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Blue Moon July 31, 2015

There are two definitions of a Blue Moon in astronomy; 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
The Mt. Karaktoa-eruption caused blue moons.
When Mt. Karaktoa erupted in 1883, the moon looked blue.

The term 'once in a blue Moon' means that that something is very rare. But just how rare, depends on your definition.

In astronomy, a Blue Moon is a full Moon, which doesn't quite fit in with the months in our calendar. However, there are two completely different ways of calculating which full Moon is a Blue Moon.

Blue Moon: The Full Explanation

Two definitions

  1. Blue Moon = The third full Moon in an astronomical season with 4 full Moons (versus the normal 3)
  2. Blue Moon = The second full Moon in a month with two full Moons.

Find Time & Date for Both

Blue Moon (2):

(Second full Moon in a month)

On our Moon Phase Pages, you'll find your local and worldwide times and dates for both of the Blue Moon definitions, along with Moon Phases and lunations, Supermoons, and Black Moons.

Why Two Definitions?

The second definition came about after an article by hobby astronomer James Hugh Pruett (1886-1955) was published in the American magazine Sky and Telescope in 1946. He made a miscalculation, and claimed that any second full Moon in a calendar month is called a blue moon. His mistake was easily refuted, and a correction was printed soon after. But his mistake spread worldwide, possibly because his definition was much simpler than the original one.

Today, it's so commonly used that it would only be fair to call it a second definition, rather than a mistake.

Blue Moon (1):

(Third full Moon in a season with 4)

Defining the Original Blue Moon

The correct, original definition is that a Blue Moon is the third full Moon in an astronomical season with four full Moons. A normal year has four astronomical seasons - spring, summer, fall (autumn), and winter - with three months and normally three full Moons each.

When one of the astronomical seasons has four full Moons, instead of the normal three, the third full Moon is called a Blue Moon.

Astronomical or Meteorological Seasons?

Blue Colored Moon

Astronomical Blue Moons happen either once every two to three years or so, depending on which of the two definitions you apply.

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 0.7 micron, for instance after a dust storm or a volcanic eruption.

Eruptions like on Mt. Krakatoa, Indonesia (1883), El Chichon, Mexico (1983), Mt. St. Helens (1980) and Mount Pinatubo (1991) are all known to have caused blue moons.

Are Red Moons Rare?

Red Moons, which can be caused by other sizes of dust particles or by Total Lunar Eclipses, are much more common than Blue Moons.

Topics: Astronomy, Moon

In this Article


All about the Moon

  1. Moonphases worldwide
  2. Phases of the Moon
  3. What is a Supermoon?
  4. Micro Moon versus Supermoon
  5. Is a Blue Moon blue?
  6. The Moon's orbit
  7. What is a Black Moon?
  8. What are Moonbows?
  9. Full Moon names

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