I'm sorry if this is a dumb question. Recently I was learning that it's impossible for us to measure the speed of light in one direction. We can only measure it in two directions and we assume that it's speed is the same in both directions by convention.


But we have been able to directly observe the effects of the finite speed of light since the 1600s, when astronomers tried to use Io as a way to measure time without a clock and found that it appeared to orbit slightly faster while the Earth was moving towards it and slightly slower while moving away from it, for a total maximum discrepancy of 16 minutes. This delay is not from light bouncing off of Io's surface, since it does so in both situations. The delay is only from the light needing to travel an extra 186 million miles between the two measurements.

So my question is, is this second example an actual measurement of the speed of light in one direction? I don't see why one source says we can't measure light in one direction while the second source suggests that we can. Please help me clear up this contradiction.


Yes, it is often assumed that Rømer measured the speed of light in one direction. It may seems trange, but Rømer velocity is also the velocity obtained under the tacit assumption of the equality of the speeds of light in opposite directions. The fact of the matter is that Rømer and Cassini were speculating about the movement of Jupiter’s satellites, automatically assuming that the observers’ space was isotropic.

The Estonian - Australian physicist Leo Karlov showed that Rømer actually measured the speed of light by implicitly making the assumption of the equality of the speeds of light back and forth.

L. Karlov, “Does Roemer's method yield a unidirectional speed of light?” Australian Journal of Physics 23, 243-258 (1970)


L. Karlov “Fact and Illusion in the speed of light determination of the Roemer type” American Journal of Physics, 49, 64-66 (1981)

Some reflections on the one-way speed of light are here.

Another interesting method to measure one - way speed of light that you may discover soon or later was so - called Double Fizeau Toothed wheel. That is two toothed wheel attached to opposite sides of long rotating shaft and a beam of light between the teeth. This method was employed (probably without proper due - diligence) by S. Marinov and M. D. Farid Ahmet.

However, Herbert Ives in his 1939 article "Theory of Double Fizeau toothed wheel" predicted that outcome of the measurement will be exactly c due to relativistic twist of the rotating shaft.


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