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Descartes, lunar eclipses, and the speed of light

  1. May 3, 2009 #1
    Hello all.

    I was doing some research on something or other (i can't rightly remember what right now) and ran across Descartes' claim that the speed of light had to be infinite because if it wasn't the co-linearity of the sun moon and earth during a lunar eclipse wouldn't exist. (that is to say that during a lunar eclipse, unless the speed of light was infinite, the sun moon and earth would be out of alignment.) We all know now that Descartes was mistaken, but my question is why did he make that claim to begin with? Pretending that it takes light one hour to reach the earth from the moon (as Descartes did), it would be true (although, to my knowledge unverifible as no information can travel faster than light (whatever that speed may be)) that at the time we observed the eclipse, the moon would be out of alignment, but we would have no way of observing that. A star that collapsed into a black hole 10 years ago still looks like a star to us as long as we are more than 10 light-years away.

    What was Descartes' line of thinking? Or am I missing something?


  2. jcsd
  3. Dec 18, 2010 #2
    yes please! I am doing a project on the speed of light and when I came across that I also was very puzzled. Even thought many websites talk about the descarte eclipse thing, they all say something like "but the celestial bodies were in a straight line" without explaining how any person could possibly come to this conclusion.
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