What Is a Blue Moon? The Meaning Behind 2026's Extra Full Moon
The moon doesn't actually turn blue. A blue moon's meaning is a calendar quirk, not a color, and 2026 happens to have one. Here's what the term means, where it came from, and when the rare cases of an actually-blue-looking moon happen.
The two definitions
Astronomers actually use two different definitions:
- The modern, more common definition: the second full moon to fall within a single calendar month. Because a full moon cycle (about 29.5 days) is just shy of most months' length, an occasional month fits two.
- The older, traditional definition: the third full moon in an astronomical season that has four full moons instead of the usual three.
Both are "blue moons" by their own definition, and they don't always land on the same date. This site's full moon calendar uses the more common, calendar-month definition.
2026's blue moon
May 2026 has two full moons: the first on May 1, and the second on May 31 -- a blue moon by the calendar-month definition. See May's full moon page for the exact dates and times, or the full moon calendar for the rest of the year.
Why call it "blue" if it isn't?
The phrase predates the astronomy. "Once in a blue moon" was already a way of saying something rare happens long before the specific two-full-moons-in-a-month definition got attached to it in the 20th century. The name stuck because it captured the right feeling (something uncommon), not because anyone was describing the moon's actual color.
When the moon actually looks blue
It does happen, just not because of the calendar. A moon that genuinely looks blue-tinted needs a specific kind of particle in the atmosphere, roughly a bit wider than 0.7 microns, which scatters red light more than blue and shifts the moon's apparent color. Large volcanic eruptions are the classic cause: both Krakatoa's 1883 eruption and Mount Pinatubo's 1991 eruption produced real blue-looking moons for people downwind of the ash.
How often do blue moons happen?
Calendar-definition blue moons happen roughly once every two to three years, which is where "once in a blue moon" gets its practical meaning: uncommon, but not once-in-a-lifetime.
The definitions, history, and frequency above are checked against a live astronomy reference and match the two full-moon dates already sourced and cross-checked on this site's own May 2026 full moon page.
Source: timeanddate.com, What Is a Blue Moon and When Is the Next One?