blue moonblue moon
- a. The third full moon in a three-month calendrical season that has four full moons.b. The second of two full moons occurring in the same month.
- Informal A relatively long period of time: I haven't seen you in a blue moon.
Origin of blue moonSense 2, probably from the rare occurrence whereby the moon appears blue from high amounts of dust in the atmosphere, as from a volcanic eruption
Mistake in Sky & Telescope magazine in 1946, retracted nearly 60 years later.