A Blue Moon is a rare occurrence that depends on the timings of full moons during the year.

When is the next Blue Moon? The next Blue Moon will occur on Aug.19, 2024, at 2:26 p.m. ET (1826 GMT).

Related: Last Super Blue Moon until 2037 lights up skies around the world (photos)

There are two types of Blue Moons but unfortunately, neither has anything to do with color.

A seasonal Blue Moon is the traditional definition of a Blue Moon and refers to the third full moon in a season that has four full moons according to NASA. Whilst the second definition — borne out of a misunderstanding of the first — is a monthly Blue Moon which refers to the second full moon within a single calendar month. The monthly Blue Moon is nowadays considered the second definition of a Blue Moon rather than a mistake, according to Time and Date.

Related: What is a blood moon?

What is a Blue Moon?

As the cycle of the phases of the moon lasts approximately one month, we typically experience 12 full moons each year. Many cultures have given distinct names to each month's full moon. 12 months, 12 full moons, 12 names. Simple right?



Well, not quite. Here's where the Blue Moon comes into the equation.

The moon phases actually take 29.5 days to complete which means it takes just 354 days to complete 12 lunar cycles. So every 2.5 years or so a 13th full moon is observed within a calendar year.

This 13th full moon doesn't conform to the normal naming scheme and is referred to as the Blue Moon.

How often do Blue Moons occur?

Contrary to the popular phrase "once in a blue moon", Blue Moons can occur relatively frequently, that is, in astronomical terms.

Blue Moons occur once every two to three years, according to NASA. As there are roughly 29.5 days between full moons, February will never experience a monthly Blue Moon as it only has 28 days in a common year and 29 in a leap year. Sometimes February doesn't have a Full Moon at all, this is known as a Black Moon, according to Time and Date.

When are the next monthly and seasonal Blue Moons?

Blue Moons occur once every two to three years. (Image credit: Andrew Bret Wallis via Getty Images)

The next monthly Blue Moon is on Aug. 30/31, 2023 according to Time and Date. The next seasonal Blue Moon will take place on Aug. 19/20, 2024.

When was the last Blue Moon?

The last Blue Moon was on Aug. 30, 2023.

If you're interested in trying to capture an impressive lunar image yourself and want to learn the right techniques and tools, check out our handy how to photograph the moon guide.

Can the moon ever turn blue?

Yes! But this is an extremely rare event, you could even say it happens "once in a Blue Moon".

According to NASA in 1883 an Indonesian volcano called Krakatoa erupted and spread ash as high as 50 miles (80 kilometers) into the atmosphere. The tiny ash particles — about one micron in size — acted as a filter, scattering red light and turning the moon a distinct blue-green hue.

According to NASA Science, other volcanic eruptions have also been known to cause blue moons including the 1983 eruption of El Chichon volcano in Mexico and the eruptions of Mt. St. Helens in 1980 and Mount Pinatubo in 1991.

Blue Moon FAQs answered by an expert

We asked meteorologist Joe Rao, a few commonly asked questions about the Blue Moon.

Joe Rao Meteorologist Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium.

What date is the next Blue Moon? That depends on which definition for a Blue Moon you care to use. It was during the time frame from 1932 through 1957 that the now defunct Maine Farmers’ Almanac suggested that if one of the four seasons contained four full Moons instead of the usual three, the third should be called a "Blue Moon." If you go by that rule, then the next Blue Moon will occur August 19, 2024. During the summer of 2024 there will be four full moons (June 21, July 21, August 19 and September 18). Since the August full is the third full moon of the summer series, it would get the Blue Moon branding. But thanks to a couple of misinterpretations of this arcane rule, first by Sky & Telescope magazine, then many years later by a syndicated radio program, it now appears that the second full Moon in a month is the one that’s now popularly accepted as the definition of a “Blue Moon.” If you go by the "Two full moon's in one month" rule, then we'll have a Blue Moon on August 30, 2023.

What is a Blue Moon? Either a calendrical oddity or the moon actually appearing blue to the eye. This poetic phrase refers to something extremely rare in occurrence.

Does the moon ever turn blue? Only under certain atmospheric conditions. In the aftermath of the massive 1884 volcanic eruption of Krakatoa, a tremendous cloud of ash and dust was injected into the stratosphere (5 to 30 miles above the Earth's surface); this aerosol cloud caused both the moon and sun appear blue from many locations in the Northern Hemisphere for many months after the explosion. And on September 24th, 1950, a 200-mile-wide swath of smoke from a series of smoldering fires in the forests of Northern Alberta in Canada cast an awesome pall over the Great Lakes, parts of New York State and Southern New England. The smoke produced an unusual midday darkness and caused the disk of the sun to shine in eerie hues of pink, blue and even purple!

How often is a Blue Moon? To see the moon actually turn blue, requires either a massive volcanic eruption or a giant pall of airborne smoke (such as in 1950). But such events are few and far between . . . perhaps occurring only about half a dozen times per century. Calendrical blue moons are much more frequent, occurring (on average) about every 2.5 to nearly 3 years.

Additional information

Explore Blue Moons in a nutshell with ESA's useful Blue Moon infographic. Check out this cool image of a Blue Moon captured by the European weather satellite MSG-3 just before the moon disappeared out of sight. Discover the difference between types of full moons with NASA.

Bibliography

Mohon, L. (2021, August 20). Blue Moon – Watch the skies. NASA. Retrieved April 21, 2022, from https://blogs.nasa.gov/Watch_the_Skies/tag/blue-moon/

NASA. (n.d.). Blue Moon. NASA. Retrieved April 21, 2022, from https://science.nasa.gov/science-news/science-at-nasa/2004/07jul_bluemoon

Hocken, V., & Kher, A. Look up for a blue moon! What Is a Blue Moon? Retrieved April 21, 2022, from https://www.timeanddate.com/astronomy/moon/blue-moon.html#:~:text=The%20next%20monthly%20Blue%20Moon,are%20traditionally%20called%20Sturgeon%20Moons.&text=There%20is%20no%20Blue%20Moon%20in%202022.