Blue moon: what the term actually means, and why there are two definitions

There are two blue moons, they are not the same event, and both are legitimate. This page gives you the dates for each, computed from the same lunar solver used everywhere else on this site, and then explains why the word means two things.

Definition one: the second full moon in a month

This is what most people mean today. A calendar month is 30 or 31 days; a lunation is 29.53 days. Most months fit exactly one full moon. Occasionally one fits two, and the second gets the name.

Blue moon	Dates	Note
May 2026	May 1, May 31	The May 31 full moon is also the farthest of the year
December 2028	December 2, December 31	The second falls on New Year’s Eve
September 2031	September 1, September 30	—

The gap between them is not regular — 31 months, then 33 — because it depends on where the 29.53-day cycle falls against months of different lengths. On average, about once every 2.7 years.

Definition two: the third full moon in a season of four

This is the older rule, and the one the Maine Farmers’ Almanac used for the names it published. A season normally holds three full moons. When it holds four — which happens because 12 lunations fall 11 days short of a year — the third of them is the blue moon.

Why the third and not the fourth? Because the almanac’s names were tied to dates: the full moon nearest the equinox had to keep its name, and the extra one had to be the one that did not disturb the sequence.

Both rules are internally consistent and they disagree in any given year, sometimes by months. If you are checking a claim, the first question is which definition the claimant is using — and most popular sources do not say.

The arithmetic behind it

Twelve lunations come to 354.37 days. A tropical year is 365.24. The shortfall is 10.87 days, which accumulates: after about two and a half years it adds up to a whole lunation, and a 13th full moon appears inside a calendar year. In 19 years there are 235 lunations and 228 calendar months, an excess of seven — which is why blue moons cluster in a Metonic pattern and why the intervals between them alternate between roughly 2.5 and 3 years.

The same 19-year cycle underlies the rule for when to add a leap month in the Hebrew calendar and the computation of Easter. It is not an astronomical coincidence so much as a statement about how close 19 tropical years and 235 lunations happen to be — within about two hours.

Where the color myth comes from

A blue moon has never meant a blue-coloured Moon, and the phrase “once in a blue moon” predates the calendar definitions by centuries — it meant something that does not happen, and the calendar rule gave it a precise meaning afterwards.

Genuinely blue Moons do occur, and the cause is atmospheric. When the air holds particles slightly smaller than the wavelength of red light, red is scattered out and the transmitted light shifts blue. Volcanic eruptions, large forest fires and dust storms can do it. Krakatoa in 1883 produced blue and green sunsets for months. The Moon after Mount Pinatubo in 1991 was reported blue in some locations for over a year.

The one thing worth checking

If a “blue moon” date matters to you, the check is arithmetic: find the full moon instants for the month. The phase calendar lists them for the coming year, and the phase tool gives the instant for a single date. If two full moons fall inside one calendar month, the definition-one blue moon is real and you can stop there.

Dates computed in UTC; a full moon near midnight can fall in different months in different time zones, which is the one place these dates can legitimately move.