Halo, 22-degree halo, Solar Halo, 22 degree solar halo - aka (incorrectly) Rainbow, Full Circle, 360 Degree, Round, Circular, Whole. Directly overhead. Morro Bay, CA. 12 June 2009.
Halo, 22-degree halo, Solar Halo, 22 degree solar halo - aka (incorrectly) Rainbow, Full Circle, 360 Degree, Round, Circular, Whole, SUnbow. Directly overhead. Morro Bay, CA. 12 June 2009. Photo by Michael "Mike" L. Baird, mike at mikebaird d o t com, flickr.bairdphotos.com, Canon 1D Mark III, Canon EF 16-35mm f/2.8L II USM Ultra Wide Angle Zoom Lens handheld, circular polarizer (not to much effect). Used Liveview to shelter eyes from the sun.
Tech note: ChuckThePhotographer asked "ISO=50?" at the circular rainbow at www.flickr.com/photos/mikebaird/3621017728/ - that was due to the fact that I was shooting directly into the center of the sun, and was doing everything I could to not overexpose the shot... turns out at f/22 I got 1/80th and could have gone for a faster capture - but it was almost impossible to see anything without burning my eyes out. I basically went on manual, set it to lowest ISO and tightest shutter I could get, and dialed thru 10 different duration exposures and picked the best. BTY the polarizing filter was on but did not seem to do much - may have had some value as a neutral density filter to mask the bright light.
Mystical note: en.wikipedia.org/wiki/Circular_rainbow says of the special way of Wu-Wo Tea Ceremony and its Seven Principles - its symbol:
A "circular rainbow" is the symbol for the Wu-Wo Tea Ceremony. A rainbow is made of seven basic colors which, when combined together, become blank white light. The Wu-Wo Tea Ceremony pursues the circular rainbow and goes towards the blank light or empty circle in the flag's center. The blended white light or empty circle of the flag’s rainbow can be thought of as “wu”. The Wu-Wo Tea Ceremony follows this concept: for example, people have knowledge, wealth, and appearance, and try to forget these things and transcend.
Science Note: Please see below where EarthSky: A Clear Voice for Science
says "strictly speaking, this isn't a rainbow. There is such a thing as a full-circle rainbow, but this isn't it. You only see what's technically called a 'rainbow' on the side of the sky that's opposite the sun. What you have here is on the same side of the sky as the sun - so purists wouldn't call it a rainbow. Purists and experts would call this a 'halo.' It's actually what's called a 22-degree halo. Wikipedia has more: en.wikipedia.org/wiki/22%C2%B0_halo "
23 Nov 2009 Creative Commons use note in a cool tech-oriented blog by high school teacher Larry Baker.