Astronomy Essentials

We’ll have a Blue Moon this weekend

Star chart showing the nights of August 19 through 22, 2021, with 4 positions of moon near Jupiter and Saturn.
Okay, so the moon won’t be blue in color on August 22, 2021. But it’ll be near Jupiter and Saturn! Read more.

Next Blue Moon August 21-22, 2021

Our last Blue Moon came on the night of Halloween, 2020. That night, the Blue Moon was near red Mars. But that full moon – like most Blue Moons – was blue in name only. We called it a Blue Moon because it was the second of two full moons in a calendar month. Now another sort of Blue Moon is coming up. It’s a seasonal Blue Moon, the third of four full moons in a season (the time between a solstice and an equinox). The upcoming Blue Moon will crest on August 21-22, 2021. And, by lucky coincidence, the moon will be near planets again, this time Jupiter and Saturn.

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 – like the ones on this page – are usually made using special blue camera filters or in a post-processing program such as PhotoShop. Usually … but not always.

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.

Blue moon: Crescent moon colored blue with blurred blue streaks of clouds.
Most Blue Moons aren’t blue in color. JV Noriega created this photo of a blue-colored moon among fast-moving clouds, using special blue filters.

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 upcoming seasonal Blue Moon of August 22, 2021, happens because June’s full moon falls just a few 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, 2021. 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 between June 2021 solstice and September 2021 equinox:

June solstice: June 21, 2021

June full moon: June 24, 2021
July full moon: July 24, 2021
August full moon (a Blue Moon): August 22, 2021
September full moon: September 20, 2021

September solstice: September 22, 2021

The Harvest Moon’s cousin

For those of us in the Northern Hemisphere, this August Blue Moon is a harbinger of the upcoming autumn season. It happens considerably closer to the equinox than the solstice. So, in the Northern Hemisphere, this late summer Blue Moon shares characteristics with the upcoming Harvest Moon in September. By definition, the Harvest Moon is the closest full moon to the autumn equinox. The true Harvest Moon will come on September 20, 2021.

Any full moon rises around sunset. On the average, the moon rises some 50 minutes later each day thereafter. The lag time between successive moonrises is reduced to a yearly low when the full Harvest Moon arrives. A month from now, at mid-northern latitudes, the moon will rise about 25 minutes (instead of 50 minutes) later daily, providing several nights of dusk-until-dawn moonlight.

At mid-northern latitudes, this August Blue Moon will show this same characteristic (a moonrise about 25 minutes later on successive evenings, instead of closer to 50 minutes later), although to a lesser degree. Like September’s Harvest Moon, it’ll showcase a few to several nights of dusk-until-dawn moonlight.

Meanwhile, in the Southern Hemisphere, the September equinox is the spring equinox. Therefore, this August Blue Moon acts as a harbinger of springtime, exhibiting longer-than-average lag times between moonrises.

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 now – in the year 2040 – the full moons will fall on June 24, July 24, August 22, and September 20.

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. We list upcoming seasonal Blue Moon dates – following the August 22, 2021 Blue Moon – below:

1) August 19, 2024
2) May 20, 2027
3) August 24, 2029
4) August 21, 2032
5) May 22, 2035
6) May 18, 2038
7) August 22, 2040

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

Huge full moon colored blue against silhouetted desert vegetation.
Desert Blue Moon from our friend Priya Kumar in Oman, August 2012. Thank you, Priya! Photos of blue moons, like this one, are made using blue filters.

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.

Full moon half blue and half orange.
Blue Moons don’t really look blue in color. Greg Hogan got this shot of a Blue Moon (blue in name only!) on July 31, 2015. He wrote: “Having some fun with the blue moon idea … I blended the same image twice: one with a blue tint, and one normal. :) ” Thank you Greg!

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.

Closeup of moon showing craters and dark areas, all in indigo colors.
It’s very rare that you would see a moon that’s actually blue in color. This photo was created using special filters. Most Blue Moons you hear about are Blue in name only. Image via our friend JV Noriega.


Phases of the moon: 2001 to 2100

Solstices and equinoxes: 2001 to 2100

Two people standing facing each other reaching toward large overexposed full moon.
What most call a Blue Moon isn’t blue in color. It’s only Blue in name. This great moon photo is from EarthSky Facebook friend Rebecca Lacey in Cambridge, Idaho.

Bottom line: Modern folklore has defined two different kinds of Blue Moons. The last Blue Moon – second full moon of a calendar month – came on October 31, 2020. The other sort of Blue Moon – third of four full moons in a single season, with a season being between a solstice and equinox – will come on August 22, 2021.

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

EarthSky’s moon phase calendar for 2021

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August 19, 2021
Astronomy Essentials

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Bruce McClure

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