A blue moon can refer to the third full Moon in a season with four full moons. Most years have twelve full moons that occur approximately monthly.
In many world folklore, eachmoon are named according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining the correct seasonal timings for future moons.
A "blue moon" is also used colloquially to mean "a rare event", reflected in the phrase "once in a blue moon".
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere.
The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre) — and no other sizes present. This is rare, but volcanoes sometimes produce such clouds, as do forest fires. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes, with most smaller than 1 micrometre, and they tend to scatter blue light. This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons.
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