A 1:2 billion scale model of the solar system laid out through the city as a walking route. The sun is the gilt ball on top of the spire of the Jacobikirche; Mercury, Venus, Earth and Mars are clustered near the market square; the outer planets get progressively further out, with Neptune and Pluto on the city's northeastern edge. A nice way to see Göttingen if you have a couple of hours and like the conceit.
A leaflet and map are available at the tourist office in the Altes Rathaus.