Vaguely interesting.
http://www.phrases.org.uk/meanings/once-in-a-blue-moon.html
Very occasionally, the moon actually does appear to be blue. This sometimes
occurs after a volcanic eruption, like that of Krakatao in 1883. Dust
particles in the atmosphere are normally of a size to diffract blue light,
making the moon appear reddish at sunset. Larger volcanic dust particles
diffract red light, making the moon appear bluish.
