Full moon explanation with diagram

We only see the Moon because sunlight reflects back to us from its surface.

Depending on earth's, moon's and sun's positions we see a different area of the moon being illuminated by the sun. The changing positions appear to us as different moon phases.

When the earth, the moon and the sun are almost aligned, most of the surface of the moon (as seen from the earth) is illuminated by the sun. Therefore, the moon appears as "full". This occurs approximatelly every 29.53 days

Full moon, earth, sun diagram
Diagram/image credits: StarChild team at Nasa

Moon phase image If you are interested in the phases of the moon, download the StarMessage screensaver for Microsoft Windows and Apple Mac OS X.
The screensaver displays the moon in its current moon phase, reminds you about the upcoming full moon dates, shows you the names of the full moon, and does much more.