I'm reading @standupmaths 's new book "Love Triangle" (not available yet in the states, but I bought a copy from him in the UK), and I get to the chapter on angles, where he refers to an erroneous picture of a rainbow as "more of an oval". But it's not an oval (two arcs connected by straight lines) but an ellipse, which would be possible in a universe where optical properties varied by direction. Or, a real photo with an anamorphic lens! Now I'm motivated to take such a photo.