Jimmy Westlake: December’s blue moon
December 5, 2009
Have you ever seen a blue moon hanging up in the sky? Well, this month you will have an opportunity to see a rare "blue moon." You might be surprised to learn that a blue moon has nothing at all to do with the moon's color. Allow me to explain.
As the moon orbits around the Earth, it waxes and wanes through a cycle of phases that repeats itself each month. In fact, our word "month" is derived from the word "moon," because a complete cycle of phases, from full moon to full moon, takes about 30 days. To be precise, the lunar month is 29.53 days long, just shy of 30 days, so that the time of full moon happens about a half a day earlier each month. This small difference can add up throughout time so that the date of the full moon slowly works its way forward to the first day of the month.
When this occurs, it's possible to have a full moon early in the month and still fit in a second full moon 29.53 days later.
For reasons that are not clear, the second full moon that falls in a calendar month is called a "blue moon."
This would represent a 13th full moon during the calendar year, for which there is no given name, such as October's "Harvest Moon," or November's "Hunter's Moon." Calendar-makers of yesteryear used little red moon symbols to mark the three named full moons of each season but used a little blue moon symbol for the odd fourth full moon. At least, that's one story about the origin of the term. In recent years, the popular meaning of the term "blue moon" has changed to denote the second full moon in any calendar month instead of the fourth full moon of any season.
The phrase "once in a blue moon" is often used to describe a rare or unusual event. Blue moons are, indeed, quite uncommon, occurring about seven times out of every 19 years. That's about one "blue moon" every 2.7 years, on average.
The first full moon in December this year, called the "Long Night Moon," fell on Dec. 2, and because December has 31 days, there still is time for a second full moon 29.53 days later Dec. 31.
This New Year's Eve full moon will be a "blue moon," though you might not know it by looking at it.
You'll have your chance to see an equally unusual red moon next December. In the wee morning hours of Dec. 21, 2010, the full moon will slip into the shadow of the Earth, creating a total lunar eclipse. Sunlight filtering through the Earth's atmosphere will cast an eerie reddish glow onto the darkened moon.
So, this poses an interesting conundrum: What would we call the second full moon in a calendar month that happened to also be totally eclipsed? If a blue moon and a red moon happened at the same time, I guess we'd have to call it … a purple moon! Now, that would be a most unusual event.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. His "Celestial News" column appears weekly in the Steamboat Pilot & Today. His "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out Jimmy's Web site at http://www.jwestlake.com.