the.com/blue moon
the calendar's way of admitting it doesn't divide evenly into the sky.
means a second full moon in one calendar month, or in the older sense, the third of four full moons in a season that usually only gets three.
from the phrase predates astronomy nerds: 16th century english used once in a blue moon to mean something absurd, since moons aren't actually blue. a 1946 sky and telescope magazine article misread an old farmer's almanac rule and invented the modern once-a-month definition, which stuck so hard it became the real one.
actual coloralmost never blue, despite the name
frequencyhappens roughly every two to three years
true blue moonscaused by volcanic ash or smoke particles
error made canona magazine mistake became the accepted definition
for instance
august 2023 — the most recent widely photographed blue moon, also a supermoon
krakatoa 1883 — volcanic ash actually turned the moon blue for months
halloween 2020 — a rare blue moon fell on halloween night