In the quiet hours after sunset, an atmospheric spectacle occasionally appears, mirroring the familiar daytime rainbow but with an ethereal glow. This rare event occurs when conditions align perfectly, allowing light to be refracted and reflected through water droplets in the darkness. The phenomenon relies on the same physical principles as its daytime counterpart, but the source of illumination is vastly different, leading to a much fainter and more elusive display.
The Name of the Nighttime Rainbow
The correct name for this nighttime arc is a moonbow, also referred to as a lunar rainbow. It is a true rainbow, caused by the dispersion of light within airborne water droplets. The distinction is that the light source is the Moon, rather than the Sun. Moonbows are significantly rarer than solar rainbows because they require an extremely specific alignment of astronomical and weather conditions.
The Mechanics of Lunar Rainbow Formation
The formation of a moonbow is governed by the same laws of optics that create a solar rainbow, involving the refraction and internal reflection of light inside water droplets. Moonlight is reflected sunlight, which must pass through moisture like rain, mist, or spray. The light enters the droplet, bends, reflects off the back surface, and bends again upon exiting, dispersing the light into its component colors.
A fundamental requirement is that the Moon must be low in the sky, typically less than 42 degrees above the horizon. The observer must be positioned with their back to the Moon, facing the water droplets. This geometry ensures the light rays are scattered back toward the observer’s eye at the specific angle required to form the arc. The Moon must also be at or near its brightest phase, meaning a full or nearly full moon is necessary to provide enough illumination.
Why Nighttime Rainbows Appear White
Despite the underlying presence of a full spectrum of colors, moonbows typically appear colorless, or a ghostly white, to the unaided human eye. This visual effect is due to the low intensity of the light source, as moonlight is far dimmer than direct sunlight.
Human vision relies on two types of photoreceptor cells in the retina: rods and cones. Cone cells detect color but require high light intensity to be activated. The dim light of a moonbow is usually insufficient to stimulate these cones. Instead, the highly sensitive rod cells, which cannot perceive color, are primarily responsible for the visual sensation. This scotopic, or night vision, causes the arc to be perceived as a monochromatic, white band.
Optimal Conditions for Viewing
Seeing a moonbow requires a precise combination of astronomical timing and local meteorological conditions. The highest probability of visibility occurs when the Moon is within a few days of being full, maximizing the reflected light. It is also necessary to have a dark sky, meaning minimal light pollution that could overwhelm the faint lunar light.
The essential environmental factor is the presence of airborne water droplets, which can come from steady rain or a dense mist. Moonbows are most reliably observed near large waterfalls, where a constant, heavy plume of spray is guaranteed.
Well-known locations for frequent sightings include Yosemite National Park in California, Cumberland Falls in Kentucky, and Victoria Falls on the border of Zambia and Zimbabwe. These sites offer the necessary mist and clear, dark skies.