Retroreflection
A subtlety of the Lunar surface is that it reflects light better directly back at the source, or within a degree or two. this is why the lunar limb is not darkened so much by curvature and why the full moon is so bright.
Before Apollo they didn't understand why, but now we know that lunar surface dust has a complex structure from micrometeorite impact that makes it pitted, rough, o0r even "tunnelled".