Passage IV

Elusive Lunar Rainbows

People have always been fascinated by rainbows. In fact, nearly every culture has legends about them. The first person initially to offer a scientific explanation of rainbows was René Descartes. In 1637 he correctly hypothesized that rainbows occur when suspended water droplets refract light, separating it into an arc-shaped spectrum of colors. Typically you look for rainbows when the sun shines through rain, but under the right circumstances, a bright full moon can supply enough light to create a lunar rainbow.

Lunar rainbows, or "moonbows," are little known and, in that way, are like some other aspects of the natural world. The moon must be full and low in the sky, and rain must be falling. Also, because moonlight is less powerful then sunlight, lunar rainbows are faint and easily obscured by surrounding lights and air pollution. (52)

Just as the sun is behind you when you see a solar rainbow, the moon is behind you when you see a lunar rainbow. When a moonbow appears in front of you, its center lies in the direction exactly opposite that of the moon. Although moonlight strikes the falling rain and separates its light into the same spectrum of color present in a solar rainbow, a lunar rainbow is likely to be perceived as a delicate white arc against the dark night sky, and which poorly illuminated objects appear colorless at night. In fact, the human eye loses its sensitivity to color at low levels of illumination, a camera with a very long exposure setting can capture a lunar rainbows' colors.

Like solar rainbows, the observers of lunar rainbows are easier to find near waterfalls. Tourist bureaus in places such as Victoria Falls in Zimbabwe list lunar rainbows as local attractions. Moonbows were reportedly once a common sight at both the New York and Canadian parts of Niagara Falls before bright surrounding lights made them impossible to see. You can still witness them at Yosemite Falls in California and at Cumberland Falls in Kentucky.