Update 7/22/11 - This photo has been shortlisted as a finalist in the the Astronomy Photographer of the Year 2011 competition!
The light of the full moon creates a lunar rainbow in the mist of Yosemite Falls in Yosemite National Park, California. The moon only creates these night rainbows on a few days per year, when it is close to full, the waterfall is flowing high enough to produce adequate small droplets of mist, and when the angles line up just right.
Fortunately astronomer Don Olson of the University of Texas forecasts when the conditions and alignments are most likely to be favorable.
I already had a number of moonbow photos from Yosemite Yalley, so for this image I decided to go a step or two further and include star trails. To provide maximum control over momentary issues such as the headlights of autos, I captured over 500 individual exposures over the course of several hours to produce this resulting image.