Thomas Pynchon has suggested a rainbow’s true shape is not parabolic, but circular.  Well, thanks to this photo taken from the window of a Thai Airways jet, we now have some documentation:

The picture shows the ring-shaped spectrum against a backdrop of cumulocirrus clouds.  Rainbows are formed when sunlight strikes the curved inside of a raindrop at a specific angle and is reflected back through the water, creating a prism effect.  The apparent semicircle of a normal rainbow is only limited by the horizon.  The full circle could be seen if the viewer were standing on a sufficiently high cliff, although it is more easily seen from aircraft.

Rainbows are long said to have had a profound religious and mythological significance.  Before they were explained scientifically, they were described in the Bible as a symbol of God?