Science Puzzle
Why Does the Moon Change Shape?
Over a month the Moon seems to change shape: a thin crescent, then a half, then a full bright disc, then back down to nothing. Something is clearly changing.
What actually causes the phases of the Moon?
The Answer
The Sun always lights exactly one half of the Moon, the half facing it. That never changes. What changes is our viewing angle as the Moon travels round Earth once a month.
When the Moon sits between us and the Sun, its lit half points away from us and we see a dark new moon. When Earth sits between the Sun and the Moon, the whole lit half faces us: full moon. In between, we catch the lit half side on and see a half or a crescent. The Moon is not changing shape at all; we are simply seeing its sunlit side from a different direction each night.
The common wrong answer is Earth's shadow. That does happen, but only rarely, and we call it a lunar eclipse. It is not what makes the ordinary monthly phases.
The principle: Lunar phases come from geometry, not shadow. Half the Moon is always lit; the phase is how much of that lit half we can see from Earth.