Why isn't a "blue moon" blue?

CHARLOTTE, NC (WBTV) - In short, it can be, but a "blue moon" has nothing to do with the color of the moon.

Roughly once every 2.7 years, two full moon will occur within the same calendar month.  This is referred to as a blue moon.

According to a NASA article, the expression originally had nothing to do with astronomy.  It was simply an expression that meant rare or ridiculous.  The modern definition came about in the 1940s when Sky & Telescope magazine published an article written by James Hugh Pruett entitled "Once in a Blue Moon."  Pruett cited the 1937 Maine Farmer's Almanac, which gave a convoluted definition of the term.  He then stated that the "second [full moon] in a month, so I interpret it, is called the Blue Moon."  

While not technically correct, the meaning stuck, and it's been the modern definition ever since.

It takes just over 29.5 Earth days for the moon to completely move through its entire cycle.  It's known as the synodic period, and it's the reason that a blue moon in February will never occur, even in a leap year.  There simply aren't enough days.

Even though a blue moon is typically the same gray and white color we usually see in the night sky, on rare occasions it CAN be blue.  Blue "blue moons" are usually blue when large volcanic eruptions send huge amounts of ash and dust into the atmosphere.  Several volcanic eruptions have resulted in color changes in both the moon and the sun.

When some volcanoes erupt, very small particles of about 1 micron in diameter are sent into the atmosphere.  Particles this size are about the same as the wavelength of red light, in effect filtering the red color out and allowing the blue to pass through.  We saw it with Krakatoa's eruption in 1883 and in 1980 when Mt. St. Helens exploded.  Forest fires have also been known to cause blue moons.

The next blue moon occurs on July 31, 2015. We get two blue moons in 2018 when they fall within January and March.

For the technical meaning of a blue moon, click here.