An orrery is a clockwork device that's meant to show the rotation of the earth, moon and sun - although it might include other planets, depending on its size. Extremely hard to pronounce for people who can't say their 'r's, orreries became popular in the 18th century as a conversation piece. No English gentleman's library was complete without one. Also, try to say the name out loud five times fast. It's not easy. And you'll look insane.