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.
The name 'orrery' comes from the man who commissioned the first one - Charles Boyle, earl of Orrery. Oddly, it's not named after the man who invented it - George Graham - or the man who built it - John Rowley. Another example of the rich taking credit for something because they had the money to buy it - not because they had the brains to think it up in the first place.
Also, try to say the name out loud five times fast. It's not easy. And you'll look insane.