A Blue Moon is an extra full moon: the second full moon in one calendar month, or the third of four full moons in one astronomical season. Each kind comes around about every 2.4 to 2.7 years on average.
Key facts
- A monthly Blue Moon is the second full moon in one calendar month.
- A seasonal Blue Moon is the third of four full moons in one astronomical season.
- The monthly definition began with a 1946 misreading of the Maine Farmers' Almanac.
- Monthly Blue Moons depend on time zone; seasonal Blue Moons are the same worldwide.
- The Moon can look truly blue only when smoke or volcanic dust fills the air.
What are the two kinds of Blue Moon?
There are two Blue Moon definitions: a monthly Blue Moon is the second full moon in a calendar month, and a seasonal Blue Moon is the third full moon in an astronomical season that has four.
| Monthly Blue Moon | Seasonal Blue Moon | |
|---|---|---|
| Rule | Second full moon in one calendar month | Third of four full moons between an equinox and a solstice |
| Months it can fall in | Any month except February | February, May, August or November |
| Occurrences, 2000–2099 | 41, about every 2.4 years | 37, about every 2.7 years |
| Same date worldwide | No, it depends on time zone | Yes |
A Blue Moon looks like any other full moon, and the next full moon page gives its exact time when one is due. On this site a Blue Moon also keeps the month’s traditional name. The second full moon of May 2026, on May 31, was both a Flower Moon and a Blue Moon. The guide to full moon names explains how each name is assigned.
When is the next Blue Moon?
The next Blue Moon is the full moon of , a seasonal Blue Moon. Upcoming monthly and seasonal Blue Moons are listed below, with dates and times in UTC.
| Full moon (UTC) | Blue Moon type | Also |
|---|---|---|
| Thursday, May 20, 2027, 10:59 | Seasonal (3rd of 4 in the season) | Flower Moon |
| Sunday, December 31, 2028, 16:48 | Monthly (2nd full moon in the month) | Cold Moon |
| Friday, August 24, 2029, 01:51 | Seasonal (3rd of 4 in the season) | Sturgeon Moon |
| Tuesday, September 30, 2031, 18:58 | Monthly (2nd full moon in the month) | Harvest Moon |
| Saturday, August 21, 2032, 01:47 | Seasonal (3rd of 4 in the season) | Sturgeon MoonSupermoon |
| Monday, July 31, 2034, 05:54 | Monthly (2nd full moon in the month) | Buck Moon |
Monthly Blue Moons are counted in UTC; in some time zones the month boundary can move a Blue Moon to a different month.
Monthly Blue Moons in the table use UTC dates. Far east or west of UTC, a monthly Blue Moon can fall on a different date or disappear from that month, as the time-zone section below explains.
How often is there a Blue Moon?
A Blue Moon happens about once every 2.4 to 2.7 years, because 12 lunar cycles last about 354 days, about 11 days less than a calendar year.
The lunar cycle from one full moon to the next lasts 29.53 days on average. Twelve cycles take 354.4 days, so the full moons arrive about 11 days earlier each year. After two or three years, the drift squeezes a 13th full moon into the year. Sky & Telescope puts this at about seven times in every 19 years, which matches the seasonal Blue Moon rate of once every 2.7 years.
Monthly Blue Moons are slightly more common, 41 times between 2000 and 2099 by UTC date. February explains the difference. It is shorter than every lunar cycle, so it never has two full moons and sometimes has none.
When February has none, two other months that year can each gain a second full moon. As a result, the gap between monthly Blue Moons ranges from about 2 months to almost 3 years.
Why does a monthly Blue Moon depend on your time zone?
A monthly Blue Moon depends on the calendar date, and the date of a full moon changes with time zone, so a Blue Moon in one country can be an ordinary full moon in another.
The Blue Moon of October 31, 2020 shows the effect. It came at 14:49 UTC, which put it on October 31 across the Americas, Europe, Africa and most of Asia. In Sydney the same instant was 1:49 a.m. on November 1, and in Auckland 3:49 a.m. on November 1.
Sydney and Auckland therefore had no Blue Moon in October. They had one on November 30 instead, the second full moon of their November.
Seasonal Blue Moons do not have this problem. The full moon and the equinox or solstice are both instants, so every place counts the same number of full moons in a season. This site flags monthly Blue Moons by UTC date. The moon phase calendar shows full moon dates month by month.
Where did the second-full-moon definition come from?
The monthly definition comes from a misreading: a 1946 Sky & Telescope article misinterpreted the Maine Farmers’ Almanac, which had used Blue Moon for the third full moon in a season with four.
The almanac’s rule was seasonal. Most years contain 12 full moons, three in each season, but some contain 13, so one season gets four. The almanac called the third of those four a Blue Moon, which kept its other moon names in step with the equinoxes and solstices. Its 1937 edition placed a Blue Moon on August 21.
In 1946 James Hugh Pruett wrote in Sky & Telescope that a year with 13 full moons has “11 months with one full moon each and one with two.” Sky & Telescope later noted that he must not have had the 1937 almanac at hand. That almanac’s Blue Moon fell on August 21, clearly not a second full moon in its month, and 1937 had only 12 full moons. Pruett’s reading spread, and the second full moon in a month became the more popular meaning.
What does “once in a blue moon” mean?
“Once in a blue moon” means very rarely, but the idea of a blue moon is older than either astronomical definition and once meant something absurd or impossible.
Sky & Telescope traces the phrase back more than 400 years. In the 16th century, saying someone “would argue the Moon was blue” meant they would argue that black is white. Later, “when the Moon is blue” meant never. The modern sense of “rarely” fits the calendar Blue Moon, which arrives only every few years.
Can the Moon actually look blue?
The Moon can look truly blue, but only when smoke or volcanic dust fills the air with particles slightly wider than 0.7 micron, which scatter red light and let blue light through.
These events are rare and can happen at any phase. Sky & Telescope describes three:
- Krakatoa, 1883. When the Indonesian volcano exploded, its dust turned sunsets green and the Moon blue around the world for the best part of two years.
- India, 1927. The monsoon arrived late, and the long dry season raised enough dust to turn the Moon blue.
- Western Canada. Smoke from huge forest fires turned the Moon blue over northeastern North America.
timeanddate also links blue-tinted moons to the eruptions of Mount St. Helens in 1980 and Mount Pinatubo in 1991.
What is a super Blue Moon?
A super Blue Moon is a Blue Moon that is also a supermoon, a full moon within 90% of perigee, so it looks slightly larger and brighter than an average full moon.
The monthly Blue Moon of August 31, 2023 was a supermoon at about 357,333 km (222,036 mi) from Earth. The seasonal Blue Moon of August 19, 2024 was a supermoon too, at about 361,959 km (224,911 mi). On January 31, 2018, a Blue Moon, a supermoon and a total lunar eclipse coincided, a combination often called a super blue blood moon. The guide to the blood moon explains why the eclipsed Moon turns red.
The opposite also happens. The Blue Moon of May 31, 2026 was a micromoon, at about 406,126 km (252,355 mi), near the farthest point of the Moon’s orbit.