Moonbow - Lunar rainbow


  Simulated naked eye view    
We have all seen rainbows during the day but what about rainbows at night? Thomas Thies of Gearhart, Oregon photographed this one after the sun had set on December 8th 2003. He says, "The rainbow, which lasted for about 20 minutes, was out over the ocean with the moon rising behind us." The several bright yellow lights near the horizon are crab boats.

The moon is the key, just as sunlight produces rainbows during the day, moonlight can produce rainbows at night. This is a lunar rainbow or 'moonbow'.   
Another moonbow image.

Moonbows are rare because moonlight is not very bright. A bright moon near to full is needed, it must be raining opposite the moon, the sky must be dark and the moon must be less than 42º high. Put all these together and you do not get to see a moonbow very often! To the unaided eye they usually appear, as in the small image, without colour because their light is not bright enough to activate the cone colour receptors in our eyes. Nonetheless colours have been reported and might be seen when the moon is bright.