In the same vein as Harry Potter's "you-know-who", "he-should-not-be-named", and such, sometimes certain words become taboo, even when in private conversation.
In a tribe, what could cause celestial object name become a taboo?
I'm planning to have a tribe with many restrictions on referring to celestial objects. They know the existence of sun, moon, and stars, and have the words for them, but refuse to use it even in writing and conversation. They simply refer to them with pronouns, such as "it" or "them".
"Do you want to watch it set?" "No, we better watch it rise tomorrow."
"They are so misaligned this night. Something bad is bound to happen soon."
I imagine the tribe has nomadic life in a desert. They encounter the sun, moon, and stars everyday, but never to refer to them directly.
Note: This is set in a fantasy world, and there's quite substantial magic around. Mentioned just in case it is relevant to your answer.