The speed of light in opposite directions with respect to rotating observers is greater or less than c. Because the effect involves rotation and therefore non-inertial reference frames, there is no conflict with Special Relativity. As this discussion demonstrates, however, something still strikes some people as "funny" about it, if I may put it that way. One of the funny things is how some authors like to avoid saying the "speed" is not c, that it's a change of path length or time interval, etc. I prefer those authors who don't do that funny dance, but come right out and admit that for rotating observers the speed of light is generally not c.
A key to the problem is the fact that the light path returns to itself without "reflection," but rather by maintaining its "withness" or "againstness" with respect to the rotation, from emission to return. So, in a sense, it is a "one-way" speed measurement...actually an AVERAGE of a one way speed. A Michelson interferometer mounted on the rim of the rotating body will always get a null result because it measures a TWO-WAY light path by reflection.
Another funny thing is how, even though the general, global circumstance demonstrates the non-c speed of light for rotating observers, the special, LOCAL circumstance (demonstrated by the Michelson interferometer) is that the speed of light is isotropically c. What's funny is that logic usually works the other way around. If something is generally, globally so, then specially, locally it may SOMETIMES not be so, but cannot always be so. In Relativity, the Special Theory is regarded as always locally valid regardless of the global circumstance. I'm not saying that it's not, of course. I'm just saying that the logic leading to this conclusion is a little bit funny. Seeing this, one can more readily understand why this subject comes up over and over again.
