D&D 5E How would you best represent astronomy skills in D&D?

[Ancalagon]

Aye. I would not add more skills.

But I find tools are far more nebulous in their place in the rules, that adding in endless numbers of them doesn't matter. Especially since they can be easily trained in downtimeand it's not like any of them other than thieves' tools seems to have any significant impact on the "average" game.

Yup, I think it's a decent avenue to solve this.

The Sage - Astronomer background and skill check of d20 + Intelligent + Proficiency Bonus.

Without the background, I would just do an Intelligence check. No proficiency bonus

Perhaps a background would be the best?