A blue moon primarily refers to a rare calendrical event involving full moons. There are two main definitions:
- The traditional definition is the third full moon in a season that has four full moons instead of the usual three. This definition comes from older almanac practices and is considered the original meaning
- The more popular modern definition, which emerged from a misinterpretation in the mid-20th century, is the second full moon occurring within a single calendar month. This happens because the lunar cycle is about 29.5 days, slightly shorter than most calendar months, allowing for two full moons in one month roughly every 2.5 years
Despite its name, a blue moon does not usually appear blue in color. The term "blue moon" originated from a 16th-century phrase meaning something impossible or very rare. Occasionally, the moon can appear bluish due to atmospheric conditions such as volcanic ash or forest fire smoke filtering light, but this is unrelated to the calendrical blue moon
. In popular culture, "once in a blue moon" signifies something that happens very rarely. Astrologically, a blue moon is often seen as a time of heightened energy, clarity, and opportunity for growth and setting intentions
. In summary, a blue moon means either an extra full moon in a season or a second full moon in a month, both of which are relatively rare occurrences, but the moon itself is not usually blue
