A blue moon is an additional full moon that appears in a subdivision of a year, either the third of four full moons in a season or, a second full moon in a month of the common calendar.
The phrase has nothing to do with the actual color of the moon, although a literal "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions; e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere. This phenomenon is specific to calendars.
The suggestion has been made that the term "blue moon" for
"intercalary month" arose by folk etymology, the "blue" replacing the no-longer-understood belewe, 'to betray'. The original meaning would then have been "betrayer moon", referring to a full moon that would "normally" (in years without an intercalary month) be the full moon of spring, while in an intercalary year, it was "traitorous" in the sense that people would have had to continue fasting for another month in accordance with the season of Lent.
In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month. There was a full moon on July 2, 2015. There’s another full moon today (July 31). So the July 31 full moon will be called a Blue Moon, according to this definition.
The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.
The suggestion has been made that the term "blue moon" for
Moon 8-31-2012 by Priya Kumar Muscat Masqat Oman |
In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month. There was a full moon on July 2, 2015. There’s another full moon today (July 31). So the July 31 full moon will be called a Blue Moon, according to this definition.
Moon Blue via EarthSky Facebook friend Jv Noriega |
The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.
To see the next full moon wait by 2018:
- January 31, 2018
- March 31, 2018
- October 31, 2020
- August 31, 2023
- May 31, 2026
- December 31, 2028
- September 30, 2031
- July 31, 2034
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