MOON: What is a “Moonbow”?

30 September 2014

ATTENTION: There will be a Full Eclipse of the Moon on October 7-8, 2014.  This is the second full eclipse of the moon in a “tetrad.”  A tetrad is a series of 4 eclipses of the moon each of which is spaced exactly 6 months apart.  The first eclipse of this tetrad happened on April 14-15, 2014 and got the popular name “blood moon.”  The 3rd eclipse will happen on April 4, 2015. 

See: What is a Blood Moon? & Blood Moons, Eclipses, and Tetrads

The Short Answer (TSA)

The light of the sun makes rainbows.  The light of the moon makes moonbows.  The basic principle at work in a rainbow and moonbow are the same.  Really small droplets of water in a really moist — almost cloudy — sky split (or “refract”) the white light of the sun or moon into the colors of a rainbow or a moonbow.

Rainbows are common.  But moonbows are rare because the moon isn’t nearly as bright as the sun.  In perfect conditions, it takes all of the sunlight reflected by a full moon to make a moonbow.  Even then, the low light of the moon may not be able to make a rainbow of colors, but only a faint white arch.

If a moonbow appears at all, it will always be faint.  And moonbows always appear in the opposite side of the sky from the moon.  “Moonbow” isn’t the only name given to rainbows made by moonlight.  Sometimes, a moonbow is called a lunar rainbow, black rainbow, white rainbow, lunar bow, or space rainbow.  At least, this is what a “true” moonbow is called.  There are also “false” moonbows.

What are these mysterious things appearing in the sky and pretending to be moonbows?  Well, a “false” moonbow is formed when the moisture in the air isn’t natural, but geological.

What does that mean?

If the moisture that makes the moonbow doesn’t come from natural weather conditions, it’s a “false” moonbow.  So, if the spray, mist or fog that makes the moonbow is from from a waterfall, the moonbow is a “false” one.  “False” moonbows are easy to photograph because they are almost always visible at certain places such as Yosemite Falls in California, Cumberland Falls in Kentucky, and Victoria Falls in Africa.

There’s a way to tell if a moonbow is “false.”  Watch and see what happens after the moon sets, and the sun rises.  If the arching band of colors is always there – day or night — it’s “false.”   In other words, after the moon sets, the moonbow fades away.  But the mist from a large waterfall is always there.  So, as soon as the sun rises, a rainbow will appear.   Just as the moonbow made by the mist from a waterfall is considered “false,” the rainbow appearing in the same place during the day, is also called a “false” rainbow.

By the way, true moonbows are rare and hard to photograph.  So, there aren’t so many pictures of true moonbows.  But “false” moonbows are a lot easier to find and predict.  So, again, most photographs are of “false” moonbows.

M Grossmann of Hazelwood, Missouri & Belleville, Illinois

30 September 2014

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