As we will show in Sec. II D, the direction of the phase velocity
up is the same as the direction of the wave vector k
and hence from Eq. (12) is the same in all inertial frames. On
the other hand, both the speed and direction of the group velocity
ug depend on the velocities of the receiver, emitter,
and medium (for waves that travel in a medium). Aberration
is a property of the group velocity, not the phase velocity,
and hence there is no contradiction between the aberration
effect and Eq. (12). A derivation of non-relativistic aberration
is given in appendix A.