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A Blue Moon refers to a moon that actually appears blue, which is caused by a peculiar rare kind
of dust in the air. When you look at the moon through ordinary dust, as at moon rise or set when
the dust is the common variety, it looks orange or red — just as the sun looks red at sunset.
But on rare occasions, such as very distant forest fires raging or a major explosive-type volcanic
eruption not too far upwind, the dust is of that special size (just barely bigger than the
wavelength of visible light) and the extinctions of light colors are reversed, giving rise to
a moon that really, truly looks blue.
Most people never have that experience, since the combination of events necessary is quite rare —
just the right size of dust particles (and no others) and, of course, you have to be there at the
right time.
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