What Is a Blue Moon? Is a Blue Moon Actually Blue?

Blue moon.

The term Blue Moon, actually has nothing to do with the color of the moon. It is nothing more than the succession of two full moons within the same calendar month, something that happens approximately every three years.

Two full moons are possible in the same month due to the fact that the full moon cycle is approximately every 29.5 days. Therefore, if the full moon occurs on the first or second day of that month, there is a probability that a second full moon will appear at the end of the month. This full moon is commonly known as the blue moon.

Does the moon appear blue? As opposed to Blood Moon when it appears red, it does not. For the moon to appear blue, it must be clouded by ash and smoke. For example, in 1883 the eruption of the Krakatoa volcano in Indonesia caused its ashes to spread to the limits of the atmosphere. Ash clouds scatter red light but allow other colors to pass through. For that reason, the moon appears to be blue during ash falls caused by volcanoes for example.

Types of Blue Moon

There are actually two types of blue moon. The most easy-to-understand type occurred on 31 October 2020, while the other type is much rarer. 

  1. Monthly Blue Moon: is the second full moon in a calendar month that contains two full moons.
  2. Seasonal Blue Moon: Third full moon in an astronomical season that contains four full moons.

The Next Blue Moons:

The most recent Blue Moon took place on 31st October 2020. A monthly Blue Moon occurs approximately every 3 years:

  • March 2023
  • May 2026
  • December 2028
  • September 2031

What Is the Origin of the Term “Blue Moon”?

The notion of a Blue Moon first appeared in 16th century writings of Thomas Wolsey, an English archbishop, statesman and a cardinal of the Catholic Church. Cardinal Wolsey wrote that his intellectual enemies “would have you believe that the moon is blue”. Apart from the occasional blue-tinged moon that can appear through a cloud of volcanic ash, the moon almost always appears in shades of white to yellow. Thus, the term Blue Moon is a kind of metaphor for absurdity or impossibility. In 19th-century London the term Blue Moon was used as street slang for “a long time”. Although it had probably been in use for a while, the slang first appeared in print in an 1821 book about working-class London.

No one knows why the term blue moon became associated with rare lunar events. However, the association most likely originated in the state of Maine. Since each season is three months long, seasons typically have three full moons. However, once in a few years, the dates align in such a way that a season will have four full moons. Farmers’ Almanac published in Maine coined the term Blue Moon for the third full moon in a season with four full moons.


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