“Once in a blue moon,” is an expression we use to mean something very rare. Actually a “Blue Moon” has several different definitions.
A Blue Moon can mean the second of two full moons in one month.
An older definition says a Blue Moon is the third of four full months in a single season.
An actual Blue Moon can occur by a rare type of dust in the atmosphere. The Moon, full or in 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. This can happen after a dust storm, forest fire or a volcanic eruption. Mt. Krakatoa, Indonesia (1883), Mt. St. Helens, USA (1980), El Chichon, Mexico (1983), and Mount Pinatubo, Philippines (1991) all caused Blue Moons.
Of course, there’s the great song by The Marcels, Blue Moon!
The next Blue Moon will be Wednesday, January 31, 2018.