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]
Back to main page