"De Sitter pointed out that some... pairs of stars have orbits aligned so that each star is either coming towards us or going away from us as it goes round its orbit. If the speed of light is modified by the motion of the star then 'faster' light from positions when the star is approaching the Earth could catch up with 'slower' light emitted earlier when the star was moving away from us in its orbit. There would then arise the possibility of multiple ghost images. Such ghost images have not been observed... de Sitter concluded that Einstein was right... it has been argued that one needs to be careful in accepting de Sitter's binary star argument... because of a phenomenon called 'extinction.' ...To avoid this problem, the binary experiment has... been repeated with stars emitting X-rays... experiment conclusively confirmed Einstein's postulate."