What Is A Moonbow? All About Lunar Rainbows

Mar 2, 2022advice

Tiffany Means for the Farmer’s Almanac on Moonbows.

Next time you’re out enjoying the splendor of the full Moon, see if you can spot this other night sky wonder: the Moonbow!

Full Moons. Supermoons. Blue Moons. Each of these gives us a welcome excuse to gaze up at the nighttime sky. But while you’re enjoying the view, don’t forget to look beyond the Moon for a chance to glimpse another night sky wonder: the Moonbow.

How Does A Moonbow Form?

In the late 1700s, English poet William Cole put his description of a Moonbow into verse:

 The atmosphere with humid vapors flow/ And the moon displays her lunar bow.

Today, we refer to Moonbows—or “lunar rainbows” as they’re sometimes called—as rainbows that occur at night.

Like rainbows, a Moonbow forms when light—moonlight rather than sunlight—shines on water droplets. As the light passes through the droplet, it bends or “refracts,” bounces or “reflects” off the back of the droplet, and finally, exits the droplet after bending a second time. It is this bending of light which causes it to split into its individual color wavelengths of red, orange, yellow, green, blue, indigo, and violet.

However, don’t expect these colors to appear as vividly in Moonbows as they do in daytime rainbows. Since the Sun is 400,000 times brighter than a full Moon, a Moonbow’s colors tend to be fairly dim. So dim, in fact, that the light is often too weak to be picked up by the color-detecting cells in our eyes, which means we see more of a white rainbow (white being the combination of all of light’s visible colors).

How To Spot One

Moonbows are fairly rare, but if you happen to catch a full Moon when the following conditions are also in place, you’ll increase your chances of seeing one:

  • The Moon is near the horizon.
  • There’s rainfall or mist (spray from waterfalls and oceans) nearby.
  • There’s a not-too-cloudy sky (clouds can obscure moonlight and your view).
  • The sky is dark (city lights can drown out a dimly lit Moonbow).

Because they provide a constant spray of mist, the world’s waterfalls, including Victoria Falls in South Africa, are prime spots for Moonbow-watching.

Moonbow Look-Alike

Lunar Haloes

Lunar halos are other circles of light around the Moon. But don’t be fooled! While halos are also created when Moonlight refracts off of water in the atmosphere, this water is in the form of frozen ice crystals in cirrus clouds—not liquid droplets. Another difference between the two is that a halo creates a ring around the Moon while the center of a moonbow’s arc appears in the direction opposite of the Moon.


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