Philosophy 167: Class 11 - Part 12 - The Eclipses of Io: Cassini and the Speed of Light.

Smith, George E. (George Edwin), 1938-


  • Synopsis: Observations of eclipses of Io caused the first speculation that planetary observations must be corrected for a measurable speed of light. Cassini resisted this idea for a long time.

    Opening line: "The tables were saying the eclipse should happen at a certain time, and the kind of error was eight minutes in time."

    Duration: 5:42 minutes.

    Segment: Class 11, Part ... read more
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This is in the late 1670s. This is what Cameron ended up writing a senior essay when he was an undergraduate at Tufts. Summa Cum Laude and I keep pushing him to turn it into a book. This started it, Romer had picked up the fact that the eclipses of the inner most satellite of Jupiter Io were not satisfying Cassini 1668 Tables.
The tables were saying the eclipse should happen at a certain time, and the kind of error was eight minutes in time. And he then figures out that if you look at the pattern of the delays, it's entirely a function of how far Jupiter is from the Earth. And that makes him propose light travels at a finite speed, and to propose a value for the speed of light.
That's what Cameron was referring to. Had he used his numbers honestly, he would have gotten a pretty good number. He did some rounding and some self-protecting and as a result he has light travelling from the Sun to the Earth, I think, it is 11 and a fraction minutes.
He actually does it in around eight minutes. But he could have done somewhat better. The key thing is he's announcing is that a new correction is needed on all astronomical observations for the speed of light. One that nobody had noticed before. Cassini rejected this. Actually Cassini was the first to propose that there was a speed of light, but then he couldn't see the same faults in the other three satellites.
And so he decides, no, no, something's going on with the IO. And he puts in a little epicycle to correct it. Huygens strongly supports the idea that this is actually speed of light, this is a real effect. By then, Huygens is a strong advocate of the wave theory of light, and waves don't travel instantaneously.
So, all of this makes him very happy. It's striking how long Cassini holds on. The best I can tell and I learned most of this from Cameron. More of this from Cameron than anyone else. So Steve Weinberg in the book I was helping him with. Has a whole half a chapter on this.
I did send him your essay. And we had back and forth on why the three outer satellites weren't showing the same effect. But it's almost certainly a penumbra effect. That is, the inner most satellite is going to have a sharp shadow that it goes through. As you move further out, of course you have rays crossing.
And so there's not a single sharp shadow, there's an intermediate shadow between the two. The penumbra and the umbra. And that was probably making it very uncertain exactly when the eclipses were happening. That was at least Huygens' suggestion and I think Roma proposed it too as I Recall in their exchanges.
Cassini continued to hold on that there's no need to correct for speed of light, all the way into the 1690s. There's an article by Halley and Phil transactions when Cassini comes out with new tables for Jupiter's satellites. And in so many words says, how can Cassini be this dumb to continue to insist that light doesn't have a finite philosophy.
And continued to mess up all of astronomy by insisting on that rather than changing things. But it's not small thing now to realize Now what you're looking at happened in the case of Saturn, ten or 12, 15 minutes before you're looking at it. And adjustments have to be made for that.
So that's a whole new correction that has to be put in, that the elder Cassini's very, very slow to adopt, for what it's worth. But this is a major achievement for Romer and it had a significant effect on Cartesian Philosophy because Descartes had proposed light is an instantaneous velocity.
And its instantaneous transfer and that just killed that theory entirely and Huygens again emphasized that quite strongly at the time. So that's another major step happening in astronomy, and notice what made it happen. Cassini had such good orbital theory for the satellites of Jupiter that you could pick up a discrepancy.
And once you had the discrepancy, then it's telling you something about the world you didn't know before. That's what we are going to keep seeing the better the orbital theory the more you discover from its inadequacies things you didn't know before. But its gotta get more and more precise in order to do that.
Fair enough? Bromer ends up going back to Denmark. I think, I'm guessing here, at least in part, because Cassini and he differed on this, and that meant it made him less popular with Cassini. he ends up being one of the people instrumental in developing the first thermometer, Fahrenheit his countryman, gets the primary credit, but Romer was working on it as well.
So he's a more important figure than you may have ever heard of.