Blue Moon

A blue moon is an additional full moon that appears in a subdivision of a year. A seasonal blue moon is the third full moon of an astronomical season that has four full moons. A monthly blue moon is the second full moon in a calendar month with two full moons.

The most literal meaning of blue moon is when the moon appears to a casual observer to be unusually bluish in colour, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, after forest fires or volcanic eruptions.