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Blue Moons: What is a Blue Moon

Full Blue Moon OFA with text
Photo Credit
Colleen Quinnell

What is the Next Blue Moon?

Written By: Bob Berman Astronomer

A “Once in a Blue Moon” happens on May 31, 2026. The Moon will not actually be the color blue, so don’t be disappointed. So, what is a Blue Moon, exactly? Read all about the different types of Blue Moons and tell us what you think.

What Is a Blue Moon?

Need a refresher on what a Blue Moon is? We get a lot of questions about the term, including:

  • Is “Blue Moon” a scientific term used in astronomy?
  • Did this term originate with Native American folklore, like a number of the other full Moon names?
  • Does a Blue Moon really look blue?

In truth, the answer to all of these questions is “no.”

Read Next

The modern understanding of “Blue Moon” only took off in the 1980s. It was a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine, and since then, the term has gone viral in the media.  

Next Blue Moon on May 31, 2026

The next Blue Moon is on Sunday, May 31. Technically, it crests at 4:45 A.M., near moonset.

Two Types of Blue Moons

There are two definitions of the term, though we commonly use the second term (a second full moon in one month).

  • Seasonal Blue Moon: The extra full Moon that occurs within an astronomical season. One season—defined by the dates of the solstices and equinoxes—typically has three full moons occurring within it. If a season instead has four full moons, then the third full moon (not the fourth) in the season may be called a Blue Moon.
  • Calendrical Blue Moon: The second full moon to occur in a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon), so if a full Moon occurs on the first of a month, there will be a second full Moon—a Blue Moon—at the end of the month, too (except in February).

Although the latter definition is the one more commonly followed today, the former actually came first. As mentioned above, a misinterpretation of the seasonal definition in the 1940s led to the calendrical definition, which was later popularized in the 80s and has persisted to today.

blue moon in the sky with a tree
A Blue Moon is when TWO full moons are seen in the same month.

How Often Does a Blue Moon Occur?

Most months have one full Moon, not two.

Since the Moon’s period of phases is 29 1/2 days, while months usually have 30 or 31 days, it’s obvious that if a full Moon lands on the first day of any month except February, it will repeat again at the end. 

Turns out, calendrical Blue Moons happen every 30 months on average. Two and a half years. Seasonal Blue Moons happen at a similar rate: about once every two to three years. So maybe “Once in a Blue Moon” isn’t so rare after all! 

Two Blue Moons in a Single Year

How often do we have two Blue Moons in a single year? (As in 2018, when there were Blue Moons in both January and March and no full Moon in February.)

This won’t happen again until 2037, when we’ll have another Blue Moon in both January and March.

Learn More

Want to learn more about the Moon? See the Almanac’s Moon Phase Calendar and Full Moon Calendar!

Full Moon Names
About The Author
Bob Berman

Bob Berman

Astronomer

Bob Berman is an astronomer, author, and science communicator with decades of experience making the wonders of the universe accessible and engaging. Known for his wit and insight, Bob has captivated a...
Read More from Bob Berman

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Comments

<span>Mary Ann Stowe</span> Fri, 09/01/2023 - 21:30
I LOVE all of your moon articles. Ans I have read the Farmer's Almanac for over 60 years - my Daddy bought one as soon as they came out. BUT I have one little suggestion. I can't believe how many asked/told me "What time will the moon would turn blue?", "The moon didn't turn blue?", "Is the moon blue around the world?", and so on! I am not sure if there was more publicity this year or what but more people seem to know about it this year/time- but not understand the reference to blue. So my suggestion: don't color the moon blue! Keep up the good work!
<span>Cindy</span> Sun, 08/06/2023 - 19:55
I just love reading these articles you wrote about the moon! So interesting!
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 08/30/2023 - 16:40

Thanks, Cindy!

 <span>Naimagarcia</span> Fri, 05/12/2023 - 12:16
i never herd about the blue moon what is the blue moon i wondor what the blue moon does
<span>Courtney Anne Jones</span> Wed, 03/14/2018 - 14:38
We should change the expression to "Once in a Super Blue Blood Moon"!!!
<span>Fredrick M. Smith</span> Tue, 01/16/2018 - 16:32
Hey Bob, I noticed that both January and March of 2018 have a "Blue Moon," and as a consequence, sandwiched in between is February with no full moon at all. It might be fun to calculate the odds of this occurrence and come up with a fun name for it. :) Keep up the good work and Cheers! -Fred
<span>Vern</span> Sun, 01/07/2018 - 11:45
A Blue Moon is the forth full moon in a quarter,not the second in a month.
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 01/09/2018 - 10:04

Generally, both definitions are accepted.

<a title="View user profile." href="/author/bob-berman">Bob Berman</a> Mon, 08/10/2015 - 13:19

Whenever the moon is low it looks absolutely enormous. This is the famous "Moon Illusion." I think maybe we'll do a blog about it soon, right here!

    Bob B

 <span>Tarbaby</span> Sat, 08/01/2015 - 08:55
Last night me and my 5 year old grandson went out to see the moon. He said it is so big about that time he grabbed me and said why is the mooonn moving it appeared to be jumping left and right also up and down what causes this?