Here’s a good article on a Blue Moon by The Farmer’s Almanac.
For more than half a century, whenever two full Moons appeared in a single month (which happens on average every 2 1/2 to 3 years), the second has been christened a “Blue Moon.” In our lexicon, we describe an unusual event as happening “Once in a Blue Moon.” This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare.
On past occasions, usually after vast forest fires or major volcanic eruptions, the Moon has reportedly taken on a bluish or lavender hue. Soot and ash particles propelled high into the Earth’s atmosphere, can sometimes make the Moon appear bluish. But the second full Moon of any month doesn’t turn blue in color.
Blue Moon Dates for 2020
In 2020, we will have a Blue Moon in October:
- Full Harvest Moon: October 1, 2020
- Full Hunter’s Moon: October 31, 2020 (Blue Moon)
In this case, the October 31st full Moon will be a Blue Moon by both definitions: For being the second full Moon in a single month, and being the third of 4 full Moons in a single season (Autumn).
Why “Blue” Moon?
For the longest time, nobody knew exactly why the second full Moon of a calendar month was designated as a “Blue Moon.” One explanation connects it with the word belewe from the Old English, meaning, “to betray.” Perhaps, then, the Moon was “belewe” because it betrayed the usual perception of one full Moon per month.
However, in the March 1999 issue of Sky & Telescope magazine, author Phillip Hiscock revealed one somewhat confusing origin of this term. It seems that the modern custom of naming the second full Moon of a month “blue,” came from an article published in the March 1946 Sky & Telescope magazine. The article was “Once in a Blue Moon,” written by James Hugh Pruett. In this article, Pruett interpreted what he read in a publication known as the Maine Farmers’ Almanac (no relation to this Farmers’ Almanac, published in Lewiston, Maine), and declared that a second full Moon in a calendar month is a “Blue Moon.”
A Blue Moon By Another Definition
However, after reviewing the Maine Farmer’s Almanac, Hiscock found that during the editorship of Henry Porter Trefethen (1932 to 1957), the Maine Farmers’ Almanac made occasional reference to a Blue Moon, but derived it from a completely different (and rather convoluted) seasonal rule. As simply as can be described, according to Trefethen’s almanac, there are normally three full Moons for each season of the year. But when a particular season ends up containing four full Moons, then the third of that season is called a Blue Moon!
To make matters more confusing, the beginning of the seasons listed in Trefethen’s almanac were fixed. A fictitious or dynamical mean Sun produced four seasons of equal length with dates that differed slightly from more conventional calculations. So, basically the current use of “Blue Moon” to mean the second full Moon in a month can be traced to a 55-year-old mistake in Sky & Telescope magazine!
We can have two instances when we can have a “Blue Moon”: One when there are two full Moons in a single calendar month, and one when there are four full Moons in a single season; the third full Moon being the Blue Moon.
Past Blue Moon Dates
In 2019, spring in the Northern Hemisphere ran from March 20th to June 21st. During that time span of slightly more than three months, these were the full Moon occurrences (in Eastern Time zones):
- Full Worm Moon: March 20, 2019
- Full Pink Moon: April 19, 2019
- Full Flower Moon: May 18, 2019 (Blue Moon)
- Full Strawberry Moon: June 17, 2019
That final full Moon fell before the summer solstice, so it wasn’t the first full Moon of summer, but rather the fourth full Moon of spring. That made the May 18th Moon—the third of the four full Moons of spring—a “Blue Moon.”