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When Is the Next Blue Moon?

There are two definitions of a Blue Moon; both are a type of Full Moon and have nothing to do with color. If the Moon looks blue, it's because of dust in the atmosphere.

Illustrating a blue moon. Manipulated photo of silhouetted trees in front of a blue glowing full moon in a very blue sky.

Dust particles can make the Moon look blue.

When volcanoes erupt, the dust particles can cause the Moon to appear blue.

©bigstockphoto.com/Kamira

How Rare Is a Blue Moon?

The term once in a Blue Moon suggests something happens very rarely. So, how often do Blue Moons occur? The answer depends on how you define a Blue Moon.

Two Definitions

There are two ways of calculating the date of a Blue Moon.

  1. Seasonal Blue Moon = The third Full Moon in an astronomical season with four Full Moons (versus the usual three).
  2. Monthly Blue Moon = The second Full Moon in a month with two Full Moons.

In the 1100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons. This means that either Blue Moon occurs roughly every two or three years, although the monthly ones are a little more frequent than the seasonal ones.

Blue Flower Moon in May

The seasonal Blue Moon is the original astronomical definition of a Blue Moon. Usually, there are three Full Moons between each astronomical season, which is the time between each solstice and equinox. But some years, there are four Full Moons in a season. When this happens, the third Full Moon is called a Blue Moon.

In 2019, May's Full Moon was a seasonal Blue Moon. The astronomical season started with the equinox on March 20, and the first Full Moon alignment was less than four hours later; on March 21. The second Full Moon was on April 19, and the third, the Blue Moon, on May 18. The fourth and last Full Moon before the June solstice was on June 17.

Not the Same Worldwide

All dates mentioned are based on UTC time. There is some variation due to time zone differences. Look up local times for Moon Phases along with information about lunations, Supermoons, Micromoons, and Black Moons. You will also find Full Moon names for the Northern Hemisphere.

Two Full Moons in the Same Month

The calculation of the dates for monthly Blue Moons is based on the 12 12 months in our calendar rather than astronomical seasons.

A lunar month – the time between two Full Moons – is around 29.5 days long, while most calendar months are longer. Because of this, most months have only one Full Moon. Some years, however, there are two Full Moons in the same month, and the second one is often called a Blue Moon.

In many Northern Hemisphere cultures, each month's Full Moon is named after a specific seasonal or natural phenomena, for example, Harvest Moon. When there are two Full Moons in a month, the second one doesn't have a proper name. The term Blue Moon has over time become a placeholder name for the extra Full Moon. This way, the other 12 Full Moons keep their rightful place in relation to the solstices and equinoxes.

Why Are There Different Definitions?

The reason the second definition of Blue Moon exists is due to an error originally made by amateur astronomer James Hugh Pruett (1886–1955). He misunderstood the basis for calculating the seasonal Blue Moon and wrote that a Blue Moon was the second Full Moon in a month in an article published in Sky & Telescope magazine in 1946. This erroneous definition since spread, particularly after it was quoted in a popular radio program called StarDate in 1980 and then appeared as an answer in a 1986 version of the board game Trivial Pursuit. Today, it is considered a second definition rather than a mistake.

Double Blue Moon

In 2018, there were two Full Moons in January and March in most time zones. This is sometimes called a double Blue Moon and takes place only about three to five times in a century. This will happen next in 2037.

Other combinations of Blue Moons also exist. Between 1550 and 2650 there are 20 years that have one seasonal and one monthly Blue Moon. The next time is in 2048 while the previous time was in 1934. Triple Blue Moons, a combination of one seasonal and two monthly Blue Moons in the same calendar year, happens 21 times in the same time span. The next is in 2143, while the last time was in 1961.

There can never be a double seasonal Blue Moon, as that would require 14 Full Moons in the same year, which is not possible because the time between two Full Moons is approximately 29.5 days.

Some years, February has no Full Moon at all, which is called a Black Moon.

The Rarest Blue Moon

A Moon that actually looks blue, however, is a very rare sight. The Moon, full or any other phase, can appear blue when the atmosphere is filled with dust or smoke particles of a certain size: slightly wider than 900 nm. The particles scatter the red light, making the Moon appear blue. This is known as Mie scattering and can happen for instance after a dust storm, a forest fire, or a volcanic eruption.

Eruptions like the ones on Mt. Krakatoa in Indonesia (1883), El Chichon in Mexico (1983), on Mt. St. Helens in the US (1980), and Mount Pinatubo in the Philippines (1991) are all known to have made the moon look blue. Some people even suggest the term once in a Blue Moon is based on these rare occasions, rather than the Full Moon definitions above.

Red Moon

The Full Moon can look red if other sizes of dust particles fill the atmosphere or during Total Lunar Eclipses, which are much more common than Moons that look blue.

Super Blue Blood Moon Eclipse

On the night of the Blue Moon on January 31, 2018 (UTC), there was a total lunar eclipse. A totally eclipsed Moon usually looks red, and because of this coloring, it is sometimes called a Blood Moon. So, this was a rare opportunity to see a red Blue Moon. If this wasn't enough, it was also almost a Supermoon, earning it the nickname Super Blue Blood Moon.

Blue Sky and Red Sunset

A similar phenomenon is Rayleigh scattering, which occurs when light strikes a molecule of air. This scattering causes the sky to look blue in the daytime and red at sunrise and sunset.

Topics: Astronomy, Moon, Calendar, Months

Recent/Upcoming Blue Moons

YearDateType
2019May 183rd Full Moon in season
2020Oct 312nd Full Moon in a month
2021Aug 223rd Full Moon in season

Blue Moons can vary by time zone. Dates above are based on the local time in Fullerton. Change location

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