Jimmy Westlake: Blue moon due this month
August 13, 2013
Have you ever seen a blue moon hanging in the sky? This month, you will have an opportunity to witness an unusual blue moon, but don't expect to go outside and see a blue-colored moon staring back at you. Allow me to explain this unusual event.
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 one full moon to the next, 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-day earlier each successive month. This small difference adds up throughout the months, so the date of the full moon slowly works its way forward to the first day of the month or the first day of a new season. When this occurs, it's possible to fit in a second full moon in a month 29.53 days later.
Full moons traditionally have seasonal names that can be traced back to early American or Native American lore. Typically, each season of the year has three full moons, each with its own name. For example, summer has its Thunder Moon, Green Corn Moon and Fruit Moon. Occasionally, a troublesome fourth full moon will fall in a single season. What shall we call this one?
Calendar-makers of yesteryear used red-colored moon symbols to mark the three named full moons of each season but used a blue-colored moon symbol for the unnamed full moon. By tradition, the third of the four full moons in any season was denoted as blue. 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 a single calendar month instead of the third full moon of any season with four.
This year's Thunder Moon happened June 23, just two days into the summer season. The Green Corn Moon fell on July 22. That allows enough time for two more full moons this summer, making the full moon Aug. 21 a blue moon and the full moon Sept. 19 the traditional Fruit Moon.
The phrase "once in a blue moon" is used to describe a rare or unusual event. Blue moons are quite uncommon, occurring about seven times out of every 19 years. That's about one blue moon every 2.7 years on average.
You'll have two chances next year to see an equally unusual red-colored moon. On April 15 and again Oct. 8, the full moon will slip into the shadow of the Earth, producing a total lunar eclipse. Sunlight filtering through the Earth's atmosphere will cast an eerie, reddish glow onto the eclipsed moon.
So this poses an interesting conundrum: What would we call a blue moon that also happened to 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. His Celestial News column appears weekly in the Steamboat Today. Check out Westlake’s astrophotography website at http://www.jwestlake.com.