A blue moon occurs when there are two full moons within a single calendar month, which happens because the lunar cycle (about 29.5 days) is slightly shorter than most calendar months. This means if a full moon happens early in a month, there can be enough time for a second full moon at the end of the same month. This is the most common modern definition of a blue moon. Another definition, rooted in traditional almanac naming, is the third full moon in a season that has four full moons instead of the usual three. The term "blue moon" originally meant something rare or impossible, but it now refers to these extra full moons happening roughly once every two to three years. The moon does not literally turn blue in these cases; however, on very rare occasions, atmospheric conditions caused by particles from volcanic eruptions or forest fires can scatter light and make the moon appear bluish. Thus, a blue moon is mainly about the timing of lunar phases and calendar months, not the moon's actual color.