The colour of the dark side of the Moon

Aims. “Earthshine” is the dim light seen on the otherwise dark side of the Moon, particularly when it is close to new. “Earthlight”, or reflected sunlight from the Earth, is the source of Earthshine. Using B and V band CCD images of both the dark and bright sides of the Moon, we aim to estimate the Johnson photometry B − V colour of the Earthshine for the first time since the late 1960s. From these measurements we are also able to quantify the colour of Earthlight. Methods. We present images of the Moon taken with a small refractor in Hawaii, in B and V bands and taken under favourable conditions so that scattered light in both bands almost completely cancels, yielding a map of the surface in B − V colour. Co-addition of 100 such images taken in rapid succession substantially improves the signal-to- noise ratio, and several sources of photometric bias are eliminated by use of relative methods. Results. The earthlit dark side of the Moon is observed to be 0.150 ± 0.005 mag bluer in B − V than the sunlit bright side, in good agreement with the only known previous measurement of this quantity from 1967. Arguing on the basis of the change in B − V for sunlight reflected once off the Moon, we derive a colour for earthlight of B − V = 0.44 ± 0.02 mag (without applying a small, uncertain, phase-dependent reddening correction). The absence of a colour-gradient in the B − V image implies that the scattering properties of the atmosphere+optical system are almost exactly matched in the two wavelength bands, the consequences of which are discussed