  It is shown that the complete description of the propagation of light in a
gravitational field and in non-inertial reference frames in general requires an
average coordinate and an average proper velocity of light. The need for an
average coordinate velocity of light in non-inertial frames is demonstrated by
considering the propagation of two vertical light rays in the Einstein elevator
(in addition to the horizontal ray originally discussed by Einstein). As an
average proper velocity of light is implicitly used in the Shapiro time delay
(as shown in the Appendix) it is explicitly derived and it is shown that for a
round trip of a light signal between two points in a gravitational field the
Shapiro time delay not only depends on which point it is measured at, but in
the case of a parallel gravitational field it is not always a delay effect. The
propagation of light in rotating frames (the Sagnac effect) is also discussed
and an expression for the coordinate velocity of light is derived. The use of
this coordinate velocity naturally explains why an observer on a rotating disk
finds that two light signals emitted from a point on the rim of the disk and
propagating in opposite directions along the rim do not arrive simultaneously
at the same point.
