In order to determine the relative motion between the Earth and the ether (the medium through which light supposedly propagated. It has zero density and complete transparency), scientists used the concept of the aberration of starlight. If the Earth frame of reference was not dragging the ether frame of reference, then the direction of starlight as seen from both frames of reference separately would be different. Whereas if the Earth frame of reference was indeed dragging the ether frame of reference, then the starlight observed from both frames of reference separately would appear to come from the same direction. This is all intuitive. But how do we prove whether either is the case, when we are unable to identify the ether frame of reference itself, and conduct experiments within this frame of reference to determine the observed direction of starlight from within this frame of reference?