OK? But we're not calling archery bows and violin bows circular; we're calling them bows i.e. curved. And we're calling the rainbow a bow, i.e. curved, which it is. Curved does not imply circular, but circular does imply curved.

Besides, I don't think the proto-indo-europeans were out there with calipers measuring the precise curvature of objects they decided to label with the *bheug- root.