Rare Blue Moons: Once in a Blue Moon Explained

A Blue Moon is the term for the second full moon in a single calendar month.
Despite the name, the Moon doesn’t actually turn blue—it looks the same as usual.
The phrase “once in a blue moon” means something rare or unusual.
Blue Moons occur about once every 2–3 years.
Another definition is the third full moon in a season that has four full moons.
The color of the Moon can appear bluish during events like volcanic eruptions or wildfires, but that’s unrelated to Blue Moons.
The most recent Blue Moon occurred on August 30–31, 2023.
The next seasonal Blue Moon will appear in August 2024.
Blue Moons add an extra twist to the regular rhythm of lunar cycles.
They remind us how astronomy and language intertwine in both science and culture.