An observatory is a special building where scientists watch the sky — the moon, the stars, planets, and sometimes the weather. It often sits on a hill or a mountain, and the roof may open so a big telescope can look up. Though actually, not every observatory is for stars. Some watch the weather; some listen for radio signals from space. In everyday English, if someone says observatory they almost always mean the star-watching place. It is a place for looking carefully, not a museum and not a school, even if school groups visit.