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Once in a Blue Moon

I grew up with the idea that a Blue Moon was a rare meteorological phenomenon in which the Moon appears to be blue, and that the phase Once in a Blue Moon meant very rarely.  Once when I was working at Adelaide University, I was talking to one of our post graduate students and realised that her understanding of the phase was that it meant never.

I consider that what many people mean when they say a word or phase in English is a correct meaning. Of course this means that words can change their meaning with time.  I have heard other people use the phase Once in a Blue moon to mean never, so her understanding of the phase was not wrong, just different from mine. 

But what makes the Moon appear Blue on rare occasions?  Apparently it is caused by dust of the right size being suspended in the atmosphere.  If a very high proportion of the dust is about 1 micron in diameter, the light is scattered by the Tindal Effect and makes the Moon look Blue.

The Tindal effect is also the thing that makes the Moss GreenTiger Barb appear to be

greenish.  Some other animals also have their apparent colours caused by the same effect.  A micron is one millionth of a metre, or, if you prefer; a thousandth of a millimetre. This is quite small dust, but can occur after some volcanic eruptions and occasionally after fires.

Apparently to other people, a Blue Moon means something completely different.  A Blue Moon can also mean the second full Moon of a month, or the fourth full Moon of a calendar season.