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Here's an image that describes the phenomenon I'm asking about. The very thin sliver to the left is of course the surface of the moon as lighted by the sun. But the rest of the moon is also faintly lighted -- by what?
I'm guessing that some amount of "earthshine" -- sunlight reflected from the earth -- is the culprit here, but I've found no authoritative confirmation online. It would be great to have a link to a source that explains what's really going on.
The dark parts of the Moon are partially illuminated by "Earthshine". That is sunlight which is reflected from Earth to the Moon. Just like the ground on Earth is lit up a bit by the Moon at night, so is the ground on the Moon lit up a bit by the Earth in the lunar night. And even more so because Earth is much larger in the lunar sky, than is the Moon in our sky. And the surface of Earth is actually more reflective per area unit than is the surface of the Moon.
It's poetically called The Old Moon in the New Moon's Arms.
Project Earthshine used measurements of the brightness of the non-sunlit portion of the moon to accurately determine earth's albedo:
A global and absolutely calibrated albedo can be determined by measuring the amount of sunlight reflected from the Earth and, in turn, back to the Earth from the dark portion of the face of the Moon (the "earthshine" or "ashen light"). For more than a decade we have been measuring the Earth's large-scale reflectance from BBSO. The observations are now done remotely utilizing our earthshine coronagraph under the small dome appearing to the right of the NST dome in the Figure 1. To get full coverage of the Earth, we have installed a carefully calibrated copy of the BBSO earthshine telescope in Tenerife.