the.com/blue moon
The second full moon in a month, proof calendars and orbits don't quite agree.
means A rare occurrence — literally the second full moon in a single calendar month, which happens only once every two or three years.
from The phrase predates the astronomical definition by centuries and originally meant something absurd or impossible, as in 16th-century English sayings about things that would happen 'when the moon is blue' — i.e., never. The modern calendrical meaning is a mix-up: a 1946 magazine article misread an old Maine almanac's more complicated seasonal rule and simplified it into 'second full moon in a month,' and that mistaken version is the one that stuck and spread through almanacs and trivia ever since.
- actual color
- usually not blue at all
- true blue cause
- volcanic ash or wildfire smoke, rarely
- frequency
- happens roughly every two to three years
- origin phrase
- once in a blue moon means rare