What is a Blue Moon, and when is the next one?

Blue Moon: Blue colored crescent moon with blurred blue streaks of clouds.
Most blue moons are not blue in color. JV Noriega created this photo of a blue colored moon among fast moving clouds, using special blue filters.

Next Blue Moon August 22, 2021

Our last Blue Moon arrived on October 31, 2020. It was Halloween night and Mars was near the Blue Moon. Mars was close to Earth and very Red then. But this full moon – like most Blue Moons – was blue by name only. We called it a blue moon because it was the second of two full moons in a calendar month. Now a different kind of Blue Moon is coming. It is the third of the four full moons of a season (the time between a solstice and an equinox). The next Blue Moon is like this. This will happen on August 22, 2021. And, by a happy coincidence, the moon will again be close to the planets, this time Jupiter and Saturn.

The second of two full moons in a calendar month? Or the third of four full moons in a single season? How can the two of them be Blue Moons? The answer comes from the nature of skylorus and folklore in general. It’s the tradition. And it’s “folk”. Suddenly, it sometimes gets messy.

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

Are moons ever blue in color?

Of course they are, and one day you might see a real blue in color moon in the sky. Blue-colored moons are rare – are not necessarily full – and occur when the Earth’s atmosphere contains dust or smoke particles of a certain size. The particles should be slightly larger than 900 nanometers.

You might find particles of this size in the air above you when, for example, a forest fire is raging nearby. Particles of this size are very effective 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 learn more about the truly blue moons, click here.

Sky map showing the nights of August 19 to 22, 2021, with 4 positions of the moon near Jupiter and Saturn.
Okay, so the moon will not be blue on August 22, 2021. But it will be near Jupiter and Saturn! Read more.

What is a seasonal blue moon?

By season we refer to the period of time between a solstice and an equinox. Or vice versa. We are talking about winter, spring, summer, autumn. Each season typically lasts three months and typically has three full moons. The next seasonal blue moon on August 22, 2021 occurs because the June full moon falls just a few days after the June solstice, at the start of the northern summer season (southern winter). And so, there is enough time to insert four full moons into the current season, which will end on the September equinox on September 22, 2021. Oddly, this is not the fourth of those four full moons. ‘will call a blue moon. It’s the the third. Go figure it out.

Full moons between the solstice of June 2021 and the equinox of September 2021:

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

Harvest Moon’s cousin

For us in the Northern Hemisphere, this blue August moon is a harbinger of the coming fall season. This happens much closer to the equinox than to the solstice. Thus, in the northern hemisphere, this late summer Blue Moon shares characteristics with the next Harvest Moon in September. By definition, the Harvest Moon is the full moon closest to the fall equinox. The true harvest moon will come on September 20, 2021.

Every full moon rises at sunset. On average, the moon rises about 50 minutes earlier each day thereafter. The latency time between successive moonrises is reduced to an annual minimum when the full harvest moon arrives. In a month, at northern mid-latitudes, the moon will rise about 25 minutes (instead of 50 minutes) later each day, providing several moonlit nights from dusk until dawn.

Right now, in northern mid-latitudes, this August blue moon will also be accompanied by earlier than average moonrises, albeit to a lesser degree. It will feature a few to several moonlit nights from dusk to dawn. This August blue moon is the harbinger of the northern hemisphere’s fall season.

Meanwhile, in the southern hemisphere, the September equinox is the spring equinox. Therefore, this August blue moon acts as a harbinger of spring, exhibiting longer than average lag times between moonrises.

How often do seasonal blue moons occur?

It turns out that the phases of the moon recur on the same or close to the same calendar dates every 19 years. This is because 235 lunar months (235 full moon returns) almost exactly equates to 19 calendar years. Indeed, in 19 years – in 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 only have three full moons per season, that makes a total of 228 full moons (76 x 3 = 228). Yet there are 235 full moons in this 19-year cycle. Thus, these seven additional full moons (235 – 228 = 7) must have seven full moon seasons. We list the next seasonal Blue Moon dates below – after the Blue Moon on August 22, 2021:

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 occur? Often as you can see.

Huge blue full moon 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 the Blue Moons as the second full moon in a single calendar month. These also happen a lot. The next Blue Moon according to this definition will come on August 31, 2023.

The time between one full moon and the next is near the duration of one calendar month. So the only time a month can have two full moons is when the first full moon occurs in the first few days of the month. It happens every two or three years, so this kind of blue moon happens a lot.

Very rarely, a seasonal Blue Moon (one-third of the four full moons in a season) and a monthly Blue Moon (the second of two full moons in a calendar month) may 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, typically, 13 full moons in a calendar year for a monthly blue moon.

This will then happen in 2048, when a monthly blue moon falls on January 31 and a seasonal blue moon falls on August 23.

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

Full moon half blue and half orange.
Blue moons don’t really look blue in color. Greg Hogan took this photo of a blue moon (blue in name only!) On July 31, 2015. He wrote: “I’m having fun with the idea of ​​the blue moon… I have mixed the same image twice : Ordinary. 🙂 “

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 arises from the March 1946 issue of Sky and telescope magazine. The magazine published an article titled “Once in a Blue Moon” by James Hugh Pruett. Pruett was referring to the 1937 Maine Farmers 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 have been – and still are – 13 full moons in one year. This gives 11 months with one full moon each and one with two. This second in a month, so I’m interpreting it, was called Blue Moon.

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

However, this fortuitous oversight gave birth to a completely understandable new definition of Blue Moon.

Blue Moons as modern folklore

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

Later, this definition of Blue Moon was also popularized by a children’s book by Margot McLoone-Basta, titled The Kids’ World Almanac of Records and Facts, published in New York by World Almanac Publications in 1985. The Definition in a Month was also used in the board game Trivial Pursuit.

Today it is part of modern folklore. As folklorist Philip Hiscock wrote in his full article Once in a Blue Moon:

It is not “old folklore”, but real folklore it is.

Close-up of the moon showing craters and dark areas, all in indigo colors.
It is very rare that you see a moon that is 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.


Moon phases: 2001 to 2100

Solstices and equinoxes: 2001 to 2100

Two people facing each other walking towards a large overexposed full moon.
What most call a blue moon is not blue in color. This is just the name blue. This awesome moon photo is from EarthSky’s Facebook friend Rebecca Lacey in Cambridge, Idaho.

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

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

EarthSky moon phase calendar for 2021

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