What is a Blue Moon?

Blue moon to light up sky ahead of Easter

January 29, 2019
Full Blue Moon

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When is the next Blue Moon? And exactly what is a Blue Moon? There’s some confusion about this popular phrase. Here’s the scoop.

Why is it a Blue Moon?

We got a lot of letters about this. Some folks imagine that it actually appears the color blue. Others assume it’s a term that originated with Native American folklore. Neither is true. Also, Blue Moon is not an astronomical term. This term only started in the 1940’s as a result of a mistake in an astronomy magazine. But it slowly went viral in the media. 

The notion has no venerable pedigree among Native Americans, ancient cultures, or anything in the actual sky. So OK, we’ll play along and call it a Blue Moon. Just so you know, it’s a new idea.

There are two sorts of Blue Moon definitions:

  • An extra full Moon that occurs in a season. One season—winter, spring, fall, summer—typically has three full moons. If a season has four full moons, then the third full moon may be called a Blue Moon.
  • The second full moon of a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon). Therefore, there will be an extra Moon in a calendar year.

The second definition has become more popular in recent decades.


When is the Next Blue Moon?

As of this writing, the next Blue Moon (as defined as the second full moon of a calendar month) is March 31, 2018.

See the March Full Moon Guide.

This March 31 Blue Moon will light up the skies ahead of Easter Sunday. Kind of neat.

How Often Do We See a Blue Moon?

Most months have one Full Moon, not two.

Since the Moon’s period of phases is 29 ½ 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, Blue Moons happen every 30 months on average. Two and a half years.  Ever heard the expression, “Once in a Blue Moon?”  Not so very rare!

After this March 31 Blue Moon, the next one will be on October 31, 2020—Halloween!

Two Blue Moons in a Single Year

But how often do we have two Blue Moons in a single year?—as happened in 2018?

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

See the Almanac Moon Phase Calendar.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe


Reader Comments

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Once In A Blue Moon?

We should change the expression to "Once in a Super Blue Blood Moon"!!!

Blue Moon

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

Blue Moon

A Blue Moon is the forth full moon in a quarter,not the second in a month.

Blue Moon Definitons

The Editors's picture

Generally, both definitions are accepted.

Whenever the moon is low it

Bob Berman's picture

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

Last night me and my 5 year

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?

See my comment, just above

Bob Berman's picture

See my comment, just above --


Last nights full moon did

Last nights full moon did apear here,East tuscaloosa Co. Al. a light blue.Around * pm CDT. Yellow afew min. later. I think This is a rare phenomonom that sarted the term. I believe I have read of this. I feel the origonal use was based on this and had mothing to do with calender days. Also we are wasting valuble sleep time debating this. Good night. Still would like other coment if others have acully seen a blue moon.

Event for August to be seen

Event for August to be seen through the telescope?

I took a excellent picture of

I took a excellent picture of the blue moon tonight. My grandchildren (the Twins)thought it was an actual blue moon, funny. Beautiful glow of brightness with the grayish crater to be seen. Can another event be seen in August through the telescope for the grand children?

I heard we won't see another

I heard we won't see another one til 2018.

The song 'Blue Moon' was

The song 'Blue Moon' was written in 1934 so the 1940's couldn't have been the source of the terminology.

I'm not sure that's what he's

I'm not sure that's what he's getting at. He's just saying that "Once in a Blue Moon" is now a common expression that means something rarely happens. But in the world of astronomy, a "Blue Moon" (the second full Moon in a month) is not really a rare event. It happens every 2 to 3 years. And it's also just a normal calendar thing since a moon cycle is about a day shorter than a calendar month. 

I understood that the

I understood that the original calender was 13 months and when we went to a 12 month calender we had to have 2 moons in one month. But if it is every 2/1/2 years then that doesn't really make sense.

I just read about the 19 year

I just read about the 19 year metonic moon cycle, that on July 31, 2015 there will be a blue moon, and every 19 years after that, this cycle will place a blue moon on July 31 (2034). But there was a blue moon on July 31, 2004- less than 19 years back. Was the 2004 blue moon special in some way that it was not part of this 19 year cycle? Can you please explain this to me?

is it true that if one

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?

Bob's column is about

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:

I have another question for

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?

My daughter was born "under"

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?

Yes, the full Moon on July 31

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!

When is the "Horns of the

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.

No idea about this folklore.

Bob Berman's picture

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

A true Blue Moon is the 3rd

A true Blue Moon is the 3rd of 4 Full Moons that fall between the seasonal markers. ~SHE

Actually, there is no true

Bob Berman's picture

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. 

What is the full story on

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.

Everyone is free to give any

Bob Berman's picture

Everyone is free to give any name to anything, and sometimes a definition sticks, sometimes it doesn't. There's rarely any logic behind it. Only two of the year's full moons are accepted by science and astronomers, the Harvest Moon around the September Equinox, and the next full moon after that, the Hunters Moon. No full moon consistently looks different from any of the others, though, let alone blue.

Thanks Bob. Love this artcle

Thanks Bob. Love this artcle especially cause I'm a moonchild

Thanks, I appreciate

Bob Berman's picture

Thanks, I appreciate that.....

"Slowly went viral"

"Slowly went viral"

i love what goes on in the

i love what goes on in the beautiful night sky

Thanks, Toni. We certainly

Bob Berman's picture

Thanks, Toni. We certainly have that in common.....


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