This is science. Moon is a reflecting object, like you and me.
Sunlight is "pure" light, it means it's got every colored light into it, explained its white color. Therefore, Moon can only show light if it's receiving light that's got its own color of light, which Sunlight got. Which explains why Moonlight is more efficace in Sun weather.
Now it doesn't make sense that the move is effective without sun in first lieu.