So I was in class the other day and my professor mentioned that we can prove that light is invariant using a binary star system. His reasoning:
Assume a binary star system 1000-light years away from Earth, with the stars orbiting counterclockwise.
Assume stars rotating at 0.003c and stars positions at top and bottom of orbit. The time light takes to get from top star to earth is longer than the bottom star.
My question is how does this show that light is invariant?