That's the Arc of 124.5°. It was a gift to Berlin from Paris, and it represents the 124.5° arc from Paris to Berlin.
So I spend the following week trying to mind my own business, because I don't know anything about Paris and Berlin, and the guide must know more than me. Right?
Nope. I can't get it out of my head. From what axis is there an arc of 124.5° from Paris to Berlin? It ain't the center of the Earth, that's for sure. We're looking at Austria or the Atlantic. Google doesn't yield much on the artwork.
So far, research suggests:
There is no significance in the degree of the arc. It is “art”."Arc of 124.5°." Feh.
Benar Venet lived in Paris.
He played with wood and tar.
He discovered bending metal was easier than bending wood.
He is not really liked by mathematicians.