A moonbow, alternatively known as a lunar rainbow or moon rainbow, occurs when moonlight, instead of sunlight, produces a rainbow. While the light source differs, the formation process is identical to that of a solar rainbow. The creation of a moonbow involves light being reflected off water droplets in the atmosphere, typically caused by rain or waterfalls. The position of a moonbow is always opposite to the moon from the observer's viewpoint.
A 10pm blood red night rainbow over Scotland! Image credit: Stefan Lee Goodwin
Moonbows have been mentioned in historical records, dating as far back as Aristotle's Meteorology (around 350 BC). These nocturnal rainbows are noticeably less vibrant than their daytime counterparts due to the moon's surface reflecting less light. This makes it more challenging for the human eye to distinguish colors in a moonbow as the light is typically too dim to stimulate our color receptors. Consequently, moonbows often appear white to the naked eye, but their colors can become visible in long-exposure photographs.
The optimal viewing conditions for moonbows occur near the full moon phase, when the moon is at its brightest and unobscured by clouds. For a moonbow to materialize, the moon should be low in the sky, less than 42 degrees above the horizon, and the night sky should be extremely dark. Given these conditions, moonbows are typically witnessed two to three hours before sunrise or after sunset. Also, the necessary atmospheric water droplets, originating from rain or mist, should be present opposite the moon.
Moonbows have been a subject of awe and wonder across cultures. In Hawaiian legends, they are considered a good omen. In other cultures, they are seen as a divine connection between the earthly and celestial realms.
These specific conditions make moonbows a much rarer occurrence than solar rainbows, with moonbows happening less than 10 percent as frequently. Moonbows can sometimes be seen during the winter months at full moonrise, when the sky is darker and rainfall is more prevalent at extreme latitudes. The clarity of the moonbow's colors depends on the size of the atmospheric water droplets; smaller droplets result in less vivid colors.
Aside from rain, moonbows can also be triggered by spray, fog, or mist. In the United States, these unique bows can be observed around various waterfalls, including Niagara Falls in New York, Yosemite National Park in California, and Cumberland Falls near Corbin, Kentucky. Victoria Falls, situated on the border between Zambia and Zimbabwe, is also well-known for its spray-induced moonbows.
How to spot a moonbow?
As noted above, moonbows are only visible for about 3 days around full moon, when viewed against a dark sky near the end of evening twilight, or before sunrise. In middle latitudes, the best time for moonbows is summer full moons, when the Moon spends more time low in the sky. In other seasons, moonbows may last only an hour.
In showery weather, always look for a pale moonbow when the Moon is low and bright. You probably won’t see much colors, but if you mount a camera on a tripod, you can capture the colors easily.
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