What is a Moonbow?
What is a Moonbow?
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. Moonbows have been mentioned at least since Aristotle’s Meteorology (circa 350 BC).
What color is a Moonbow?
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.
Where can you find a Moonbow?
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 very dark. Since the sky is still light on a rising full moon, this means they can only be observed 2 to 3 hours before sunrise, a time with few observers. 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.
Notable Moonbow Locations
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; 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.
Are there “fake” Moonbows?
A true Moonbow is lit by reflected light from the Moon, not directly by 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. Additionally, 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 mMon are a corona, a diffraction phenomenon produced by very small water droplets or ice crystals in clouds.
Some of this article uses modified material from the Wikipedia article “Moonbow“, which is released under the Creative Commons Attribution-Share-Alike License 3.0.