Moonbow: The midnight show

Carla Marie Segarra Guerra

After the sun goes down and the stars come up, the show begins. Moonbows are more than a rainbow and almost magic… 

Moonbows are sunlight reflected off the surface of the moon. This event is a very rare phenomenon. Moonbow are form when the light that is reflected from a full moon is diffracted when interacting with spherical water droplets suspended in a rain cloud or the mist of a waterfall. When the light goes through the drop, a rainbow or a moonbow is the way our eyes see that light. Since ancient time, this beautiful phenomenon has been studied. Rene Descarte in “Discours de la Method” explains this process. He writes: "Considering that this bow appears not only in the sky, but also in the air near us, whenever there are drops of water illuminated by the sun, as we can see in certain fountains, I readily decided that it arose only from the way in which the rays of light act on these drops and pass from them to our eyes.” Unlike the colorful rainbows produced by the sun´s light in a rainy day, moonbows appear white due to the loss of brightness which is not able to activate the cone color receptors in the eyes. The only way a moonbow will look like a rainbow is in a picture or sometimes under extreme clean conditions. For a moonbow to occur there must be good weather conditions with clear skies and a full moon.

Moonbows are also known, as lunar rainbow, white rainbow and space rainbow. They can only be observed on a dark night, in places far away from city lights. A moonbow is created by the indirect light from the Sun. This produces a decrease in light intensity, producing moonbows which are almost imperceptible. In addition, the loss of intensity could be further eclipsed by added light luminosity, depriving the possibility of witnessing a moonbow.

Without a full moon this would not probably happen. It is essential in order to reflect plenty of light to create the moonbow. The Moon must be positioned in a 42° angle, or less from the horizon. Moonbows usually appear in the part of the sky opposing the Moon with the viewer behind the Moon. The full moon, the dark night and the perfect angle are needed to appreciate it. Because a moonbow is a rainbow, all the natural laws governing a rainbow apply. But all this physics isn’t as important as simply understanding that your shadow always points toward the center of the rainbow/moonbow; this spectacular will only appear when the sun/moon is 42 or fewer degrees above the horizon (assuming a flat horizon) the higher the sun/moon, the lower the rainbow or the moonbow. When the sun or the moon is above 42 degrees, the “show” disappears below the horizon.

Also, the waterfall provides the medium against which the light reflected off the moon will interact in order to produce a moonbow.. There are famous places in which mooonbows are known to become visible, including Cumberland Falls and Yosemite National Park in the U.S. Victoria Falls in Africa and Waimea Canyon in Hawaii

Two people will never see the same moonbow, is a special distribution of colors in reference to the viewer point. Only in a picture two persons would see the same arc of colors. Moonbows are fascinating, magical, unexpected, special for each viewer, a one in a lifetime experience. The lunar rainbows are more rare than rainbows created by the Sun, because they depend on more factors other than just sunlight. 

References: 

The National Center for Atmospheric Research & the UCAR Office of Programs http://eo.ucar.edu/rainbows/ (Accessed: April 2013)