Science & Astronomy

Once in a Blue Moon Event Is Not Really Blue (Infographic)

Karl Tate |
Thought to be called "blue" after an old english term meaning "betrayer," a Blue Moon is an extra new moon that occurs due to a quirk of the calendar. Original Image
Credit: Karl Tate, SPACE.com

The moon can sometimes appear reddish, especially during eclipses. But what we call a "Blue Moon" has nothing to do with its color. Normally there are 12 fully lit, or full, moons per year. A season of three months should therefore contain three full moons.

Thought to be called "blue" after an old English term meaning "betrayer," a Blue Moon is an extra full moon that occurs in that span, due to a quirk of the calendar. 

On occasions where there are four full moons in a season instead of three, the third of the full moons is traditionally called a Blue Moon. A Blue Moon happens on average about once every 2.7 years.

Occasionally two full moons will fall within the same month. The second full moon is also often called a Blue Moon, but this is not the term's original meaning.

The moon can actually appear blue under certain circumstances, such as when ash is present in the atmosphere from fires or volcanic eruptions. This type of Blue Moon cannot be predicted in advance, however.

Author Bio


Karl Tate, SPACE.com Infographics Artist

Karl's association with SPACE.com goes back to 2000, when he was hired to produce interactive Flash graphics. Starting in 2010, Karl has been TechMediaNetwork's infographics specialist across all editorial properties.  Before joining SPACE.com, Karl spent 11 years at the New York headquarters of The Associated Press, creating  news graphics for use around the world in newspapers and on the web.  He has a degree in graphic design from Louisiana State University. To find out what his latest project is, you can follow Karl on .