  The cosmological expansion of the universe affects the behaviour of all
physical systems and, in the case of gravitationally bound ones, could
correspond to or mimic a time dependent Newton's constant. Here we discuss the
case of a locally spherical mass distribution embedded in a generic Robertson
Walker universe. Choosing the most appropriate metric tensor for the problem
and assuming that the local time scale is much much lower than the cosmic one,
we show that G is practically unaffected thus leaving the absolute magnitude of
type Ia supernovae unaltered at all epochs.
