The observer on Earth sees the phases of the Moon because the Moon reflects sunlight, and the amount of illuminated surface visible from Earth changes as the Moon orbits our planet.

Here's a breakdown:

* The Moon doesn't produce its own light. It reflects sunlight.

* The Moon orbits Earth. As it orbits, the angle at which sunlight hits the Moon and the angle at which we see the illuminated portion changes.

* Different phases correspond to different angles. A full moon occurs when the entire sunlit side faces Earth, while a new moon occurs when the sunlit side is facing away from Earth.

Think of it like this: Imagine holding a ball in your hand and shining a light on it. As you turn the ball, the amount of light reflecting towards you changes. The Moon is doing something similar as it orbits Earth.