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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. If the moon actually looks blue, it's caused by a rare type of dust in the atmosphere.

Illustration image

Dust particles can make the Moon look blue.

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


Double Blue Moon in 2018

In 2018, there were two Full Moons in January, and there are also two in March in most time zones. This is sometimes called a double Blue Moon. Check the Moon Phase page to see if this is the case in your location.

Two Definitions of 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.

Local Time & Date for Blue Moons

Blue Moons can vary by time zone. On our Moon Phase Pages, you'll find local and worldwide times and dates for Blue Moons, Moon Phases, lunations, Supermoons, Micromoons, and Black Moons. You will also find Full Moon names for the Northern Hemisphere.

Why Are There Different Definitions?

The reason the second definition of Blue Moon exists is down to an error originally made by amateur astronomer James Hugh Pruett (1886–1955).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.

A Full Moon Without a Name

The seasonal Blue Moon originally came about as a kind of placeholder name for a Full Moon which doesn't have a proper Full Moon name, such as Harvest Moon or Paschal Moon. This way, when there are 13 Full Moons in a year instead of the usual 12, the other 12 can keep their rightful place in relation to the solstices and equinoxes.

How Rare Is a Blue Moon?

The term once in a Blue Moon means that something is rare. Blue Moons happen once every two or three years. In the 1100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons.

Double Blue Moons happen only about 3 to 5 times in a century. The next year that has two months with two Full Moons each will be 2037, while the last time this occurred was in 1999.

Other combinations of Blue Moons also exist. Between 1550 and 2650 there are 20 years which 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.

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.

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

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 Washington DC. Change location


The Moon

  1. What Is a Supermoon?
  2. The Moon Illusion
  3. The Moon Phases
  4. The Moon's Effect on Tides
  5. What Is a Micro Moon?
  6. How Can Full Moon Be in the Daytime?
  7. Is a Blue Moon Blue?
  8. The Moon's Orbit
  9. The Far Side of the Moon
  10. What Is a Black Moon?
  11. What Are Moonbows?
  12. Full Moon Names
  13. Taking pictures of the Moon

Moon index

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