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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 Wednesday night, August 30. 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 August 30, 2023

The next Blue Moon will occur on Wednesday, August 30, 2023. 

This Blue Moon is also special because it’s the nearest supermoon of 2023. This means that this year’s Blue Moon is also the closest and brightest Moon of the year!  

A Blue Moon and a Super Moon occuring on the same night happens roughly every 10 years, and won’t occur again for another 14 years, in the night sky in January and March 2037.

A Supermoon is when the Moon is at or near its closest point to Earth in its oval-shaped orbit. A super moon can also cause higher tides than usual.

Read more about the August full Moons here.

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 gave way to the calendrical definition, which was later popularized in the 80s and has stuck around 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 will again 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>Barbara B. …</span> Fri, 07/17/2015 - 18:34
is it true that if one cuts his hair right after a full moon, his hair will grow back at a slower pace than if he cut it before the full moon?
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 07/21/2015 - 23:59

Bob's column is about astronomy. That said, there is indeed an an age-old astrology belief that the Moon affects hair growth. You can find our best days to cut hair--to encourage growth or discourage growth--here on this timetable:
www.almanac.com/bestdays/timetable

 <span>Amy W</span> Wed, 07/15/2015 - 21:44
I have another question for you: In January of 2000 (I think it was a Wednesday, maybe the 26th of January), I was living in Washington DC area at that time on the top floor of an apartment building. I looked out my big living room window to the southwest facing night sky (10:00 ish). I saw a green light arch and hook and then it was gone. I think it was a comet. How can I find out more about it, or at least verify it?
<span>Amy W</span> Wed, 07/15/2015 - 21:33
My daughter was born "under" the Blue Moon on July 31, 2004. We looked it up in the Farmers Almanac and it said that Blue Moon of July 31, 2004 was called the Thunder Moon. Now on July 31st, exactly 11 years later, there will be another July 31st Blue Moon. Will it also be the Thunder Moon, or will it have another name?
<a title="View user profile." href="/author/editors">The Editors</a> Thu, 07/16/2015 - 13:04

Yes, the full Moon on July 31 can be called a Thunder Moon. Actually, any full Moon around July can be named Thunder Moon, which was an Algonquin name, in addition to Buck Moon. The Colonial Americans also used the term Thunder Moon, as well as a few others. When two full Moons occur in one month, we list an alternate name for the second. Moons around the time of July were also known by names such as Raspberry Moon (Ojibway), Peaches Moon (Natchez), Crane Moon (Choctaw), and Moon when the Cherries are Ripe (Lakota Sioux).

Early Native American/First Nations tribes kept track of time by observing the seasons and lunar months. Learn more about full Moon names: www.almanac.com/content/full-moon-names

Hope this helps, and please extend our Happy Birthday wishes to your daughter!

 

 <span>Barbara David</span> Sun, 07/05/2015 - 20:32

When is the "Horns of the Moon" —which is supposed to be the best time to stop an addiction? A friend stopped a 4 pack a day smoking habit easily....no problem. Several months later an astrologist told him it was because he did it on the Horns of the Moon. I have wondered about this for years.

<a title="View user profile." href="/author/bob-berman">Bob Berman</a> Mon, 07/06/2015 - 20:02

No idea about this folklore. But I hope they're successful in kicking that habit.

 <span>Shelli H English</span> Thu, 07/02/2015 - 00:24
A true Blue Moon is the 3rd of 4 Full Moons that fall between the seasonal markers. ~SHE
<a title="View user profile." href="/author/bob-berman">Bob Berman</a> Mon, 07/06/2015 - 20:04

Actually, there is no true blue moon that's accepted officially by anyone. Nowadays the most widely accepted definition is the second full moon in the same calendar month. Previously, and only in Maine, a local publication had defined it the way you just did. 

 <span>Jeffrey Maslin</span> Wed, 07/01/2015 - 23:20
What is the full story on what we call a "Blue Moon"? Why call it "Blue"? The article mentions it came from a misunderstanding, it seems like there would be some other term applied. Maybe "Double Moon" or "Wishing Moon" (implying one could have a wish granted because of it's so-called rarity, like wishing on a falling star). I've tried to research it, but only found definitions.