If rainbows are rare, moonbows are even more unusual. They form from light being reflected off the surface of the moon, rather than from direct sunlight. Several factors must be in place for a moonbow to be visible. The moon must be full or within two days of becoming full (when it is brightest). With the exception of moonbows generated by waterfalls, the moon must be low in a dark sky. Water vapor must also be present in the air. Like rainbows in relation to the sun, moonbows always appear in the opposite part of the sky from the moon.
Fascination of the Lunar Rainbow
Traditionally, moonbows, or lunar rainbows, most often appear in three locations across the world: Victoria Falls in Africa, Yosemite (California) Falls and Cumberland (Kentucky) Falls in the U.S. Niagara Falls on the U.S.-Canadian border has also been the scene of a few sightings, but conditions are not frequently favorable there. Recent land moonbows have mainly been detected over Pacific islands: Tahiti, Australia and Hawaii.
As far back as 300 B.C., Aristotle wrote about witnessing the phenomenon. Americans Benjamin Franklin and John Muir described their observations of this rare and beautiful sight in their works. Naturalist Muir urged visitors to seek to view the Yosemite Falls moonbow, which he called "one of the most impressive and most cheering of all the blessed mountain evangels," in his 1912 book The Yosemite.
Why It Is Sometimes Called a White Rainbow
Many people are not able to discern all the colors in a moonbow. It depends on the number and sensitivity of the cone color receptors in an individual's eyes. If you have good night vision, you should be capable of making out most of the colors. If not, the rainbow may appear to be white. Even when the moon is at its brightest, moonbows are fainter than rainbows due to the lower quality of the moon's light. Luckily, the colors in a moonbow will show up in long exposure photographs.
Capturing a permanent record of a moonbow is the aim of most who visit the waterfalls where they are likeliest to materialize. Photography tips abound to prevent winding up with a white blur. Using a tripod, switching to manual focus, bracketing exposure time and first finding your shadow are among the suggestions.
Predicting Yosemite's Moonbows
Inspired by Muir's writings and striving to enable more people to experience this rarity of nature, a group of astronomers at Texas State University-San Marcos began in 2005 to develop a computer program to predict the dates and times favorable for the formation of moonbows at Yosemite Falls. The Texas State team, led by Professor Donald Olson, conducted extensive on-site surveying and topographical research before establishing the criteria necessary for a moonbow appearance.
Their study indicates that six criteria must be present to produce the ideal condition for manifestation of this unusual event: clear skies around the moon, abundant mist at the falls' base, dark skies, bright moonlight, moonlight not blocked by anything, and correct geometry. The importance of the last factor became apparent when they examined data about an expected appearance that seemingly failed to occur shortly before their research began. Because of the geometry involved, they discovered the moonbow did appear at 12:45 a.m. the next morning - but long after the photographers had given up and gone home.
The team's predictions for this year and the next are available online at Texas State University.
Sources
Blaschke, Jayme. "Mastering Yosemite's Moonbow Mystique." txstate.edu. Accessed March 11 2010.
Fabricius, Karl. "The Elusive Beauty of Lunar Rainbows." environmentalgraffiti.com. Accessed March 7 2010.
Walklet, Keith S. "How to Photograph a Lunar Rainbow." anseladams.com. Accessed March 12 2010.
Mary Simpson - Mary Simpson holds a BA in journalism and an MA in English from the University of North Texas. After living in all of the major cities of ...
Join the Conversation