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Most people don't realize it, but a rainbow is actually a full circle. You usually can't see the full circle, since half of the rainbow lies beneath the horizon, where it is not raining. After all, there is no weather underground. However, if one is in an airplane or overlooking a waterfall, the 360-degree rainbow can be seen. I know--I have seen them twice from research airplanes that were flying through rain showers. Check out the 360-degree rainbow image at the bottom. Note that there is a separate optical phenomena, called a glory (caused by diffraction), surrounding the shadow of the airplane. Unfortunately, I only had a 23mm wide angle lens, and could not capture the entire 360-degree rainbow. To my knowledge, no one has captured a photograph of a full NATURAL
360-degree rainbow. You can easily photograph one using a sprinkler, as this photographer has done here
I challenge all you wunderphotographers to capture a 360-degree rainbow image in rain or waterfall mist. First photographer to post such a natural 360-degree rainbow image wins a free 2-year wunderground.com membership!
Update: In 2013, a photographer on a helicopter in Australia captured a beautiful picture of a 360-degree rainbow, posted at NASA's Astronomy Picture of the Day