Here's why:

* Lunar Phases: The moon doesn't produce its own light; it reflects sunlight. As the moon orbits Earth, the amount of sunlight reflecting off its surface changes, creating the different lunar phases we see.

* Full Moon: During a full moon, the entire sunlit side of the moon faces Earth. This means we see the entire disk of the moon illuminated, making it appear as a complete circle.

It's important to note that the moon isn't actually perfectly circular. It's slightly elliptical. However, from Earth, we often perceive it as a full circle during the full moon phase.