The Full Moon of November 2010 is on Sunday and will bring with it a cosmic addition: It will also be a so-called "Blue Moon." What is Blue Moon? Why will it be called as Blue Moon?

What is Blue Moon?

A 'Blue Moon' has been defined as the second Full Moon that occurs during a calendar month. Sunday's Full Moon falls on Nov 21 and it will be the only Full Moon in November 2010, then how can it be a 'Blue Moon?" In fact November's full Moon is Blue Moon according to our old rule. The current "two - Full Moons in one month" rule has superseded an older rule that would allow us to call Sunday's Moon "blue."

Back in the July 1943 issue of Sky & Telescope magazine, in a question and answer column written by Lawrence J. Lafleur, there was a reference made to the term "blue moon." Lafleur cited the unusual term from a copy of the 1937 edition of the now-defunct Maine Farmers' Almanac (NOT to be confused with The Farmers' Almanac of Lewiston, Maine, which is still in use). Its calendrical meaning was given on the almanac page for August 1937 - "... usually comes full twelve times in a year, three times for each season." But occasionally, however, there will come a year when there are 13 Full Moons during a year, not the usual 12. The almanac explanation continued: - "This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number."

And with that extra Full Moon, it also meant that one of the four seasons would contain four Full Moons instead of the usual three."

"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the almanac, ending on the comment that, "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon'."

But while LaFleur quoted the almanac's account, he made one very important omission: He never specified the date for this particular blue moon. As it turned out, in 1937, The Blue Moon occurred on Aug 21. That was the third full moon in the summer of 1937, a summer season that would see a total of four Full Moons.

Names were assigned to each Moon in a season: For example, the first moon of summer was called the Early Summer Moon, the second was the Midsummer Moon, and the last one was called the Late Summer Moon. But when a particular season had four Moons, the third was apparently called a Blue Moon so that the fourth and the final one can continue to be called the Late Moon.

So where did we get the "two Full Moons in a month rule" that is so popular today?
So once again, we must turn to the pages of Sky & Telescope.

If you see the page 3 of the March 1946 issue, James Hugh Pruett wrote an article, "Once in a Blue Moon," in which he made a reference to the term "Blue Moon" and referenced LaFleur's article from 1943. But because Pruett had no specific Full Moon date for 1937 to fall back on, his interpretation of the ruling given by the Maine Farmers' Almanac was highly subjective. Pruett ultimately came to the conclusion - "Seven times in 19 years there were – and still are – 13 full moons in a year. This gives 11 months with one full moon each and one month with two Full Moons. This second Full Moon in a month, so I interpret it, was called Blue Moon."

How unfortunate that Pruett did not have a copy of that 1937 almanac at hand, or else he would have almost certainly noticed that his "two Full Moons in a single month assumption" would have been totally wrong. Because the Blue Moon date of Aug. 21 was most definitely not the second full moon that month.

Blue Moon Myth Runs Wild

Pruett's 1946 explanation was, of course, the wrong interpretation and it might have been completely forgotten were it not for Deborah Byrd who used it on her popular National Public Radio program, "StarDate" on Jan. 31, 1980. In the aftermath of her radio show, the incorrect Blue Moon rule "went viral".

Over the next decade, this new Blue Moon definition started appearing in diverse places, such as the World Almanac for Kids and the board game Trivial Pursuit. It was made referenced in the Dec. 1, 1982 edition of The New York Times, in the newspaper's "New York Day by Day" column.

But by 1988, the new definition started receiving international press coverage. Today, Pruett's misinterpreted "two Full Moons in a month rule" is recognized worldwide and meanwhile, the original Maine Farmers' Almanac rule had been all but forgotten.

Play by the Old Rules

According to the old Almanac rule, Nov 21 Full Moon would technically be a Blue Moon, because in the Fall season of 2010, there are four Full Moons:  Sept 23, Oct 22, Nov. 21, and Dec. 21. Although you might say, "Dec 21 is the first day of winter" ad is not the day of Fall. And you would be correct, but only if you live in the northern part of the equator in the Northern Hemisphere. In south of the equator it's the first day of Summer.

In 2010, the solstice comes at 6:38 pm EST (2338 UT). But the Moon turns Full at 3:13 am EST (0813 UT). That's 15 hours and 25 minutes before the solstice occurs. So the Dec 21 Full Moon occurs during the waning hours of Fall and qualifies as the fourth Full Moon of the Fall season. This means that under the original Maine Almanac rule – the one promoted by Lafleur and later misinterpreted by Pruett – the third Full Moon of the 2010 Fall season on Nov 21 would be a Blue Moon.

Choose Your Blue Moon

So what Blue Moon definition will you follow? Is it the second Full Moon in a calendar month, or (as is the case on Sunday, Nov 21) the third full moon in a season with four? Maybe it's both. The final decision is solely up to you. Sunday's full moon will look no different than any other Full Moon.

The Moon can change color in certain conditions - after forest fires or volcanic eruptions, the moon can appear to take on a bluish or even lavender hue. Soot and ash particles, deposited high in the Earth's atmosphere, can sometimes make the moon appear bluish.

We could even call the next Full Moon (on Dec. 21) a "Red Moon," but for a different reason: On that day there will be a total eclipse of the Moon and, for a short while, the Moon will actually glow with a ruddy reddish hue.

Dates of Blue Moons

2007, May 31, Blue Moon Day - The first Full Moon of May 2007 was on May 2nd in the USA. What about Europe? Because of time zones, this week's Full Moon occurred over Europe on June 1st rather than May 31st. In Europe, therefore, it is the month of June which has two Full Moons, and a Blue Moon on June 30th. In North America, May has two Full Moons and a Blue Moon on May 31st.

2012, August 31, Blue Moon Day - For the second time this month, the Moon is about to become full. There was one full Moon on August 1st/2nd, and now a second is on August 31st. According to modern folklore, whenever there are two full Moons in a calendar month, the second one is called "Blue Moon."