Bewitched.

Harry looked upward and saw a velvety black ceiling dotted with stars. He heard Hermione whisper, “Its bewitched to look like the sky outside. I read about it in Hogwarts, A History.”

It was hard to believe there was a ceiling there at all, and that the Great Hall didn’t simply open on to the heavens.

J.K. Rowling, The Sorcerer's Stone