Although a bluish-looking moon can result from effects of Earth’s atmosphere, the term “blue moon” does not refer to the color of the moon. For example, the phenomenon was widely observed in North America on September 26, 1950, due to Canadian forest fires that had scattered high-altitude dust, which refracted or absorbed certain wavelengths of light.

The popular definition of a “blue moon,” is the second full moon in a single calendar month. Based on this definition, a blue moon occurs, on average, every 2.72 years. Since 29.53 days pass between full moons (a synodial month), there is never a blue moon in February. On rare occasions, a blue moon can be seen twice in one year, but only in certain parts of the world. Blue moons will next occur:

• August 31, 2012
• July 31, 2015
• January 31, 2018
• March 31, 2018
• October 31, 2020
• August 31, 2023
• May 31, 2026
• December 31, 2028
• September 30, 2031
• July 31, 2034

However, this is a new definition of a blue moon based on an article that appeared in Sky & Telescope in 1946. The older definition of a blue moon, found in the Maine Farmer’s Almanac dating back to 1819, describes a blue moon as the third full moon in a season of four full moons. To further complicate the calculation of a blue moon, astronomers define the start of a season based on the actual position of the sun throughout its annual orbit. The seasons will not be of equal length according to this definition. The Maine Farmer’s Almanac preferred to define each season as being of equal length. Therefore, determining the occurrence of a blue moon based on the traditional definition will depend on whether the astronomical definition of a season or the Maine Farmers Almanac definition is used.