The moon itself won't be blue this week, but still...

There is a full moon to end 2009. (December 31, 2009, 19:13 GMT, 2:13 PM EST, 1:13 PM CST, 12:13 PM MST, 11:30 AM PST. (Don’t be shocked that the moon doesn’t become full based on your particular idea of night.)

It is also the second full moon this month. And, it is a “Blue Moon.”

Defining a blue moon is a bit confusing. The third full moon in a season with four full moons is actually what is called a blue moon.

People often think that the second full moon in a month is a blue moon, but it turns out that idea was erroneously reported in an issue of Sky & Telescope magazine way back in 1946 and it has taken on a life in the media ever since. (Like those stupid emails you get.)

Prior to that article, a “blue moon” traditionally referred to an extra moon in a season. If a season had four full moons (rather than the more common three), then the third of the four moons was known as a blue moon. (A “season” is defined here by solstices and equinoxes.)

But, the idea that it is just the second full moon in a calendar month is a common one. Since there are 12.37 full moons in a year, a “blue moon” would occur on the average every 2.7 years, by either definition.

Unlike the other full moons during the year, Native Americans did not have a particular name for this moon. Why? Well, the idea of a “month” was not part of their timekeeping. Another moon simply signaled the start of another lunar period.

All this leads us today to use the idiom “once in a blue moon” to mean something which occurs very infrequently.

In researching the etymology of this term, I discovered an interesting alternative interpretation which is based on the old English meanings of “belewe” which were “blue” or “betrayer.”  The church was responsible for the calendar and it based church events, such as Easter, on the full moon. Lent falls before Easter starting at the beginning of the Lent moon cycle (late winter moon). The next moon is the egg moon (early spring moon), and Easter usually falls on the first Sunday after the full egg moon. Every one to three years the Lent and egg moons would come too early, so the clergy would have to tell people whether the moon was the Lent moon or a false one, which they may have called a “betrayer moon”.

As a kid, I thought the term just referred to a literal color of the moon. Ask a bunch of people to define a blue moon and someone will say something about the color. This actual rare event can occur because of smoke or dust particles in the atmosphere, such as after forest fires or the eruption of a volcano. It is a causal event much like the appearance of a red moonrise due to particles and pollution in the atmosphere, and not connected to any astronomical event.

The next seasonal blue moon won’t occur until August 21, 2013. But, if you want to plan some blue moon parties, you can also go with the calendar version (two full moons in one month) and plan celebrations for 2012 (August 2, August 31) and 2015 (July 2, July 31).

Sounds like this New Year’s Eve might be a good night to listen to Elvis’ “Blue Christmas” and break out the Blue Moon beer. Send me an invite and save me a bottle of Grand Cru…