  We determine the MS-bar charm quark mass from a charmonium QCD sum rules
analysis. On the theoretical side we use input from perturbation theory at
O(alpha_s^3). Improvements with respect to previous O(alpha_s^3) analyses
include (1) an account of all available e+e- hadronic cross section data and
(2) a thorough analysis of perturbative uncertainties. Using a data clustering
method to combine hadronic cross section data sets from different measurements
we demonstrate that using all available experimental data up to c.m. energies
of 10.538 GeV allows for determinations of experimental moments and their
correlations with small errors and that there is no need to rely on theoretical
input above the charmonium resonances. We also show that good convergence
properties of the perturbative series for the theoretical sum rule moments need
to be considered with some care when extracting the charm mass and demonstrate
how to set up a suitable set of scale variations to obtain a proper estimate of
the perturbative uncertainty. As the final outcome of our analysis we obtain
m_c(m_c) = 1.282 \pm 0.006_stat \pm 0.009_syst \pm 0.019)_pert \pm 0.010_alpha
\pm 0.002_GG GeV. The perturbative error is an order of magnitude larger than
the one obtained in previous O(alpha_s^3) sum rule analyses.
