A blue moon can refer to two different lunar events

:

• The second full moon in a single calendar month

• The third full moon in a season that has four full moons

The moon is not actually blue during a blue moon

. However, the moon can appear to have a blue hue because of particles in the atmosphere, such as dust or smoke

. Blue moons happen with some regularity because the lunar cycle (approximately 29.5 days) is slightly shorter than the average length of a calendar month

. A blue moon occurs about every two to three years

. The term "blue moon" has also been used to describe something that is rare