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

Moonbows: What Is a Lunar Rainbow?

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

Bob Berman
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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!