What is a Blue Moon?

Have you ever used the phrase, "once in a blue moon"? If so, you and the person you were talking to probably knew exactly what you meant. Most people understand it to mean, "not very often."

There are three types of blue moons

But there is a formal definition of this kind of moon. Actually, there are three different definitions. And you got it; none of the three types occurs very often.

Definition #1: Two full moons in a single month

The most familiar definition calls it the second full moon occurring in a single calendar month. This happens approximately every 2-1/2 years. In June of 2007 we'll have a full moon on June 1 and a blue moon (second full moon) on June 30 (if you live in Greenwich Mean Time).

Most of us think this is based on some ancient folklore. Turns out this definition only came about 60 years ago.

By the way, February is the only month that can sometimes have no full moon. This only happens 4 or 5 times in a hundred years. And 2006 had no blue moons. Check out the following website for more details and a "blue moon" calculator to tell you when they will be occurring in the future. www.obliquity.com/astro/bluemoon.html

Definition #2: Third full moon in a season that has four

This definition has been around for at least 400 years. Philip Hiscock, a folkloreist, is credited with tracing the use of the term back to the Maine Farmer's Alamac. He has verified that the original use of the term meant the third full moon in a season that had four full moons and that definition #1 came about because of a mistake by Sky and Telescope Magazine. Their article called "Once in a Blue Moon," from March, 1946, explains how a misunderstanding started what became a piece of modern folklore.
www.skyandtelescope.com/observing/objects/moon/article_377_1.asp

For the fascinating details of Mr. Hiscock's research, see his article "Once in a Blue Moon" at the following link. http://skytonight.com/observing/objects/moon/3305141.html

Definition #3: The moon looks blue in color

Sometimes atmospheric conditions make the moon look blue to our eyes. The precise conditions needed don't occur very often, but they do occur.

These conditions can be caused by huge amounts of dust or smoke being put into the air by things like volcanoes or forest fires. If the dust or smoke particles are just the right size (about 1 micron or one millionth of a meter), red light is scattered by the particles while other colors of light pass through. This results in the moon looking blue or sometimes green. Maybe we should say "once in a green moon" since those are even more rare!

Anything else?

What else do we know about blue that might explain why we link it to the moon?

Blue has been shown to be a calming color, slowing respiration and heart rate.

We live on the "blue planet." We associate blue with life as we know it.

What about the way we associate the color blue with feeling sad? Such as singing the blues, feeling blue.

We also associate howling at the moon with loneliness and sadness. So a really pathetic coyote would be one howling at a blue moon?

And what about blue in the face? That can mean tired and speechless, as in, "I told her to pick up her stuff till I was blue in the face." Or it can be someone who's not breathing and is in danger of expiring. That's kind of the opposite of life, isn't it?

What about "out of the blue," which means suddenly and unexpectedly. How does that fit in?

At this point it really doesn't matter exactly where the term came from. Common usage has cemented it firmly into our everyday language. You can use it safely, knowing you will always be understood.

If you want to start a fight, ask someone where the term "blue moon" comes from. The odds are good that if they know at all, they'll give you definition #1 above. Then you can with great glee tell them they are sadly mistaken and explain definition #2 to them.

Just be prepared to experience another variation of the blue theme: a black-and-blue eye.