1. Clouds cover the part of the moon that we cannot see.
2. Planets cast shadows on the part of the moon that we cannot see.
3. The shadow of the sun falls on the moon, blocking our view of it all.
4. The shadow of the earth falls on the moon, blocking our view.
5. The phases are explained in terms of the portion of the illuminated side of the moon visible
from the earth (scienticfic viewpoint).