Moonbows, also known as lunar rainbows, are indeed rare atmospheric phenomena. These nocturnal displays are created by moonlight instead of the direct light from the sun. The scarcity of a moonbow sighting is directly related to the specific environmental and celestial conditions that must align for the light to become visible. A lunar rainbow demands a precise combination of factors, which is why most people live their entire lives without seeing one.
The Formation of Moonbows
A moonbow forms through the same optical principles as a solar rainbow. The light source is the moon, which reflects the sun’s light toward Earth. When this moonlight encounters a high concentration of water droplets suspended in the atmosphere, the light refracts as it enters the droplet. It then reflects off the back inner surface and refracts again as it exits, spreading the white light into its component colors. The observer must be positioned between the light source and the water droplets to see the resulting arc. Since the moon’s light is significantly dimmer than the sun’s, the resulting moonbow is much fainter.
Why They Are Infrequently Observed
The infrequency of moonbow sightings stems from the requirement that three specific conditions must occur simultaneously. First, the moon must be at or very near its fullest phase, typically within two or three days of the full moon. This maximizes the light intensity needed to illuminate the water droplets sufficiently for human perception. The sky must also be exceptionally dark, free from any competing light sources.
Light pollution or thin cloud cover can easily wash out the moonbow’s faint light, making remote locations with minimal artificial illumination ideal. A source of water droplets, such as rain, mist, or sea spray, must be present directly opposite the moon. The moon itself must also be low in the sky, positioned less than 42 degrees above the horizon, otherwise the arc will be cast below the horizon.
Prime Viewing Locations and Timing
Due to the need for consistent atmospheric moisture, the most reliable places to see a moonbow are near powerful, mist-generating waterfalls. Cumberland Falls State Resort Park in Kentucky is one of the few locations in the Western Hemisphere where moonbows are regularly predicted. The heavy, consistent spray from the falls provides the necessary water screen opposite the moon.
Victoria Falls, on the border of Zambia and Zimbabwe, is another location for predictable lunar rainbows. These sites offer a natural advantage that overcomes the dependence on random rainfall. The optimal time for viewing is during the two to three nights surrounding the full moon, often observed a few hours after sunset when the moon is at its lowest angle.
How the Human Eye Perceives Lunar Color
While a moonbow contains the full spectrum of colors, it typically appears white or silvery to the naked eye. This is a physiological effect related to how the human eye processes low-intensity light. The retina contains two types of photoreceptor cells: rods, which detect shape and brightness, and cones, which are responsible for color perception.
Moonlight is approximately 400,000 times less bright than sunlight, providing insufficient illumination to stimulate the cone cells effectively. Under these dim conditions, the rod cells take over, causing the visual experience to be almost monochrome. If captured with a camera using a long exposure, the sensor collects enough light over time to reveal the hidden colors of the spectrum.