Last spring I took an atmospheric science class. I still have the book, because it was too interesting to sell back. I was looking through it, and read this:

"Although solar radiation travels in a straight line, the gases and dust particles in the atmosphere can redirect this energy. This process, called scattering, is responsible for illuminating a shaded are of a room when direct sunlight is absent. Scattering also produces the so-called diffused light that accounts for the brightness of the daytime sky. In contrast, bodies like the moon and Mercury, which are without atmospheres, have dark skies and "pitch black" shadows, even during daylight hours." [all bold, italic and quotations are from the text, not added by me]

Wow. I had forgotten about that. Apparently, though, I had stumbled across that almost a year ago, because, for the benifit of the next owner if I sold it, I had written, "Not true. Dust and gases are not responsible for shadow illumination. Illuminated objects around the shadows are responsible. The moon and Mercury do have black skies, but not all the shadows are pitch black."

If anyone cares, it's from the textbook "The Atmosphere" by Fredrick K. Lutgens and Edward J. Tarbuck, sixth edition.