why does a blue moon happen

A blue moon happens mainly due to two definitions: it is either the second full moon in a single calendar month , or the third full moon in a season that has four full moons instead of the usual three. The phrase "once in a blue moon" reflects how infrequent this event is, occurring roughly every two to three years. This happens because the lunar cycle (about 29.5 days) is shorter than most calendar months, so sometimes two full moons fit into one month. The term "blue moon" itself is more metaphorical and does not mean the moon will actually appear blue in color in typical cases. However, the moon can very rarely appear blue due to atmospheric conditions when particles like dust or smoke from volcanic eruptions or forest fires scatter red light, making the moon look bluish. These actual visually blue moons are uncommon and unpredictable. In summary, a blue moon mostly denotes a calendar or seasonal peculiarity of full moons rather than a blue-colored moon, though the moon can sometimes appear blue due to special atmospheric conditions.