A planetarium is a special building, or a round hall inside a museum, where you sit in the dark and look up at a curved ceiling. Lights on that ceiling make a picture of the night sky: stars, planets, the Moon, and how they move through the year. It is not the real sky. It is a show, often with a teacher or a recorded voice telling you what you are seeing. People go there to learn about space on a rainy night, or in a bright city where real stars are hard to see. Though actually, speakers also use planetarium for the machine that throws the lights, not only for the building. It is a school-and-museum word. You would not use it for standing in a field and looking up.