The sun is the most reliable way to create the gorgeous light display we call a rainbow, but it's not the only way. After all, the moon illuminates the Earth, too — and rainbows are essentially an optical illusion caused when water droplets split light into its ROY G. BIV components. But seeing a "moonbow" isn't exactly easy.
How rare is this nighttime meteorological phenomenon? Well, Aristotle wrote around 350 BCE in his treatise Meteorologica that "it was formerly thought that [rainbows] never appeared by night as a moon rainbow. This opinion was due to the rarity of the occurrence… we have only met with two instances of a moon rainbow in more than fifty years." So, pretty rare. That's because for a moonbow to form, you need a variety of conditions to be Goldilocks-level perfect. First, the moon must be low in the sky, and can't exceed 42 degrees from the horizon. The moon must be full or near full, and you can't be hanging around any artificial light — sorry, no moonbows in cities. Finally, just like rainbows, moonbows need water droplets in the atmosphere, so waterfalls are often a good spot to go hunting for moonbows. Just don't expect to see the dazzling array of color typical of a daytime rainbow. Because the moon isn't as bright as the sun, less light is refracted, and a moonbow usually looks white (at least to human eyes). But if you have a camera handy, long exposure photos will reveal a moonbow in all its colorful glory.
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