G'morning Herman, 

 HT> Not the reason why.  Blue moons ARE blue.  They have
 HT> nothing to do with the number of full moons in a calendar
 HT> month.  The moon (full or otherwise) appears blue due to
 HT> atmospheric scattering.  The longer wavelengths are
 HT> scattered by particles (e.g. particles of smoke) leaving
 HT> the shorter (i.e. blue) wavelengths.

Err ... my OED sees `blue moon' as `very rare',  and fitting a 
lunar cycle inside a calendar month is indeed a rare event...
justifying the common usage of the term, one would think.

Blue perceptions of the month may have both physical and 
psychological explanations - atmospheric scattering modified by an 
assortment of pollutants certainly happens (one sees green rays 
from sunsets over Auckland City from the Coromandel penisular 
here),  and impressionable persons asked to verify this month's 
`blue moon' here on talkback excitedly report delicate shades of 
blue - contradicted by fuming observers who have seen none.

Mind you, seeing a `blue' moon for either reason is also a `rare 
event'  -  but I'd go for the mathematical rarity myself.

:-)


 
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