1. A phenomenon caused by high-altitude dust in the Earth's atmosphere, which scatters red light preferentially, making the Moon appear blue (see Mie Scattering) Blue Moons were seen in 1883 after the eruption of the volcano Krakatoa, and in 1950 because of forest fires in Canada. The phenomenon is rare, hence the expression ‘once in a blue Moon’.
2. In modern US usage, the second full Moon in a calendar month. This occurs every 2–3 years on average.
"blue Moon" A Dictionary of Astronomy. 1997. Encyclopedia.com. 1 Oct. 2009
--from *A Dictionary of Astronomy,* 1997, on Encyclopedia.com acessed Oct. 1, 2009
Mie scattering Scattering of light by particles whose size is similar to the wavelength of light. It occurs in
interstellar space and in the Earth's atmosphere. For wavelengths much shorter than a given particle size, Mie
scattering is a complex function of wavelength. Scattering drops to a minimum at a wavelength half the particle size,
rises to a maximum when it is the same as the particle size, and then decays to zero towards longer wavelengths.
Mie scattering can thus make objects appear either redder or bluer according to the size of the scattering particle.
In the Earth's atmosphere there is usually a wide range of particle sizes, so that these colour effects are blurred
into a nondescript grey. Very occasionally, atmospheric effects will combine so that the dust is mostly 900?nm in
size and the minimum in scattering is at 450?nm. This is the wavelength of blue light that causes the phenomenon
of the blue Moon
I don't know of any folklore specfically attached to the the second full moon in a calendar month which is called a "blue moon" or "blue Moon." However, since it only happens about two or three years, it would seem to m me to be a very auspicious time to petion the luner Goddess.
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