Astronomical Blue Moons happen either once every two to three years or so, depending on which of the two definitions you apply.
A Moon that actually looks blue, however, is a very rare sight. The Moon, full or any other phase, can appear blue when the atmosphere is filled with dust or smoke particles of a certain size; slightly wider than 0.7 micron. The particles scatter the red light making the Moon appear blue in color, this can happen for instance after a dust storm, forest fire or a volcanic eruption.
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Uploaded: Sep 24, 2015