The moon was so bright last night, that it could be photographed hand-held. 1/200 of a second at f/8, to be exact. For those at home keeping score, that's bright. There's no denying it's beauty, but there's also a burning question: if it's so bright, why can't the full roundness of the moon be seen? Perfect! Glad you asked. Last night, the moon was at about 3/4 visible, as you can see in the photo. This happens when the moon is more than half past earth to either side, as figure 3.1 illustrates.
Now, add the moon's position to our vantage point from earth, and then throw in some curvature, and the result is that only 3/4 of the moon is hit by the sun's light. So, here it is: the difference between what is in the light and what isn't is so large that what's not in the light becomes invisible. Let's look at another example.
This image was taken in a studio. We can all agree that there are at least four walls that make up this studio. The reason they can't be seen behind the kind gentleman above is because they're well outside of the light needed for this specific exposure. They are there, though, just as the moon is round. Capisci?