Moonbow: The Rainbow That Rises by Moonlight

Wait at the base of the falls on a clear night when the moon is full, and a pale arc will lift out of the spray, colorless at first glance, a rainbow with the volume turned almost all the way down. You will not be sure you are seeing it until you are. People who catch their first moonbow tend to whisper, as though speaking might scatter it.

A moonbow is exactly what the plain reading suggests: a rainbow produced by moonlight instead of sunlight. The physics is identical. Light enters water droplets, bends, reflects off the back of the drop, and bends again on the way out, splitting into a spectrum and arcing across the sky opposite the light source. The only difference is the lamp. The Moon does not make its own light; it reflects the Sun, and it reflects only a sliver of it, which is why a moonbow is roughly hundreds of thousands of times fainter than its daytime cousin.

Why moonbows look white

The faintness is the whole story of how a moonbow appears. Human eyes have two kinds of light sensor: cones, which see color but need decent brightness, and rods, which work in the dark but are effectively colorblind. In the dim light of a moonbow the cones barely fire, so the rods do most of the seeing, and the rods report a pale, ghostly band with little color. The colors are physically there; a long-exposure photograph will pull out the full spectrum that the naked eye cannot. That gap between what the camera records and what you can see is part of why moonbows feel slightly unreal.

Where to find one

Moonbows need a rare stack of conditions: a bright, near-full moon low in the sky, a dark night, and water in the air opposite the moon, which is why the reliable spots are big waterfalls. Cumberland Falls in Kentucky is famous for producing moonbows regularly on clear full-moon nights, one of the few places on Earth where they can almost be scheduled. Victoria Falls on the Zambia and Zimbabwe border throws them from its enormous spray, and Yosemite's waterfalls generate lunar spray-bows in spring. Rain-based moonbows happen too, but they are matters of luck.

Old sky, old meanings

Because they are dim, strange, and easy to miss, moonbows have long invited more than optics. Aristotle described them more than two thousand years ago. Across cultures a rainbow at night, appearing where the mind does not expect one, has been read as an omen, good or ill, a sign hung in the dark. The honest correction is that a moonbow foretells nothing; it announces only that the moon is bright, the air is wet, and you happen to be standing in the right place at the right hour.

The optics cover the arc completely. They are quieter about the particular nights people describe, the moonbow that appeared the evening something changed for them, the one a whole group swears stood brighter and more colored than any camera should allow. Those are stories, not spectra. They are what our readers sort through.

Below, people have filed their own moonbow sightings and the nights that came with them, voting each True, Fake, or Not sure. Watch the pale arc, then decide.