What is a 'blue moon?'

BATON ROUGE, LA (WAFB) - The full moon that will be seen in the sky Friday night is labeled by many as a "blue moon," but what does that mean exactly?

Let's get this said first:

"No, the moon doesn't turn blue, look blue or act blue during a blue moon." (More on that later.)

Normally, there are 12 full moons each year: one per month. In fact, the term "month" is derived from the word "moon." That means that usually there is one full moon each month and that there are normally three full moons for each season.

Ah, but wait! A lunar cycle (the time to cycle from full moon to full moon) is only 29.5 days long, which means that 12 lunar cycles (354 days) is a little shy of our solar year (365 days). As a result, that 11-day difference starts to add up until we end up with 13 full moons in a calendar year. In fact, we end up with 13 full moons every two to three years.

So, the term "blue moon" - as in "once in a blue moon," which means something somewhat rare - must be a way of accounting for that "13th" full moon.

Folklorists say one of the earliest written references to "blue moon" dates back to the 1500s. Apparently, the original term was the archaic word, "belewe," which means to betray. Interpretation of that original reference suggests that the "belewe moon" ("traitor moon?") was the third full moon to occur in a season with four full moons.

This "third full moon in a season with four full moons" definition for blue moon was still being used routinely by the Maine Farmers' Almanac into the 1800s and 1900s.

However, a misinterpretation of the "blue moon" definition used by the Maine Farmers' Almanac was published in 1946 by a much more widely-read magazine, Sky and Telescope. The Sky and Telescope article fumbled the original definition by simplifying it to mean "the second full moon to occur within a calendar month."

Frankly, this "modern" definition of blue moon is far easier to track! And, by the way, for the purists, Friday's blue moon would not qualify as a "belewe moon." Friday's full moon would only be the second full moon since the summer season began on June 21.

Okay, so does the moon ever appear blue?

Yes, it can happen, but this is truly a rare event. There are documented cases of the moon having a blue tint, with most, if not all, of these cases associated with great fires or volcanic eruptions.

Most people are familiar with an orange or red-tinted moon when it is low in the sky (the same is true for the early morning and evening sun). This orange tint is the result of fine atmospheric particles (mostly fine airborne dust and water) that "remove" the blue and green wavelengths in white light (scatters is the technically-correct term) and allows the reds, yellows and oranges to pass through.

For a "true blue" moon, the particulates must be a little larger and yet, the very fine dust must be at a minimum. Larger particulates (that might result from fires or volcanoes) can scatter the red and yellow wavelengths but allow the blues to pass through, resulting in a blue tint for the moon.

Copyright 2015 WAFB. All rights reserved. Wikipedia and EarthSky contributed to this report.