Astronomy Essentials

Blue moon? What is it and when is the next one?

Blue moon: Sky chart: Full moon next to dot for Saturn.
The full moon – and a monthly blue moon – will be overnight on August 30-31, 2023. It’s also a supermoon – the 3rd of 4 in a row – and the closest supermoon of 2023. Chart via John Jardine Goss/ EarthSky.

See photos of the August 30-31 blue supermoon here!

Next Blue Moon on August 30-31, 2023

Our last Blue Moon came on August 21-22, 2021. That night, the Blue Moon was near the planets Jupiter and Saturn. But that full moon – unlike most Blue Moons – was a seasonal blue moon. A seasonal blue moon is the third of four full moons in a season (the time between a solstice and an equinox). The Blue Moon on August 30-31, 2023, is a monthly Blue Moon because it’s the second of two full moons in a calendar month.

The second of two full moons in a calendar month? Or third of four full moons in a single season? How can both of them be Blue Moons? 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 are usually made using special blue camera filters or in a post-processing program such as PhotoShop. Usually … but not always.

Full moon with a blue hue with black background.
A full “blue” moon. Image Photo via Eileen Rollin/ Unsplash. Used with permission.

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

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 August 19-20, 2024, happens because June’s full moon falls about two days after 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 current season, which will end at the September equinox on September 22, 2024. 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 June 2024 solstice and September 2024 equinox:

June solstice: June 20, 2024

June full moon: June 22, 2024
July full moon: July 21, 2024
August full moon: August 19, 2024
September full moon: September 18, 2024

September solstice: September 22, 2024

How often do seasonal Blue Moons occur?

As it happens, 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 August 31, 2023.

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.

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.


Phases of the moon: 2001 to 2100

Solstices and equinoxes: 2001 to 2100

Bottom line: Modern folklore has defined two different kinds of Blue Moons. The last Blue Moon – third of four full moons in a single season, with a season being between a solstice and equinox – came on August 22, 2021. The other sort of Blue Moon – second full moon of a calendar month – will come on August 30-31, 2023.

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

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August 1, 2023
Astronomy Essentials

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Bruce McClure

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