blue moon

The name English speakers give to the second full moon occurring within the same month, which happens every two and a half years, on average. This name is also given to the bluish or greenish-colored moon observed when a major volcanic eruption occurs on Earth. The injection of enormous quantities of very fine particles into the upper atmosphere attenuates the fraction of moonlight of longer wavelengths (red, orange, yellow), enhancing the shorter-wavelength colors (green, blue, violet). It is an optical phenomenon similar to that observed in cigarette smoke, whose bluish color is a consequence of the selective scattering of light reflected by the particles making up the said smoke.