I confess I didn't follow the reasoning as Sagan presented it. Even if the Earth is flat you'd still get a shadow from any tower that the sun isn't directly above. He said you'd get no shadow from both towers on a flat Earth, which is wrong. The sun can only be over one of them. He was probably stoned again.
Maybe dude couldn't reconcile the excessive length of shadow with trig and posited a curved surface hypothesis. In addition to the fact that ships disappear over a horizon and any night you go outside and look up you see a big ass round moon with shadowing that indicates a sphere, so it's not that far out a possibility to come up with.
