Moonbow

Photograph of a spray moonbow (lunar rainbow)

A moonbow (also known as a lunar rainbow, black 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) refracting off of moisture in the air. 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.

True 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.

Moonbows have been mentioned at least since Aristotle's Meteorology (circa 350 BC), and also in an 1847 publication.

False moonbows

Not a moonbow—partial corona formed by moonlight diffracted by droplets in a cloud

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 minuscule water droplets or ice crystals in clouds.

Moonbow viewing

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 spray moonbow locations

Spray moonbow at the Lower Yosemite Fall

Few places in the world frequently feature spray moonbows. Such sites in the United States include several waterfalls in Yosemite National Park, California and Cumberland Falls, near Corbin, Kentucky. Victoria Falls, in Africa on the border between Zambia and Zimbabwe, is also widely known for spray moonbows. Moonbows have also been spotted at Waimea on the Big Island of Hawaii.

See also

• Moon ring
• Moon dog

External links

• High resolution images in Wikimedia Commons
• "Maui's Night Sky" Time elapse photography by Wally Pacholka. Maui No Ka 'Oi Magazine Vol.14, No.3 (May 2010)
• Lunar Rainbow photos from Victoria Falls
• Moonbow picture
• Moonbow picture made with long exposure
• Moonbow in New Zealand
• Moonbow predictions – Lower Yosemite Falls – as calculated by Don Olson at Texas State
• Moving rainbow over Patagonia, all night time lapse movie
• Kentucky State Park Moonbow Dates At the bottom of the page under "Moonbow" tab
• Moon Light Effects: Moon Rings, Moon Dogs And Other Moon Light Phenomena...
• Moonbow on Big Island Hawaii
• Moon Rainbow picture taken in the middle of Nevada during a rain storm.