I’d say that it has to be far enough from the ground that you wouldn’t notice the parallax effect if you walked around below it, it has to be above the horizon. Also, it can’t be an airplane or something. I’m not sure why exactly that last rule is there, given that meteors and such count. Maybe most people would consider it part of the sky. I’d say it’s in the sky, but not part of it.
I’d say that it has to be far enough from the ground that you wouldn’t notice the parallax effect if you walked around below it, it has to be above the horizon. Also, it can’t be an airplane or something. I’m not sure why exactly that last rule is there, given that meteors and such count. Maybe most people would consider it part of the sky. I’d say it’s in the sky, but not part of it.