  Astrophysical cosmology constrains the variation of Newton's Constant in a
manner complementary to laboratory experiments, such as the celebrated lunar
laser ranging campaign. Supernova cosmology is an example of the former and has
attained campaign status, following planning by a Dark Energy Task Force in
2005. In this paper we employ the full SNIa dataset to the end of 2013 to set a
limit on G variation. In our approach we adopt the standard candle delineation
of the redshift distance relation. We set an upper limit on its rate of change
|G dot / G| of 0.1 parts per billion per year over 9 Gyrs. By contrast lunar
laser ranging tests variation of G over the last few decades. Conversely, one
may adopt the laboratory result as a prior and constrain the effect of variable
G in dark energy equation of state experiments to delta w < 0.02. We also
examine the parameterization G ~ 1 + z. Its short expansion age conflicts with
the measured values of the expansion rate and the density in a flat Universe.
In conclusion, supernova cosmology complements other experiments in limiting G
variation. An important caveat is that it rests on the assumption that the same
mass of 56Ni is burned to create the standard candle regardless of redshift.
These two quantities, f and G, where f is the Chandrasekhar mass fraction
burned, are degenerate. Constraining f variation alone requires more
understanding of the SNIa mechanism.
