The different phases of the moon are caused by the changing positions of the moon, Earth, and sun. As the moon orbits Earth, the amount of sunlight that reflects off of it changes, causing the moon to appear to change shape.

When the moon is between Earth and the sun, the side of the moon facing Earth is not illuminated, and we see a new moon. As the moon moves around Earth, more of the side facing Earth becomes illuminated, and we see a waxing moon. When the moon is on the opposite side of Earth from the sun, the entire side facing Earth is illuminated, and we see a full moon. As the moon continues to move around Earth, less of the side facing Earth becomes illuminated, and we see a waning moon.