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A moonbow (also known as a lunar rainbow or white rainbow), is a rainbow produced by moonlight rather than sunlight. Other than the difference in light source, its formation is exactly the same as for a solar rainbow: It is caused by the refraction of light in many water droplets, such as a rain shower or a waterfall, and is always positioned in the opposite part of the sky from the moon relative to the observer.
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Moonbows are much fainter than solar rainbows, due to the smaller amount of light reflected from the surface of 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, a moonbow often appears to be white. However, the colors in a moonbow do appear in long exposure photographs.
Moonbows have been mentioned at least since Aristotle's Meteorology (circa 350 BC), and also in an 1847 publication.
Viewing
Moonbows are most easily viewed when the moon is at or nearest to its brightest phase full moon. For moonbows to have the greatest prospect of appearing, the moon must be low in the sky (at an elevation of less than 42 degrees, preferably lower) and the night sky must be very dark. Since the sky is not completely dark on a rising/setting full moon, this means they can only be observed 2 to 3 hours before sunrise (a time with few observers), or 2 to 3 hours after sunset. And, of course, there must be rain falling opposite the moon. This combination of requirements makes moonbows much rarer than rainbows produced by the sun. Moonbows may also be visible when rain falls during full moonrise at extreme latitudes during the winter months, when the prevalence of the hours of darkness give more opportunity for the phenomenon to be observed. One good location for viewing 'true moonbows' is Waimea 'Kamuela', Hawaii Island, Hawaii.
Locations
Numerous places in the world feature spray-, fog- or mist-induced bows. In the United States such bows may be seen in relation to various waterfalls including Yosemite National Park, California and Cumberland Falls, near Corbin, Kentucky. Victoria Falls, in Africa on the border between Zambia and Zimbabwe and Plitvice Lakes in Croatia is also widely known for spray moonbows.
Spray moonbows are also seen with some regularity in the cloud forests of Costa Rica, in mountain towns like Monteverde and Santa Elena. These occur when clouds of mist are blown in from the Caribbean by the Christmas Winds. The Christmas Winds happen from the end of December through late January or early February. These clouds of mists create a streaming pattern of stripes giving rise to their popular name in Spanish of Pelo de Gato or Cat's Hair. Moonbows happen in this part of Costa Rica almost every full moon in the months of December through February. The bows that are caused by Pelo de Gato are not limited to just before dawn but can happen after sunset too, but it does need a full or nearly full moon. Moonbows are also found in Kauai, with the moon rising in the east, during light rain, but must be captured by a time-exposure photo, as they appear white to the naked eye of most people.
Similar bows are occasionally seen from the Kohala districts on the Big Island of Hawaii.