Letter from Thomas Gold to Edward Eisner, Bell Telephone Laboratory, New Jersey Explaining to Mr Eisner what 'aberration' is i.e. 'Aberration is not related to the motion of the source of the light; it is not given by the relative velocity between source and receiver. Instead it is merely the change in angle of a light-ray described in one coordinated system when you change to one in a different state of motion. Thus, on the earth half a year later you are adopting a different (inertial, near enough) system and you have to make the necessary correction. That correction is aberration, and it is identically for all light-rays in your new system, since all light-rays, no matter ... what source, lie on the same light cone in your coordinate system'. 10 March 1966