Jimmy Westlake: Blue moon due Friday
August 28, 2012
One of my favorite doo-wop songs from the '60s is "Blue Moon," written by Rodgers and Hart in 1935 and recorded by The Marcels in 1961. You know the words:
You saw me standing alone
Without a dream in my heart
Without a love of my own."
Have you ever seen a blue moon hanging up in the sky? Well, this month you can, but 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 the Earth, it waxes and wanes through a cycle of phases that repeats itself every month. In fact, our word "month" comes 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. That makes the time of each full moon a fraction of a day earlier each cycle. This small difference adds up throughout time so that the date of the full moon slowly works its way forward to the first day of a month. When this happens, 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 exactly 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 denote the three named full moons of each season but used a little blue moon symbol for the oddball fourth full moon. At least that's one story about the origin of the term. In recent years, the most 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 or the 13th of the year.
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 August this year was the "Green Corn Moon," which fell on the night of Aug. 1. Because there are 31 days in the month, there still is time for a second full moon Friday. So, this year's "Fruit Moon" also will be a blue moon, though you might not know it just by looking at it.
Don't let this month's blue moon catch you standing alone.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Westlake's astrophotography website at http://www.jwestlake.com.