Extinction Shift Principle© : The Details On the Velocity of Light A clear distinction is to be made between these two very different terms:     a) The velocity of light Constant     b) The Constancy of the velocity of light The parameter c is that of the velocity of light constant which has been measured very accurately to be about 299,792,458 meters per second in vacuum.  There are additionally many issues pertaining to whether this constant has had different values at earlier times or in different regions of the universe. But, the constant c is not the issue here at all. The real issue here is that of the constancy of the velocity of light in all frames of reference.  The primary question remains:  Does the true velocity with which electromagnetic waves and gravitation move in a given frame of reference depend on the motion of its primary source of a different frame of reference?  Does the Galilean transformation of velocities apply to both electromagnetism and gravitation?  These questions have been answered with the affirmative in the mathematical proof of this emission theory.   Click anytime on Glossary of Terms for definitions.   Relativity assumes that all velocities of emission are constant in all frames of reference.  The addition of velocities  are according to the Lorentz  transformation of velocities, which is actually based on and is derived from the Galilean transformations by requiring that all velocities of emissions are constant in all frames of reference.  Taking x' and t' from the Lorentz transformations of x and t, the transformation of velocities as required by Relativity is gotten from u'=x'/t' as . The Extinction Shift Principle assumes that the emissions and all re-emissions occur at constant velocity c in the frames of reference of their most primary sources only. All emissions will have the additive velocity  c' in all other frames of reference other than that of the primary source.  Hence, the additive velocity would be c+v for an approaching primary source, which moves with velocity v relative to that chosen frame of reference. The velocity would be c-v for a receding primary source. The addition of velocities are according to Galilean transformations