I'm trying to get a better handle on the second postulate of relativity. I've read that Einstein adopted it because Maxwell's equations appeared to suggest that this was the case. I just read the below quote in another thread The speed of light is measured, or defined, as 299792458 m/s; but the 's', or "the second" in that measurement is defined in terms of the oscillations of a caesium atom, in an atomic clock, at rest relative to the earth. Does this not mean then, that the speed of light is, by definition, relative to a clock at rest on earth? I'm also, wondering, how is the [assumption of the] second postulate actually tested?