There are two full moons in December. By modern convention, the second full moon in a calendar month is called a blue moon. So, New Year’s Eve party goers can celebrate a blue moon followed by a New Year. However, this definition of a blue moon results from a mistake made in an article published in Sky & Telescope magazine in 1946. Previously, the definition of a blue moon was the third full moon in an astronomical season that has four full moons. While that short sentence provides the essential information, I can’t resist the urge to add more details.
Some cultures have names for each full moon during a year. The 1937 Maine Farmer’s Almanac lists the full moons “named by our early English ancestors.”
Winter Moons – Moon after Yule, Wolf Moon, Lenten Moon
Spring Moons – Egg Moon, Milk Moon, Flower Moon
Summer Moons – Hay Moon, Grain Moon, Fruit Moon
Fall Moons – Harvest Moon, Hunter’s Moon, Moon before Yule
The Harvest Moon is the only one I hear used any more. However, there is an occasional complication to this naming pattern. Because the lunar cycle is shorter than a calendar month, every so often, there are thirteen full moons during a seasonal cycle, giving one season four full moons. To keep the full moon names in sync with their appropriate seasonal activities, the third full moon was called a blue moon. We will have this kind of blue moon next year in 2010.
In 2010, the fall equinox occurs on September 22 and the winter solstice on December 21. Full moons occur on September 23, October 22, November 21, and December 21 (15 hours before the solstice). Therefore, the full moon on November 21 is called a blue moon under the old rule.
Given the circumstances, I think the last full moon of fall 2010 should be renamed the “Moon just before Yule.” As an added bonus it will also be a total lunar eclipse.