I subscribed to an online Page-A-Day calendar last year, and for Valentine's, they gave me a code to subscribe to one for free this year. So I opted for the Fact or Crap calendar. Browsing January, I got the answer to a puzzlement:
Why do you see the whole moon even when it's new? Y'know, you look up, see that bare line of a crescent, but can also make out the rest of the circle, the rest of the moon. Why isn't the dark part completely invisible?
Earthshine, that's why. Just like the moon, the Earth reflects sunlight, too - enough to let you make out the whole moon even when it's dark. The strength of the earthshine depends on Earth's cloud cover.
I knew the Earth reflected light, like the moon, if not as well; I just never realized that was why a new moon is visible.
UPDATE: There was a derailment in the comments; two trains of thought couldn't stay on the same track. My American pop culture references do not extend to TV-series of the 1950's so Mark had to explain, which he has done here.