        – A finite speed. Galileo and others first thought it was infinite.

        (Aristotle, by contrast, would have denied *that*, too.)

  6. I believe Benjamin Franklin was the first to measure the speed of light, using a technique involving an assistant on a distant mountain top uncovering a lantern the moment he saw Franklin’s light.

    Franklin reported the speed of light as “extraordinarily fast.”

    The speed of light is exactly 299,792,458 meters per second. It would have been fun to see if anyone could be tricked into challenging the idea that the speed could be known exactly, but unfortunately Alan Clark spoiled it.

    1. What Franklin was measuring, of course, was the reaction time of his assistant.

      299,792,458 – is that accurate to the millimetre? 😉

      (Re Alan Clarke’s note on definitions of units, I recall from university how snotty some professors would get if I talked about a ‘cc’ (cubic centimetre) instead of a ‘millilitre’ and how delighted I was to discover that, some years previously, the CGPM had redefined the litre as *exactly* 1000 cc).


      1. I tried to confirm that bit about Franklin with some research, only to discover that it was actually Galileo who tried to measure the speed of light with lanterns. It’s all here on Wikipedia. Sorry about that.

        It seem to me that even with his lack of resources, Galileo could have at least placed a lower limit on the possible value of the speed of light by determining the longest delay that was undetectable by his lantern method, but according to Wikipedia, what he actually reported was that if light wasn’t instantaneous, it must nevertheless be extraordinarily rapid.

        1. Oh yes, I knew about Galileo’s effort, but (my memory of scientific history being a bit hazy), I just assumed from your mention that Franklin had had a go at it too. In fact the one name that recurs to me from my schooldays was Michelson-Morley, but they were of course trying to establish if there was any motion through the ‘aether’.

          The one name I didn’t recall was Roemer. One of those odd lacunae in one’s knowledge, I guess.


      2. “299,792,458 – is that accurate to the millimetre?”

        It is *far* more accurate than to the millimeter, it is *exact*. The speed of light (actually causality) was previously defined in terms of the meter and second.

        The meter was defined as the length of a platinum bar kept in Paris. The second was redefined in terms of the exact number of oscillations of a cesium-133 atom while the platinum bar was far from being exactly defined due to finite measuring precision.

        Once it was established that the speed of causality in a vacuum was constant in all reference frames the speed of causality could be exactly defined to be 299,792,458 meters per second. Thus the meter is now exactly defined as a function of the exact second and exact speed of causality in a vacuum. Of course we cannot measure the exact length of an object, but we can still know the exact length of a meter.

        1. I was being a bit facetious with my question. If it’s defined that way then it is exact. Just as a mile is precisely 1.609344 kilometres (because the inch is defined as 25.4mm).

          As regards the litre, from memory it was intended to be 1000 cc’s, but then they found that the platinum-iridium cylinder that defined a kilogram* had been made a bit too big and the litre was therefore, IIRC, equal to 1000.028 cc’s. This didn’t bother anybody except chemistry teachers.
          It was later realised this was a bit daft and the litre was redefined as 1000 cc’s exactly.

          (*from which the litre was derived as the volume of a kilogram of water)

          I suppose it’s a bit more scientific than measuring a dozen peoples’ feet and taking the average.


  7. If you install the free planetarium / virtual telescope programme , “Stellarium” from then you can zoom in on the planet Jupiter and fast forward the time so that it only takes a few minutes to watch Io complete an orbit of Jupiter. There is a clock on the screen so you can see for yourself how long it takes for one orbit. It is actually quite tricky to judge exactly when Io appears to touch Jupiter i.e immersion & emergence. ( maybe i’ll try again with greater magnification ) It is also interesting to watch Jupiter spin on its axis and see the giant red spot move across.

    Stellarium is brilliant if you don’t have time to set up a telescope or if cloud and light pollution would hinder your view, or if you can’t bear to face cold frosty night time viewing. Stellarium is also great because if you saw some starry object in the sky during the day you can rewind the planetarium to the time and world location you saw it to find out what it was.

    Io orbits Jupiter at a distance of 262,000 miles – compare with the average distance of our moon from Earth of 238,900 miles. Wow Io is going so much faster ( the physicists are going to groan at that rubbish statement ) Wikipedia says average orbital speed of Io = 17,334 km/s where as our moon has orbital speed of 1.022 km/s

    1. It is actually quite tricky to judge exactly when Io appears to touch Jupiter i.e immersion & emergence. ( maybe i’ll try again with greater magnification )

      Probably won’t hep – look up the problems people have had with the “black drop effect” when trying to measure the size of the universe during (solar) transits of Venus.

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