David Palmer and Tim Kallmanfor the Ask an Astrophysicist team, http://imagine.gsfc.nasa.gov/
Question: Why does the Moon sometimes come out during the day?
Answer: It might be useful to think of the Sun as a large light bulb, and the moon as a large mirror. There are situations where we can't see the light bulb, but we can see the light from the bulb reflected in the mirror. This is the situation when the moon is out at night. We can't see the Sun directly because the earth is blocking our view of it, but we can see its light reflected from the moon. However, there are also situations where we can see both the light bulb and the mirror, and this is what is happening when we see the moon during the day. You can explore this for yourself with a light and a hand mirror. Depending on which way you face (away from the light or sideways to the light) you can see either just the mirror, or both the light and the mirror.
