Why the 'Blue Moon' Isn't Blue

Friday's full 'Blue Moon' will, no doubt, be lovely to look at, but it is not likely to be blue.

On Friday evening sky viewers will have a chance to see the 'Blue Moon,' a phenomenon that sounds much more exciting than it actually will be. 

That's because the Blue Moon is not actually blue - Friday's full moon t is called a “blue moon” because it is the second full moon in a month, according to EarthSky, which mentions other definitions of the term.  Because a full moon occurs every 29.5 days, it only happens roughly every 2.7 years that there are two in one month.

So a blue moon Friday night is not any more likely than on any other night.

“It’s very rare that you would see a blue-colored moon, although unusual sky conditions – certain-sized particles of dust or smoke – can create them,” EarthSky notes.

In fact, the entire notion of calling the phenomenon a 'Blue Moon' is a mistake, says JPL volunteer ambassador Tony Rice.  The use of the term dates back to a 1946 Sky and Telescope magazine article which "misinterpreted the usage of the term in The 1937 Maine Farmers' Almanac," Rice writes.

Still, for those who want to see the full moon, it will no doubt be a lovely viewing experience - how often do you go out and take a long look at a full moon anyway? So if you want to view it here are some good local options:

  • The Cobb Estate - The park at the top of Lake Avenue stays open 24/7 and if you follow the path that breaks left away from the junction with the Sam Merrill trail, you can very quickly get above the sight line of trees and have a clear view of the sky.
  • - Altadena's two higher elevation parks are a good bet for viewing.
  • Highway 2 - If you want to go further afield, you can quickly drive to a very high altitude and get a nice clear view without having to hike at all. 

Patch Asks: Is there another spot, South Pasadena, where you might recommend we moon gaze? Tell us in the comments!

Editor's note: An earlier version of this story incorrectly reported the frequency of a full moon. It is every 29.5 days. 


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