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What Is A Moonbow? All About Lunar Rainbows

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.

When’s the next full Moon? Be sure to check out our full Moon dates and times!

Have you ever spotted a Moonbow? Tell us in the comments below.

Tiffany Means is a freelance writer and a degreed meteorologist. She specializes in weather forecasting and enjoys making the subject of weather (and the science behind it) more relatable. She currently resides in the Blue Ridge Mountains of North Carolina. Her article, Animals with Accidental Green Thumbs appears in the 2021 Farmers' Almanac.

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Marcia Tullous

My family and I just saw our first Moonbow last night! It was amazing! I had never even heard of a moonbow before!

monique

I’ve seen moonbow just yesterday evening for the first time, in Croatia..didn’t knew it existed! It was just a spot but it was lovely.

urvi

ohhh this is so going to help me for my project!

Susan Higgins

Oh, good! We’re glad we can help, Urvi!

Queen Bee Tink

I saw a MoonBow last night. My husband and 3 boys did not.. so they thought I was crazy. It was clear to me. At one point I saw red, orange,yellow, green, blue and violet. Mostly though the red and violet I did not see. According to Chick Trainor’s comment ..there was less than a finger measurement width from moon to colored ring ( I didn’t know that would mean something .. but was trying to get my husband to see it!) well .. we just had a very bad storm with CLoud to Ground lighting!

Wondering if only certain people with eye conditions can see these Moonbows ? It was even much brighter without my glasses on but no one else saw it

Chick Trainor

Thanks for the information,Tiffany Means. Here is something related, for you. The other day a local native Eskimo here in Nome,Alaska told me about a weather rule another man told him. That man was also an Eskimo and he was for many years a weather service observer on the radio here. It relates to the ring around the moon. When at night you observe the Moon with a ring around it, extend your flat hand up and out,with fingers together in front of your face,fill the space from the edge of the Moon out to the inside of the ring. If you can only fit three fingers between those two locations then you only count three in your calculation…if 4 fingers,then you calculate using 4. Each finger will represent 4 days….so…if it’s 3 fingers you’ve used,it is 3×4(days)…or 12 days total. If it is 4 fingers then it will be 4 times 4 days or 16 days total. Whatever it comes out to be…for these examples either 12 days or 16 days, that is the time you have left before the next storm.

Lee Michael

Interesting fact – never knew that.

Chick Trainor

Indigenous people are tied to their surroundings. That historically has been a life or death decision especially in the winter.

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