In the following paper from I. Bernard Cohen, "Roemer and the First Determination of the Velocity of Light (1676)" published on Isis (1940):

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Can be read:

explaining that the delay would arise from the simple fact that astronomers considered light to be propagated instantaneously rather than gradually

I'm not sure I understand what this means, Roemer is defending that light is not instantaneous. Isn't the delay meant to prove that the light propagates gradually?

  • 1
    $\begingroup$ Yes it is. Perhaps a bit awkward in the wording, but the astronomers were not expecting the delay since they thought it would be instantaneous. Roemer did expect the delay since he thought light had a velocity of propagation. $\endgroup$
    – Jon Custer
    Commented May 24, 2023 at 20:27
  • $\begingroup$ Indeed. Is the wording what is misleading me. I read it as: "explaining that the delay would arise from [cause] rather than [cause that you are rejecting]" $\endgroup$
    – Jon
    Commented May 25, 2023 at 14:47

1 Answer 1


What Roemer was says is that the astronomer's predictions were based on an infinite speed of light, but this is not correct: The speed of light is finite and the extra time it takes light to cross Earth's orbit around the Sun results in the eclipses observed when Jupiter is near to superior conjunction (i.e., on the opposite side of the Sun from Earth) happening later than predicted based on eclipses when Jupiter is at opposition (i.e., straight out away from the Sun as viewed from earth) because of extra time the time takes to cross the earth's orbit.

He describes this as a delay because he is compared what is predicted from the timing at opposition and an infinite speed of light.

A diagram would explain this clearly, but this margin is too small to contain one.


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