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Kerri McIntire  .
Publication Date: November 18, 2010


Just what is a Blue Moon? It is a misnomer that it’s the second full moon of a month. The confusion has been sourced to a March 1946 article in Sky & Telescope Magazine. In the article, James Hugh Pruett is right to imply that Blue Moons occur seven times every 19 years, but mistakenly attributes this to only those calendar years having thirteen full moons – and going on to call any second moon to fall in one month “Blue.”

Pruett likely came to this conclusion based on an even earlier Sky & Telescope piece on Blue Moons (from July 1943). In a question-and-answer column, Laurence J. Lafleur noted that the Moon occasionally “comes full thirteen times in a year.” This statement might have lead the later S & T writer to assume that any year which had two full moons in one month would qualify as a Blue Moon year – but this is not always the case. In 2009, for example, a full moon fell on December 2nd and again on the 31st. By its true definition, though, that second full moon was NOT Blue, despite the fact it made for the thirteenth full moon of that calendar year.

So what does define a Blue Moon?

In the 1943 article, Lafleur is clearly sourcing the Maine Farmers’ Almanac. He should have qualified his thirteen full moons assertion by saying they had to fall within a Tropical or Solar Year, which is how the almanac measured time. This is based on a Winter Solstice to Winter Solstice calendar. So the older, more accurate definition of a Blue Moon is based on the tradition of having three named Full Moons per season (as opposed to one per month), for a total of twelve. This is how those twelve moons are named in the Maine Farmers’ Almanac for 1937:


Yule (Winter Solstice)
Winter Moons:
Moon after Yule
Wolf Moon
Lenten Moon

First Day of Spring (Vernal Equinox)
Spring Moons:
Egg Moon (Paschal Moon)
Milk Moon
Flower Moon

The Long Day (Summer Solstice)
Summer Moons:
Hay Moon
Grain Moon
Fruit Moon

Summer’s End (Autumnal Equinox)
Fall Moons:
Harvest Moon
Hunter’s Moon
Moon before Yule


These twelve named Full Moons were designed to fall in an appropriate time frame for the activity of the season. The Lenten Moon must fall during Lent, the Harvest Moon at the harvest, and the Moon before Yule as the final moon of the “year.”

When it happens that a thirteenth moon appears in the Yule to Yule time frame, it is named “Blue.” But which moon of the thirteen should the Blue one be? That extra full moon will fall within a season, giving that season four full moons. It was decided that the third full moon in a four full moon season would be the one to deviate from the named twelve. So why the third and not the fourth?

The Sky & Telescope web page from which I found most of my information states it well:

“When a season contains four full Moons, the third is called a Blue Moon. Why is the third full Moon identified as the extra one in a season with four? Because only then will the names of the other full Moons, such as the Moon Before Yule and the Moon After Yule, fall at the proper times relative to the solstices and equinoxes” … Read More: Sky & Telescope: “What is a Blue Moon?”

Another site that names the moons by season and not by months: EarthSky: “Can you tell me the full moon names?”


Full moon names by season:

After the December solstice
Old Moon, or Moon After Yule
Snow Moon, Hunger Moon, or Wolf Moon
Sap Moon, Crow Moon or Lenten Moon

After the March equinox
Grass Moon, or Egg Moon (also Paschal Moon)
Planting Moon, or Milk Moon
Rose Moon, Flower Moon, or Strawberry Moon

After the June solstice
Thunder Moon, or Hay Moon
Green Corn Moon, or Grain Moon
Fruit Moon, or Harvest Moon

After the September equinox
Harvest Moon, or Hunter’s Moon
Hunter’s Moon, Frosty Moon, or Beaver Moon
Moon Before Yule, or Long Night Moon

and back to the December solstice again.


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