  We use the GEANT4 Monte Carlo framework to calculate the gamma-ray albedo of
the Moon due to interactions of cosmic ray (CR) nuclei with moon rock. Our
calculation of the albedo spectrum agrees with the EGRET data. We show that the
spectrum of gamma rays from the Moon is very steep with an effective cutoff
around 4 GeV (600 MeV for the inner part of the Moon disc). Apart from other
astrophysical sources, the albedo spectrum of the Moon is well understood,
including its absolute normalisation; this makes it a useful "standard candle"
for gamma-ray telescopes, such as the forthcoming Gamma Ray Large Area Space
Telescope (GLAST). The albedo flux depends on the incident CR spectrum which
changes over the solar cycle. Therefore, it is possible to monitor the CR
spectrum using the albedo gamma-ray flux. Simultaneous measurements of CR
proton and helium spectra by the Payload for Antimatter Matter Exploration and
Light-nuclei Astrophysics (PAMELA), and observations of the albedo gamma rays
by the GLAST Large Area Telescope (LAT), can be used to test the model
predictions and will enable the GLAST LAT to monitor the CR spectrum near the
Earth beyond the lifetime of PAMELA.
