99 points + brain describe how the phases of the moon depend on the positions of earth and the moon.

The Moon’s position depends on its position relative to Earth. This means that the way we see the Moon from Earth is a product of the Moon’s position. The different phases we see are actually not from Earth’s shadow cast like many people think. The Moon reflects the Sun’s light. The sun is always illuminating one side of the Moon, but sometimes we can only see the side that is unlit. The positions then come into play because as the Moon moves along in its orbit, the sunlight falls differently on it. This is why we see different shapes, like a thin crescent.