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Moonbows: What Is a Lunar Rainbow? | The Old Farmer's Almanac

Moonbows: What Is a Lunar Rainbow?

A moonbow over the town of Kihei, seen from Kula, on Maui, Hawaii, 2016
Photo Credit
Arne-kaiser
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Learn When to Look for This Rare Nighttime Phenomenon!

Bob Berman
March 16, 2022
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Rainbows are a symbol often associated with St. Patrick’s Day. This year, St. Patrick’s Day happens to coincide with a full Moon! The light from a bright Moon can create rainbows at night, a phenomenon referred to as a moonbow! Learn more about moonbows.

What Is a Moonbow?

Just like a solar rainbow, a moonbow or lunar rainbow appears as an enormous arc in the sky when moonlight is refracted through water droplets in the air. Moonbows are much less intense than solar rainbows and usually look spooky-white rather than the well-known set of rainbow colors. All of the colors of the spectrum are actually present in a moonbow, but because moonlight is much less intense than sunlight, our eyes have trouble picking up the various colors. (If you’re a photographer, the colors of a moonbow will show up in long exposures or on high-speed film.)

The size of a moonbow depends on the Moon’s height in the sky: the lower the Moon, the bigger the moonbow. The largest moonbows occur when the Moon is within only an hour of rising or setting.

How Rare Is a Moonbow?

A moonbow is a much rarer occurrence than a solar rainbow. In order for a moonbow to appear, a few things need to happen:

  1. The Moon must be in or near its brightest phase (full) and be unobstructed in the sky.
  2. Water droplets must be present in the part of the sky opposite the Moon. Water droplets in the air can come from a recent rain shower, a nearby waterfall, or even the spray produced by the crashing waves on a shoreline.
  3. The Moon must be low in the sky—no more than 42 degrees above the horizon.

If all of these things happen together, there’s a chance that a moonbow will appear! The full Moon is nearest the horizon when it’s rising or setting, so look for a moonbow in the hours after sunset or before sunrise.

Spot a Moonbow

Now that you know where to look when the conditions are right, you can see a moonbow, too! If rain is the forecast and you have a bit of luck, keep an eye out for a moonbow this St. Patrick’s Day, as March’s full Moon rises in the evening of Thursday, March 17. 

Have you ever seen a moonbow? Let us know in the comments!

Moon

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Comments

Sophia Kehau Keller (not verified)

2 months 1 week ago

I see other comments about Hawai'i here. My moonbow viewing was also in Hawai'i. It was Summer Solstice 1998 and we had a potluck celebration in South Kohala on the Big Island. There was a misty not-quite-real-rain drifting in from the ocean. I went outside to enjoy the twilight, facing West toward the water, and the Full Moon rose behind me, shining into the mist. There was a brilliant, exquisite bow, every color perfectly visible but silvery, instead of the usual warmth. I went in to share the news, and a couple of people came outside to see. We watched for a good half hour until finally the Moon rose high enough that the colors faded. It was a one-of-a-kind memory that I've always treasured. Thanks for this article.

David Kantor (not verified)

9 years 11 months ago

Sailing from Hawaii to California I experienced a full arc moon bow. Very bright night. An amazing site. The dolphins following the boat were pretty nice also.

fooddiaryuser (not verified)

12 years 3 months ago

I was living on Oahu in Makiki on the 8th floor and I was looking west at the Downtown Honolulu city lights. We were sitting on the lanai having dinner and could see that it was raining down there, but when the Moon rose (which would have been to our backs) we saw an incredible specimen of a Moonbow. It was mostly purple but we could even see all of the other vivid colors of white, pink, green, purple and blue...it was A MAZE ING!

Thank you for asking this question, it brought back a wonder memory!

mtngeo (not verified)

12 years 3 months ago

I had always wondered about this, and as I was riding over Rocky Mt. National Park on my way home to Grand Lake via motorcycle, about 9:30 PM, on a mid-August night in 1985, a rain blew in from the West and the full moon shined down from the East. A full Moonbow lit the park just below Fall River Pass. Incredible. There are no words to describle the beauty. About four cars had stopped to watch. I stayed until it faded and continused down the mountain. Once in the switchbacks, I rode into the rain and out of it again several times. I should be so fortunate to see another.

Monica Pihi (not verified)

12 years 6 months ago

Moonbows are amazing. I observed one as a teen during a camping trip at Pohoiki, Hawaii, 1977. It was shimmery white and it extended over the entire bay, a complete moonbow. It was beautiful and it's memory has never left me.