We'll Have A Blue Moon This Weekend - EarthSky

Next Blue Moon: May 31, 2026

Our last Blue Moon came on August 19, 2024. It wasn’t blue in color. Blue Moon is (usually) just a name. That 2024 Blue Moon carried the name Blue because it was the third of four full moons in a season, with a single season being defined as the time between a solstice and an equinox. Those moons carry the name Blue Moon as well. And the August 19 Blue Moon was more. It was also a supermoon, in fact, it was the first of four full supermoons in a row in 2024.

The next Blue Moon is a calendar-month Blue Moon and will be on May 31, 2026. And it’s the most distant full micromoon of 2026. What’s a micromoon? That’s when the new or full moon reaches apogee or its greatest distance from Earth. A full micromoon appears about 12-14% smaller than a full supermoon and about 7% smaller than an average full moon.

What’s a Blue Moon?

So, why is it a Blue Moon? The answer stems from the nature of skylore, and folklore in general. It’s lore. And it’s of the “folk.” So it sometimes gets messy.

Blue-colored moons in photos – like the one at the top of this post – are often made using special blue camera filters or in a post-processing program such as PhotoShop. Usually, but not always.

What is a seasonal Blue Moon?

By season, we’re referring to the period of time between a solstice and an equinox, or vice versa. We’re talking about winter, spring, summer, fall. Each season typically lasts three months and typically has three full moons. The next seasonal Blue Moon on May 20, 2027, happens because June’s full moon falls about two days before the June solstice, early in the season of northern summer (southern winter). And thus, there’s enough time to squeeze four full moons into the 2027 March equinox season, which will end at the June solstice on June 21, 2027. Weirdly, it’s not the fourth of these four full moons that’ll be called a Blue Moon. It’s the third. Go figure.

Full moons (based on UTC date and time) between March 2027 equinox and June 2027 solstice:

March equinox: March 20, 2027

March full moon: March 22, 2027 April full moon: April 20, 2027 May full moon: May 20, 2027 June full moon: June 18, 2027

June solstice: June 21, 2027

How often do seasonal Blue Moons occur?

The phases of the moon recur on or near the same calendar dates every 19 years. That’s because 235 lunar months (235 returns to full moon) almost exactly equal 19 calendar years. Sure enough, 19 years from 2024 – in the year 2043 – the full moons will fall on June 22, July 21, August 20, and September 18.

Seasonal Blue Moons occur because there are 235 full moons but only 76 seasons (4 x 19 = 76) in this 19-year lunar cycle. If you have only three full moons in each season, then that’s a total of 228 full moons (76 x 3 = 228). Yet, there are 235 full moons in this 19-year cycle. So, these seven extra full moons (235 – 228 = 7) have to showcase seven 4-full-moon seasons in this 19-year period. We list upcoming seasonal Blue Moon dates – following the August 19, 2024, seasonal Blue Moon – below:

May 20, 2027 August 24, 2029 August 21, 2032 May 22, 2035 May 18, 2038 August 22, 2040 August 20, 2043

How often do seasonal Blue Moons happen? Often, as you can see.

What is a monthly Blue Moon?

In modern times, most of us know Blue Moons as the second full moon of a single calendar month. These happen a lot, too. The next Blue Moon by this definition will come on May 31, 2026.

The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every two to three years, so this sort of Blue Moon comes about that often.

Very rarely, a seasonal Blue Moon (third of four full moons in one season) and a monthly Blue Moon (second of two full moons in one calendar month) can occur in the same calendar year. For this to happen, you need 13 full moons between successive December solstices for a seasonal Blue Moon and, generally, 13 full moons in one calendar year for a monthly Blue Moon.

This will next happen in the year 2048, when a monthly Blue Moon falls on January 31, and a seasonal Blue Moon on August 23.

Then 19 years later, in the year 2067, there will be a monthly Blue Moon on March 30, and a seasonal Blue Moon on November 20. In this instance, there are 13 full moons between successive December solstices, but only 12 full moons in one calendar year and no February 2067 full moon.

Calendar-month Blue Moons

Calendar-month Blue Moons happen when two full moons fall within a single calendar month. The second of the month’s two full moons is popularly called a Blue Moon. This sort of Blue Moon happens seven times in every 19 years.

Let’s take a look at the eight calendar-month Blue-Moons in the present 19-year Metonic cycle:

1. March 31, 2018 2. October 31, 2020 3. August 31, 2023 4. May 31, 2026 5. December 31, 2028 6. September 30, 2031 7. July 31, 2034 8. January 31, 2037

Also, in a year where February has no full moon at all, as in the year 2018, you can have two full moons in January and two full moons in March. Thus, during those years there are two Blue Moons in single year. The next time we have two Blue Moons in one year is 2037.

Why call them Blue Moons?

The idea of a Blue Moon as the second full moon in a month is more recent – more modern – than the idea of a Blue Moon as the third of four full moons in a season. It stemmed from the March 1946 issue of Sky and Telescope magazine. The magazine published an article called “Once in a Blue Moon” by James Hugh Pruett. Pruett was referring to the 1937 Maine Farmer’s Almanac, which defined Blue Moons as the third of four full moons in a season. But he inadvertently simplified the definition. He wrote:

Seven times in 19 years there were – and still are – 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.

Had James Hugh Pruett looked at the actual date of the 1937 Blue Moon, he would have found that it had occurred August 21, 1937. Also, there were only 12 full moons in 1937. You generally need 13 full moons in one calendar year to have two full moons in one calendar month.

However, that fortuitous oversight gave birth to a new and perfectly understandable definition for Blue Moon.

Blue Moons as modern folklore

The notion of a Blue Moon as the second full moon of a calendar month was buried for decades. Then, in the late 1970s, EarthSky’s Deborah Byrd happened upon a copy of the old 1946 issue of Sky and Telescope in the stacks of the Peridier Library at the University of Texas Astronomy Department. Afterward, she began using the term Blue Moon to describe the second full moon in a calendar month on the radio series StarDate, which she wrote and produced.

Later, this definition of Blue Moon was also popularized by a book for children by Margot McLoone-Basta, called The Kids’ World Almanac of Records and Facts, published in New York by World Almanac Publications in 1985. The second-full-moon-in-a-month definition was also used in the board game Trivial Pursuit.

Today, it has become part of modern folklore. As the folklorist Philip Hiscock wrote in his comprehensive article Once in a Blue Moon:

‘Old folklore’ it is not, but real folklore it is.

Are moons ever blue in color?

Sure, they are, and someday you might see a true blue-colored moon in the sky. Blue-colored moons are rare – aren’t necessarily full – and happen when Earth’s atmosphere contains dust or smoke particles of a certain size. The particles must be slightly wider than 900 nanometers.

You might find particles of this size in the air above you when, for example, a wildfire is raging nearby. Particles of this size are very efficient at scattering red light. When these particles are present in our air, and the moon shines through them, the moon may appear blue in color.

To read more about truly blue-colored moons, click here

Bottom line: What is a Blue Moon? The 2nd of 2 full moons in a calendar month? Or the 3rd of 4 full moons in a single season? The answer is … both!

Phases of the moon: 2001 to 2100

Solstices and equinoxes: 2001 to 2100

Possible to have only 2 full moons in a single season?

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