Here's why:

* Lunar Phases: The moon doesn't emit its own light; it reflects sunlight. As the moon orbits the Earth, the angle at which sunlight hits the moon changes, creating different phases.

* Full Moon: During a full moon, the entire face of the moon that faces Earth is illuminated by the sun. This makes it appear as a bright, round circle.

Other times, the moon might appear as a crescent, a half-circle, or a gibbous shape, depending on its position in its orbit.