You’ve probably heard the phrase “once in a blue Moon”, usually referring to a rare event. But where does the phrase come from? And does the Moon ever actually appear blue?
The Meaning of “Blue Moon”

Dating back to the 1800’s the term “blue moon” was used by the Farmer’s Almanac to denote the appearance of a third full Moon in a season where four full Moon’s will occur. Since there are normally 12 full Moon’s a year (one per month), this works out to three per quarter (three month period).

However, occasionally a quarter will see four full Moon’s.

The naming arises because each Moon in a given season has its own name. This tradition of naming full Moons has existed for hundreds of years across many cultures. The names generally accepted today are those coming from the Farmer’s Almanac.

For instance, during the second quarter of the year, the Moons are named Pink (April), Flower (May) and Strawberry (June). However, should another full Moon appear during this quarter it would be called, by default, a Blue Moon. Since the third of the four full Moon’s in a quarter is called the Blue Moon, the order of Moon’s in the second quarter of the year would be Pink, Flower, Blue and Strawberry.

Since Blue Moons only occur about once every three years, it became becomes a convenient measure of a long period of time, therefore giving rise to the cliche “once in a blue moon”.
A Misunderstanding Leads To A New Definition

While the above is the generally accepted meaning for Blue Moon, a misunderstanding lead to a new definition being adopted that today is the more often quoted.

In an article, “Once in a Blue Moon”, that appeared in the March 1946 edition of Sky and Telescope Magazine the author, James Hugh Pruett incorrectly stated that the Blue Moon was the name given to the second full Moon that appeared in any given month.

He had drawn this conclusion by looking at the 1937 Farmer’s Almanac. His analysis of the data therein lead him to draw the conclusion that, “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 with two. This second in a month, so I interpret it, was called Blue Moon.”

While the definition put forth by Pruett could coincidently be correct, it is not strictly true. For instance if we examine the example from earlier, it is possible that May could have two full Moons. Therefore the second full Moon in May would be called the Blue Moon (since it is always the third full Moon in the quarter). So, Pruett’s definition would be consistent with the traditional description. But, if two full Moons fall in June, the first full Moon would be the Blue Moon, rendering the two definitions inconsistent.

So, even though Pruett’s definition isn’t strictly correct, it became the adopted definition most widely used. In fact, it is so widespread that it is not uncommon for books to use Pruett’s definition over the correct one.

While actual Blue Moons only occur about once every three years, occurrences of two full Moons on a month are much more frequent. The years 2009 and 2010 saw multiple examples of this phenomenon, while 2011 will go without a single such event.
Does the Moon Ever Actually Appear Blue?

While the term Blue Moon is linked to full Moon events, it actually has nothing to do with the Moon’s color. However, under certain circumstances, the Moon can actually appear blue, even during non-full Moon nights.

The conditions under which a full Moon will occur have to do with moisture, gas and particulates in the atmosphere. When atmospheric conditions favor, for whatever reason, the existence of particles larger than 0.7 microns red light is easily scattered, while blue wavelenghts pass undisturbed. The result is that the Moon appears bluer (or some shade of grayish blue) than normal.

These conditions almost always limited to events such as volcanic eruptions or other large fires that fill the sky with smoke and dust particles. As winds carry these particles up into the atmosphere, certain regions of the world will experience these “blue” Moons.

Historical instances of visibly blue Moons include the eruption of Mt. St. Helens in 1980, and Mount Pinatubo in 1991.

Source: About.com