Blue Moon Calculator
Blue Moon Calculator
Maximum range: 200 years. Blue moons are shown for the selected range.
Definitions
Monthly: 2nd full moon in a calendar month
Seasonal: 3rd full moon in a season with 4
Note: February can never have a monthly blue moon
Enter a Year Range
Select a year range and blue moon type above, then click Calculate to see all upcoming blue moon dates with countdown, named moon labels, and special tags.
Find every blue moon — monthly, seasonal, and double blue moon years
A blue moon is one of the rarest events on the lunar calendar, yet it happens more often than most people realize. Our Blue Moon Calculator uses the Jean Meeus algorithm — the gold standard in amateur and professional astronomy — to compute every blue moon date for any year range you choose, from historical dates back to the early 20th century all the way through the mid-22nd century. There are two entirely different definitions of a blue moon, both of which have been used by astronomers, almanac publishers, and folklore traditions. The first and most commonly known today is the calendrical (or monthly) blue moon: the second full moon in a single calendar month. Because a full moon cycle (synodic month) lasts approximately 29.53 days while most calendar months have 30 or 31 days, a second full moon sneaks into a month roughly once every 2 to 2.5 years. February can never host a monthly blue moon — it is simply too short at 28 or 29 days. The second definition is the seasonal blue moon, and it is actually the older of the two. Each astronomical season (spring, summer, autumn, winter) normally contains exactly three full moons. When a season contains four full moons instead, the third of the four is called the seasonal blue moon. This happens because the 365-day solar calendar does not align perfectly with the lunar cycle. The Metonic cycle — named after the Greek astronomer Meton of Athens — describes how 19 calendar years contain exactly 235 lunar months, producing seven extra full moons per cycle. Those seven extras are what generate the seven blue moon seasons in every 19-year period. The phrase 'once in a blue moon' has been in English usage since at least the 16th century, meaning something extremely rare. Paradoxically, blue moons are not actually that rare — they occur roughly every 2.5 to 3 years. The confusion partly stems from a famous 1946 misinterpretation: writer James Hugh Pruett read the Maine Farmer's Almanac's description of the seasonal blue moon and misinterpreted it to mean the second full moon of a calendar month. That interpretation was popularized by the 1980s radio quiz show StarDate and has since become the dominant modern definition. The moon can also appear literally blue in color, though this has nothing to do with the calendar definition. A truly blue moon (the atmospheric phenomenon) occurs when large particles in the atmosphere — typically from wildfire smoke or volcanic ash — scatter red wavelengths of light and allow blue light to pass through more easily. Particles around 900 nanometers in diameter are needed. Famous historical examples include the eruption of Krakatoa in 1883 and the Mount St. Helens eruption in 1980, both of which caused reports of blue-colored moons across wide geographic areas. Our calculator shows you both definitions side by side, lets you filter for monthly only, seasonal only, or both, and annotates each result with the moon's traditional name (Wolf Moon, Snow Moon, Flower Moon, and so on), plus special tags for supermoons (when the full moon is within 362,000 km of Earth), micromoons (when the full moon is beyond 405,000 km), and lunar eclipse coincidences. You can also enter any birth date to find out whether you were born near a blue moon, and export your results to CSV or iCal for use in any calendar application.
Understanding Blue Moons
What Is a Blue Moon?
A blue moon is a supplementary full moon — an extra full moon that appears beyond the usual one per calendar month or one per astronomical season. The monthly definition (second full moon in a calendar month) was popularized by a 1946 Sky & Telescope article and later made famous by the board game Trivial Pursuit. The seasonal definition (third full moon in a season that has four) is older and appears in the Maine Farmer's Almanac going back to the early 19th century. Both definitions are used today, and years where both types occur in the same 12-month span are called double blue moon years. These are especially rare: recent examples include 2018 and the next upcoming one in 2037.
How Are Blue Moons Calculated?
Blue moon dates are computed in two steps. First, all full moon dates for a given range are calculated using the mean synodic month of 29.530588867 days, anchored to a known full moon epoch (January 25, 2005 at 10:32 UTC). This simple iteration is accurate to within a few minutes over spans of decades. For higher precision, the Jean Meeus method from Astronomical Algorithms applies a series of correction terms derived from the solar mean anomaly, lunar mean anomaly, and moon's argument of latitude. Once all full moons are known, each calendar month is checked for two or more full moons (monthly blue moon), and each astronomical season is checked for four full moons (seasonal blue moon). The third of four seasonal full moons is the blue moon.
Why Do Blue Moons Matter?
Beyond folklore and the saying 'once in a blue moon,' blue moons are genuinely interesting celestial events for skywatchers. When a blue moon coincides with a supermoon — meaning the moon is near its closest orbital point (perigee) — it appears visibly larger and brighter than an average full moon. When it coincides with a lunar eclipse, the combination is even more striking. Blue moons also matter for gardeners, fishers, and those who follow traditional lunar calendars, since the extra full moon can shift the timing of seasonal planting signals or tidal patterns. Photographers and astrophotographers plan shoots around blue moon nights, particularly supermoon blue moons. The Metonic cycle also makes blue moon years predictable far in advance, useful for long-range event planning.
Limitations and Caveats
The dates computed by this calculator are based on UTC (Coordinated Universal Time). Because the full moon is a single global instant, the calendar date of a blue moon can differ by one day depending on your local timezone. A full moon that peaks at 23:30 UTC is already the following calendar day in time zones east of UTC+0. The astronomical season boundaries used here are fixed approximations (March 20, June 21, September 22, December 21) and can vary by up to a day from year to year. The supermoon and micromoon tags use a simplified orbital oscillation model; the precise Earth–Moon distance at any given full moon requires a full ephemeris computation. For professional applications, always consult JPL Horizons or the USNO Astronomical Almanac.
How to Use the Blue Moon Calculator
Choose Your Year Range
Enter a From Year and To Year in the inputs on the left. The default range is the current year through 10 years ahead. You can look back historically (e.g. 1990 to 2000) or plan far into the future (e.g. 2025 to 2050). The maximum range is 200 years.
Select the Blue Moon Type
Choose Monthly (second full moon in a calendar month), Seasonal (third full moon in a season with four), or Both to see all types. The Both filter is the most comprehensive and will highlight double blue moon years — years where both a monthly and a seasonal blue moon occur.
Review the Results
The right column shows the next upcoming blue moon with a countdown in days, a lunar cycle progress ring, and the traditional moon name. Below that, a full table lists every blue moon in your range with UTC peak time, type, moon name, and special tags (Supermoon, Micromoon, Lunar Eclipse, Double Year).
Export or Share
Click CSV to download a spreadsheet of all blue moon dates. Click iCal to download a .ics calendar file compatible with Google Calendar, Apple Calendar, and Outlook. Use the Copy Date and Share buttons to share the next blue moon date directly, or click Print for a print-friendly view of the full list.
Frequently Asked Questions
When is the next blue moon?
As of 2026, the next monthly blue moon is May 31, 2026 at approximately 08:45 UTC. This is the second full moon of May 2026 and is also described by astronomers as a micromoon — the most distant full moon of the year. The next seasonal blue moon after that is May 20, 2027, at approximately 10:59 UTC, the third of four full moons in the spring 2027 season. Our calculator updates in real time based on today's date and shows you exactly how many days remain until each upcoming blue moon.
How often does a blue moon occur?
Monthly blue moons occur roughly once every 2 to 2.5 years — about 41 times per century — because the calendar year has 365 days but 12 full moons span only about 354 days, leaving 11 days of surplus that accumulates into an extra full moon every 2–3 years. Seasonal blue moons occur slightly less frequently, roughly once every 2.5 to 3 years, because the Metonic cycle produces exactly 7 extra full moons per 19-year cycle. Double blue moon years — where both types occur in the same calendar year — happen only about 4–5 times per century. The last double blue moon year was 2018; the next is 2037.
What is the difference between a monthly and a seasonal blue moon?
A monthly blue moon is the second full moon in a single calendar month. Because most months have 30 or 31 days and a full moon cycle is 29.53 days, it is possible for two full moons to fall in the same month. February can never have a monthly blue moon because it is too short. A seasonal blue moon is the third full moon in an astronomical season (spring, summer, autumn, or winter) that happens to have four full moons instead of the usual three. The seasonal definition is older, originating in the Maine Farmer's Almanac, while the monthly definition was popularized by a 1946 Sky & Telescope article and later spread via Trivial Pursuit.
Does the moon actually look blue during a blue moon?
No — the calendar blue moon appears exactly like any other full moon. It is simply the name for an extra full moon, not a description of the moon's color. The moon can genuinely appear blue during certain atmospheric conditions, specifically when particles of around 900 nanometers in diameter are suspended in the atmosphere. This was observed after the 1883 Krakatoa eruption, the 1980 Mount St. Helens eruption, and during major wildfire events. These atmospheric blue moons are unrelated to the calendar definitions and occur unpredictably, whenever the right particle sizes from fires or volcanic ash are present in the sky.
What is a supermoon blue moon?
A supermoon blue moon is a full moon that is both a blue moon (by either definition) and a supermoon — meaning the moon is near its closest approach to Earth (perigee), typically within about 362,000 km. Supermoon blue moons appear roughly 14% larger and 30% brighter than a micromoon (full moon near apogee). The term 'supermoon' is not a formal astronomical designation but has become widely used by the media since 1979, when astrologer Richard Nolle coined it. Our calculator tags each blue moon as a supermoon or micromoon based on its approximate orbital distance at that full moon.
What is the Metonic Cycle and how does it relate to blue moons?
The Metonic Cycle, discovered by the Greek astronomer Meton of Athens around 432 BC, is a 19-year cycle in which the phases of the Moon repeat on the same calendar dates. More precisely, 19 calendar years (6,939.60 days) are almost exactly equal to 235 synodic months (6,939.69 days) — a difference of only about 2 hours. This means that in every 19-year span, there are 235 full moons instead of the expected 228 (12 per year × 19), producing exactly 7 extra full moons. Those 7 extra full moons are what generate 7 blue moon events (seasonal definition) per Metonic Cycle. The cycle also underpins the Jewish calendar, the calculation of Easter, and various other traditional calendars.