I've seen one like this, only indoors. The point is that a free pendulum goes back and forth in a 2-dimensional plane, but the Earth revolves under it. It's even possible to time things with it. It does have something in common with a Spirograph (wonder why there's not a software version out there?..anyone know of one?..;)

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)