Moon · Evergreen

What is a blue moon? Two definitions, told honestly

Nov 16, 2026 · ~6 min read

"Once in a blue moon" is doing a lot of work in English — it means rarely, and the astronomy behind it is messier than most articles admit. There are two competing definitions of a blue moon, one original and one born from a magazine misreading in 1946, and both are in circulation today. The Moon never actually turns blue in either case. Here is the full story, sourced.

Key takeaways

Seasonal blue moon (original): the third of four full moons in one astronomical season — the Maine Farmers' Almanac rule.
Calendar blue moon (modern, more common): the second full moon within a single calendar month.
The popular monthly definition spread from a misreading of the almanac rule, published in Sky & Telescope in 1946.
The Moon looks blue only for atmospheric reasons (volcanic ash, smoke) — unrelated to either definition.

The arithmetic that makes blue moons possible

The mismatch is the heart of it: full moons arrive every 29.5 days on average, but seasons run about 91.3 days and calendar months 28–31. Twelve names, twelve full moons — most years. The extra 0.37 moon per year accumulates until a season or a month overflows:

Seasonal overflow: a season (solstice to equinox) occasionally contains four full moons instead of three.
Monthly overflow: a 30- or 31-day month occasionally contains two full moons — one on the 1st or 2nd, another on the 30th or 31st. February, shorter than the 29.5-day cycle, can never hold two (and about once every 19 years it holds none at all).

Both events happen roughly every two to three years — hence "once in a blue moon."

The original rule: third moon of four

Researching old Maine Farmers' Almanac editions (1819–1962), Sky & Telescope's editors found the almanac's rule: when a season contained four full moons, the third was called the Blue Moon. Why the third, not the fourth? So that seasonal moon names tied to church calendars — the Moon Before Yule, the Lenten Moon — stayed aligned with the solstices and equinoxes; the "extra" third slot absorbed the surplus. These seasonal blue moons can only fall in February, May, August or November. The 1937 Maine almanac's Blue Moon, the editors showed, landed on August 21 — in a year with only twelve full moons, and not a month's second full moon. Verified: 2026-11-16 — Sky & Telescope, "What Is a Blue Moon?".

How the monthly definition took over

The popular meaning traces to a chain of errors Sky & Telescope itself helped start. In 1943, Laurence Lafleur vaguely cited the Maine rule. In March 1946, amateur astronomer James Hugh Pruett, writing in Sky & Telescope without the actual 1937 almanac in hand, inferred that "the second [full moon] in a month, so I interpret it, was called Blue Moon." A 1950 Sky & Telescope note repeated the slip, radio's StarDate spread it in 1980, and a 1986 Trivial Pursuit edition immortalized it. The magazine's own historians untangled the story decades later and, in the same article, concluded there is no harm in enjoying both definitions — the newer one is fun and draws people to the sky. We agree, which is why SkyDeck pages say which definition they mean whenever the phrase appears.

Which definition should you use?

Practically: if you need a crowd-pleasing date, use the monthly rule — it is what most calendars, apps and news pieces mean. If you care about the older almanac tradition, use the seasonal rule and enjoy the obscure correctness. Either way the Moon is exactly the same shade of grey. For reference, the phase tables in our monthly guides are computed, not named — the moon-phase calculator will show you whether any given month of 2026–2027 manages a double full moon, and the full moon names guide covers the other labels.

When the Moon really is blue

Genuinely blue-tinted moons exist, but they are meteorology, not astronomy: fine particles from major volcanic eruptions or large forest fires scatter red light and let blue through, and the Moon (and Sun) can appear lavender or steel-blue for months after such events. Those moons have no schedule, appear in every phase, and have nothing to do with the blue moons of either definition. The definitions overlap only in poetry.

Disclosure

Blue moon dates differ between the two definitions and between time zones — a month's second full moon in Universal Time may be its first in your local zone. SkyDeck phase tables are published in UT; the calculator shows local time for any event.