Jimmy Westlake: Friday’s full moon — blue or not?
July 27, 2015
Have you ever seen a blue moon hanging up in the sky?
You'll have an opportunity to witness an unusual "blue moon" this month but don't expect to go outside and literally see a blue-colored moon staring back at you. The term "blue moon" has an unusual and uncertain history, but it certainly does not refer to the actual spectrum of the moon.
Allow me to explain.
As the moon orbits the Earth, it waxes and wanes through a cycle of phases that repeats itself each month. To be more precise, the lunar month is 29 1/2 days long, just shy of 30 days.
This allows the full moon to happen a little bit earlier each successive month until the date of the full moon creeps its way forward to the first day of a month. When this occurs, it's possible to fit an extra full moon into a calendar month. This is where things get interesting.
Normally, there are 12 full moons in a calendar year, three for each season. These 12 full moons have been given traditional names that can be traced back to early American or Native American lore. For example, summer has its Thunder Moon, Green Corn Moon and Fruit Moon.
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Occasionally, a troublesome 13th full moon will show up in a calendar year. What shall we call this one?
Calendar-makers of yesteryear used red-colored moon symbols on their calendars to mark the three named full moons of each season but they used a blue-colored moon symbol for the unnamed full moon. By tradition, the third of the four full moons in a 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 any calendar month, instead of the third full moon of any season with four. This year's Thunder Full Moon shone July 1, the first full moon of the summer season. That allows time for the moon to complete a cycle of phases and return for a second full moon July 31.
By the currently popular meaning of the term, this second full moon of July will be a blue moon, however, there are still only three full moons this summer season, so by the older meaning of the term, this is not a blue moon but just the usual Green Corn Full Moon of summer.
So — is our full moon July 31 a blue moon or not?
Well, it just depends on your definition of the term "blue moon." None of this blue moon lore is etched in stone. Personally, I like the popular meaning of the term, so I am going with July 31 as a blue moon.
Our next blue moon won't happen until January 2018.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper, and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Westlake's astrophotography website at http://www.jwestlake.com.