Circular Rainbow on Wendelstein Mountain.

From Earth Picture of the Day archive:

This nearly circular rainbow was observed from atop of Wendelstein Mountain (1.835 m or 6,020 ft) in Germany in March of 2000. Because we're used to seeing rainbows as graceful arcs and not circular structures, we're tricked into believing that they end when they appear to come in contact with the horizon. However, as viewed from aloft, especially from an airplane, rainbows are indeed circular structures. In the 1600s, Rene Descartes demonstrated that the rainbow is a complete circle with a radius of approximately 42 degrees, centered about the antisolar point (point in the sky opposite the Sun). The dome of the Wendelstein Observatory is seen as lower left.

http://epod.usra.edu/archive/epodviewer.php3?oid=296928