The moon is only round a few nights a month, so the scribes drew it the way they usually saw it: a crescent. Just as 日 became "day", 月 became "month" — one full cycle of the moon. When you see 月 inside other characters, it often whispers "flesh" instead, from a look-alike ancient drawing of ribs.
A piece of culture
Mid-Autumn moon The moon is family in Chinese culture. At Mid-Autumn Festival (中秋节), families gather under the full moon, eat round mooncakes, and think of relatives far away — because everyone, everywhere, sees the same moon. The poet Li Bai raised his cup to the moon 1,300 years ago and made it the most famous drinking companion in Chinese poetry.