A "blue moon" has two main meanings related to full moons:
- Seasonal Blue Moon (Traditional Definition): It is the third full moon in an astronomical season (the period between a solstice and an equinox) that contains four full moons instead of the usual three. This definition comes from the Maine Farmers' Almanac and is linked to the ecclesiastical calendar to help keep church holidays aligned with lunar phases
- Monthly Blue Moon (Modern Popular Definition): It is the second full moon occurring within a single calendar month. Since the lunar cycle is about 29.5 days, sometimes two full moons fit into one month. This definition originated from a misinterpretation by James Hugh Pruett in a 1946 Sky & Telescope magazine article and has since become the most widely known meaning of "blue moon"
The phrase "once in a blue moon" refers to something rare, as blue moons occur roughly once every 2 to 3 years. Despite the name, a blue moon rarely appears blue in color. The moon can actually look blue only under unusual atmospheric conditions, such as after volcanic eruptions or large forest fires, which scatter light in a way that tints the moon blue
. In summary, a blue moon either means an extra full moon in a season or a second full moon in a month, both relatively rare events, and the moon itself is usually not literally blue during these times.
