What a blue moon is and when to expect the next one
A blue moon is not a common visual spectacle where the Moon turns blue, but a calendar label for the second full moon in a single calendar month or the third full moon in a season that contains four full moons. The next blue moon by the monthly definition will occur on 31 August 2026. By the seasonal definition, the next blue moon is in 2025, with the astronomical event occurring on 22 August 2025. These dates are predictable based on the Gregorian calendar and the Metonic cycle, and they remain useful references for years.
Why the term blue moon exists and what it means
The phrase blue moon has been used in English since at least the 16th century, but the modern calendrical sense—referring to an extra full moon in a season or month—originated from a misinterpretation in a 1946 Sky & Telescope article that accidentally reversed the traditional definition used in the Maine Farmer’s Almanac. Today, two definitions coexist: the monthly blue moon (second full moon in a calendar month) and the seasonal blue moon (third of four full moons in a single astronomical season). Neither definition makes the Moon appear blue; the name is purely calendrical, highlighting an irregularity in how we track full moons in calendar systems.
How full moons and calendars interact
The Moon’s synodic month—averaging about 29.53 days—is shorter than most calendar months, so full moons drift about 11 days earlier each year in the Gregorian calendar. This drift allows occasional years to fit 13 full moons instead of 12. A year with 13 full moons creates the conditions for a seasonal blue moon, because the traditional calendar seasons are typically designed for four full moons each. The Metonic cycle, which repeats approximately every 19 years, helps align solar years and lunar months, making blue moon patterns repeat with high predictability. Understanding this cycle clarifies why blue moons can seem rare without being mysterious.
Upcoming blue moon dates and types
Next monthly blue moon
The monthly blue moon occurs when a calendar month has two full moons. The next instance in this pattern is 31 August 2026. This event will appear in any time zone where the full moon falls on that date, though local moonrise and moonset times affect visibility. Because August 2026 includes a full moon on 1 August and another on 31 August, the latter will be labeled the blue moon under the monthly rule. While August 2026 is notable for this pairing, other months may also host monthly blue moons on different years.
Next seasonal blue moon
The seasonal blue moon occurs when an astronomical season contains a fourth full moon instead of the usual three. By this definition, 22 August 2025 is the next seasonal blue moon, occurring in the summer season in the Northern Hemisphere. This date is widely cited in almanacs and astronomical references because it follows the traditional seasonal count adopted by the Maine Farmer’s Almanac. Unlike the visually misleading name, the Moon will not look blue; it will appear as a typical full moon. The seasonal rule is particularly useful for astronomical and cultural tracking because it ties full moons to equinoxes and solstices.
At a glance: upcoming blue moon summary
| Definition | Next occurrence | Date | Notes |
|---|---|---|---|
| Monthly blue moon (second full moon in a calendar month) | Upcoming | 31 August 2026 | Occurs in months with 2 full moons; local visibility depends on moonrise times |
| Seasonal blue moon (third of four full moons in a season) | Upcoming | 22 August 2025 | Follows traditional seasonal count used in almanacs; Moon appears normal |
Related terms and common misperceptions
Despite the name, a blue moon will not look blue under typical conditions. The Moon can appear bluish only when fine dust or smoke particles in the atmosphere—often from large fires or volcanic eruptions—scatter red light more efficiently, a rare physical effect unrelated to the calendrical definition. Calendrical blue moons are predictable by design, whereas a genuinely blue-colored Moon is a transient atmospheric event. People sometimes conflate supermoons, harvest moons, and blue moons, but these terms describe different attributes: apparent size, seasonal role, or frequency, respectively. Recognizing these distinctions prevents confusion and supports clearer skywatching expectations.
Practical tips for moon watchers
- Plan moon observations around moonrise and moonset times for your location, because a full moon labeled as a blue moon will still set during daylight in some regions.
- Use reputable almanac sources or astronomical APIs that explicitly label monthly versus seasonal blue moons to avoid definition confusion.
- Remember that the next visual chance to see a full Moon on 31 August 2026 will be a regular full moon; the calendrical label does not change its appearance.
- For photographers, treat a blue moon like any other full moon: focus on composition, foreground interest, and lighting rather than expecting a color change.
Why these dates stay useful over time
Blue moon dates are not arbitrary; they derive from well-defined astronomical cycles and calendar conventions. Once calculated using the Metonic cycle and standard lunisolar relationships, these events remain valid across decades, making them ideal for evergreen planning and educational content. Almanacs, calendars, and timeanddate systems can confidently list future blue moons because the underlying mechanics rarely change barring major calendar reforms. For this reason, explaining both the monthly and seasonal rules gives readers a durable mental model rather than a one-time fact. This supports long-term usefulness and reinforces trust in the information.
Frequently asked questions about blue moons
- Does a blue moon happen every few years? Yes, monthly blue moons occur two to three times per year on average, while seasonal blue moons occur about once every 2.7 years in the current era.
- Will the Moon actually look blue? No. The Moon’s appearance is unaffected by the calendrical rule; atmospheric conditions are required for a bluish tint, which is unrelated to blue moons.
- Why do seasons sometimes have four full moons? Because seasons vary in length and synodic months do not divide evenly into seasons, occasionally a season contains four full moons instead of three, triggering the seasonal blue moon label.
- Are blue moons rare events? They are uncommon in the sense that they do not occur annually for a given definition, but they are regular and predictable outcomes of calendar–moon interactions.