Why does a half-moon look straight?
I haven’t read the other answers. I was just struck by your “look up” comment. No one is doubting that you see a straight edge on the moon. What they mean is that the moon phases aren’t caused by the Earth’s shadow. If you look at the moon a few days after New Moon, you’ll see it’s a thin crescent following the sun down in the evening sky. If it’s in the same part of the sky as the sun, then it can’t be the Earth’s shadow blocking the sun’s light over the rest of it. The moon always has half its surface sunlit. The half-moon looks that way because it’s “next to” Earth, compared to the sun. Because of that, only half of the sunlit side is facing us.
