Then that's mostly non-mechanical. You can just say that he knows the stars--I wouldn't make you roll for it except under exceptional circumstances.
I see skills as being much more related to the use of the knowledge. Are you trying to navigate? Then Navigator's Tools. Are you trying to determine the religious significance of a pattern? Religion. Are you trying to determine the magical significance? Arcana. Are you just knowing the stars for role-play purposes? No skill needed. Just like you don't need History to know the leader of the town you grew up in.