A moonbow (also known as a lunar rainbow, white rainbow, lunar bow, or space 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.
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.[1] However, the colors in a moonbow do appear in long exposure photographs.
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A true moonbow is lit by the Moon itself, not the Sun. In contrast, a colored rainbow (or a white fogbow) seen when the sun is rising or setting, or in twilight, is not a moonbow because it is still produced by sunlight.
Moonbows have been mentioned at least since Aristotle's Meteorology (circa 350 BC), and also in an 1847 publication.[2] The term moonbow was used by Nick Whelan, who sighted one of the first documented moonbows in eastern Utah.
A moonbow occurred on July 26, 2010, at Kalapana on Hawaii's Big Island (Hawaii), seen from near an active lava flow.
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 true moonbows, other than those produced by waterfalls or sprays, 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.
Few places in the world frequently feature spray moonbows. Such sites in the United States include several waterfalls in Yosemite National Park, California; Cumberland Falls, near Corbin, Kentucky;[3][4] and Waimea on the Hawaiian Island of Kauai. Victoria Falls, in Africa on the border between Zambia and Zimbabwe, is also widely known for spray moonbows.
