Daughter loves to look at the moon. We find the window in the house that we can see it from, and we just stare at it and talk. Mostly just "It's so beautiful!" kind of stuff.
I think this is her first "obsession" that we're going to take as a learning experience. I've heard from homeschooling friends that they allow their children to follow passions and research them. This doesn't seem like "school" to the kids then and they learn a lot because it's what they want to be studying.
So, next time at the library, we're getting moon books! :)