Few astronomical terms have entered everyday language as successfully as blue moon. “Once in a blue moon” has become synonymous with something rare, yet the astronomical event itself is neither especially unusual nor related to the Moon’s color. A blue moon is simply the result of a mismatch between the Moon’s natural cycle and the way humans organize time.
The Moon cycles through each phase, from one full moon to the next, about every 29.5 days. But that doesn't line up with any of the months, creating the possibility of an extra, 13th full moon in a year, known today as a "blue moon."
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Today, a blue moon is defined as the second full moon occurring within a single calendar month. For this to happen, the first full moon must fall on the first or second of the month, and that month must contain 30 or 31 days.
The full "flower moon" occurred on Friday, May 1, so here we are in the final days of the month with the Moon again reaching full phase at 4:45 a.m. on Sunday, May 31, 2026. If it looks a little smaller, that is because it is also a "micromoon," a full moon happening near apogee, or the furthest point in the Moon's not-so-circular orbit around the Earth. This hasn't happened since the micro blue moon on October 31, 2020.
This last happened August 30, 2023, when the Moon reached full phase at 9:35 p.m. EDT. The next blue moon won't be until December 31, 2028.
"Blue moon" originally referred to the third full moon in a season that contained four full moons rather than the usual three. The Maine Farmers’ Almanac selected this seasonal definition to preserve the traditional, Native American naming of full moons that marked agricultural and seasonal activities. When an extra full moon appeared within a season, designating the third as a blue moon prevented the established sequence of names from shifting.
The transition to today's more calendrical definition happened in 1946 when amateur astronomer James Hugh Pruett published an article in Sky & Telescope magazine that misinterpreted the almanac’s explanation. Pruett described the second full moon in a calendar month, the definition we use today. His error remained largely unnoticed until it was repeated in later publications and popularized by astronomy programs during the 1980s. By then, the simpler definition had become firmly established in public usage.
Don't expect Saturday night's or Sunday night's Moon to look particularly blue. Volcanic eruptions, major forest fires, and certain dust events can fill the atmosphere with particles of just the size to scatter red wavelengths of light, leaving blue moonlight. The 1883 eruption of Krakatoa famously produced blue-colored moons across all phases recorded around the world.
