It looks like we were all wrong. I got an answer from Raymond Lee, Research Professor at the U. S. Naval Academy. His comment:
. . .Your photograph shows a rather nice circumzenithal arc (abbreviated CZA), an often spectacular
ice-crystal halo. Because all halos are caused by ice crystals rather than by raindrops, the
CZA should not be called a rainbow, although that is most observers' reasonable first guess.
For some photographs and an optical explanation of the CZA, I highly recommend my
colleague Les Cowley's Atmospheric Optics website at
http://www.atoptics.co.uk/halo/cza.htm Note that the moon is far too weak a light source
to contribute to a visible CZA when the sun is above the horizon.
Following his lead, I looked at Wikipedia which has a photo that is more intense than mine:
Nice to learn something new when it's real cool . . .
