A blue moon primarily refers to the rare occurrence of having two full moons within a single calendar month or, alternatively, the third full moon in a season that has four full moons instead of the usual three. This happens because the lunar cycle is about 29.5 days, slightly shorter than most calendar months, allowing for a second full moon to occasionally fit into one month. This type of blue moon occurs roughly every two to three years

. Despite the name, a blue moon does not usually appear blue in color. The Moon typically looks its normal gray-white. However, on very rare occasions, the Moon can actually appear bluish due to atmospheric conditions. This happens when particles of a specific size, such as volcanic ash or smoke from large fires, scatter red light and allow blue light to pass through, giving the Moon a blue tint. Historical examples include after the 1883 Krakatoa eruption and other volcanic eruptions like El Chichón in 1983 and Mount St. Helens in 1980

. In summary:

• Blue moon (calendar definition): Second full moon in a calendar month or third full moon in a four-full-moon season.
• Frequency: About every 2-3 years.
• Color: Usually not blue; actual blue appearance is extremely rare and caused by atmospheric particles filtering light.
• Special cases: Sometimes called a "super blue moon" when a blue moon coincides with a supermoon (when the full moon is closest to Earth and appears slightly larger and brighter)

Thus, during a blue moon event, you typically see an extra full moon in a month, but the Moon itself rarely looks blue unless unusual atmospheric conditions are present.