Moonbows are rare rainbows produced by moonlight instead of direct sunlight.

Moonbows, scientifically known as lunar rainbows, are formed through the same optical process as solar rainbows: refraction, reflection, and dispersion of light in water droplets. However, they are significantly rarer because they require a specific set of conditions to align. The moon must be at or near its brightest full phase and positioned low in the sky, typically at an altitude of less than 42 degrees.The surrounding sky must be extremely dark, and there must be a source of water droplets, such as rain or the mist from a waterfall, opposite the moon. Because the light intensity of the moon is much lower than that of the sun, the light is often too weak to excite the cone receptors in the human eye. This results in the scotopic vision threshold where we perceive the bow as a ghostly white arc rather than a spectrum.Long-exposure photography can reveal the hidden colors by allowing more photons to hit the camera sensor over several seconds. Famous locations for consistent moonbow sightings include Cumberland Falls in Kentucky, USA, and Victoria Falls on the border of Zambia and Zimbabwe. At Victoria Falls, the 'lunar rainbow' is a major tourist attraction that occurs for three days during each full moon when the spray from the Zambezi River is high.Research published in journals like 'Applied Optics' notes that the moon's light is roughly 400,000 times fainter than the sun's. This explains why the phenomenon is so elusive and why the human eye struggles to see the colors. Despite the lack of perceived color, the physical properties of the light spectrum remain identical to those of a standard daytime rainbow.