Blue moon
What is a blue moon?
A blue moon usually means the second full moon in one calendar month. It looks like any other full moon. An older definition counts full moons within a season instead.
Try the question
When can one month contain two full moons?
Move the first full moon through the month. Find the latest starting day that still leaves room for another.
Watch for: The Moon does nothing unusual. A calendar box happens to be slightly longer than the phase cycle.
Why a month can fit two full moons
A calendar month is a container built for the Moon’s cycle — the word comes from “moon” — but built slightly too big, rounded up to 30 or 31 days. A container one or two days roomier than the thing it holds will occasionally hold the start of a second one. February is the exception in both directions: at 28 or 29 days it is shorter than the cycle itself, so it can never hold two full moons — and some Februaries hold none at all. When that happens, January and March can each go blue in the same year.
The older, stranger definition
The original almanac rule counted full moons per season: a season normally gets three, and in a season with four, the third was the blue moon. The simpler month-based rule spread from a mid-century magazine’s mistake and stuck. This site’s list uses the monthly rule and says so on the page — a list that does not name its rule is presenting a choice as a fact.
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Has the Moon ever actually been blue?
Rarely — and for none of these calendar reasons. After enormous volcanic eruptions or forest fires, ash particles of just the right size scatter red light and let blue through, and the Moon genuinely shows blue-grey: Krakatoa’s 1883 eruption tinted moons around the world for months. The phrase “once in a blue moon,” meaning almost never, is older than either calendar rule — the calendars borrowed the phrase, not the other way round.