This is a moonbow, also known as a lunar rainbow: a rainbow produced by light reflected off the surface of the moon refracting off moisture-laden clouds in the atmosphere. Moonbows are relatively faint, due to the smaller amount of light reflected from the surface of the moon. They are always in the opposite part of the sky from the moon. Because the light is usually too faint to excite the cone color receptors in human eyes, it is difficult for the human eye to discern colors in a moonbow. As a result, they often appear to be white. However, the colors in a moonbow do appear in long exposure photographs. This particular moonbow was taken at a spot that is famous for them -- Victoria Falls, on the Zambesi at the border between Zambia and Zimbabwe. The moisture-laden clouds are regularly generated by the continuous spray from the falls.

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