Lunar Rainbows



Two letters to the New Scientist Last Word column (27th January 2001)asked about rainbows caused by moonlight. One observer saw :...."the full moon ... producing an inverse rainbow [having the red band on the inside and the blue on the outside] in the 12 o'clock position directly above the moon. as well as ....very bright spots of light in the 3 and 9 o'clock positions on either side of the moon that also showed patches of rainbow.

One respondent stated that the bright spots of light were "mock moons or moon dogs" and said that descriptions of this phenomena could be found in The Nature of Light and Colour in the Outdoors by Marcel Minnaert. All respondents attributed the phenomena to ice crystals in the atmosphere.

This phenomena would seem to be a variety of lunar halo.

The second observer described a complete rainbow, much like a solar rainbow, in that the moon was behind the observer and the arc seen in front of the observer. It was described as: ....much dimmer than a normal rainbow and looked as though it had been washed through in sepia." The time was early morning and the weather wet

Apparently the full moon is normally bright enough to produce rainbows, but these can only be seen under certain conditions:
It must be within a day or two of the full moon, the moon must be low in the sky (<42 degrees). The sky must be dark and the weather showery. Moon rainbows tend to be less colourful than solar rainbows for two main reasons:
Our eyes don't see colour well in poor light, and also because moonlight is less colourful than sunlight and contains a lot of brown (?!), according to Albert Zjilstra of the Department of Physics, University of Manchester Institute of Science and Technology[1]



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