From Wikipeadia
A moonbow (also known as a lunar rainbow, lunar bow or white rainbow) is a rainbow produced by light reflected off the surface of the moon rather than from direct sunlight. 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.
It is difficult for the human eye to discern colors in a moonbow because the light is usually too faint to excite the cone color receptors in human eyes. As a result, they often appear to be white.[1] However, the colors in a moonbow do appear in long exposure photographs.
A True Moonbow appears white, and is lit from the Moon itself. A colored rainbow when the sun is setting or when it is darker out is not a Moonbow because it is still partially lit from the remaining light in the sky. The term Moonbow was coined by Nick Whelan who sighted one of the first documented Moonbows in Eastern Utah.
A colored circle around the moon is not a moonbow—it is usually a 22° halo produced by refraction through hexagonal ice crystals in cirrus cloud. Colored rings close to the moon are a corona, a diffraction phenomenon produced by very small water droplets or ice crystals in clouds.
Moonbows are most easily viewed when the moon is near to full (when it is brightest). For other than those produced by waterfalls, the moon must be low in the sky (less than 42 degrees and preferably lower) and the sky must be dark. And of course there must be rain falling opposite the moon. This combination of requirements makes moonbows much more rare than rainbows produced by the sun.
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