blue moonblue moon
a very long time: chiefly in the phrase once in a blue moon, very seldom; almost never: so called from a rare atmospheric condition in which layers of smoke, dust, etc., as from forest fires or volcanoes, cause the surface of the moon to look blue
- 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.