Common nouns are simply words that denote people, places, things, or ideas: a car, a tree, your neighbor, a bus, a house. They are not the actual NAMES of people, places or things.
The word “state” is a common noun because it could refer to any state in America. The word “Texas” is a proper noun because if refers to a specific state in America.
When I look at my lawn, I see grass (common noun). A specific grass, Couch grass, would be a proper noun.
When I look up, the general, common noun for what I see is 'sky'. There is no proper noun for any specific name.