WRAL WeatherCenter Blog

Friday's 'blue moon' probably won't be

Posted July 27, 2015

A full moon is visible in this view above Earth's horizon and airglow from the International Space Station  (Credit:
NASA/ISS Expedition 10)

When we speak of a "blue moon," we're usually referring to the currently popular definition, which is, very simply, the second full moon occurring within a single calendar month. We had a full moon back on the evening of July 1, and the moon will be technically full again just after moonset the morning of Friday, July 31.

The current widely used definition actually seems to have supplanted an older one because of a misinterpretation of the previous one that was part of an article in Sky & Telescope magazine back in the 1940s. Prior to that, the "blue moon" was the third full moon in a series of four, when those four were all contained within a three-month season, when there would typically be three. By either means of defining the term, a blue moon of this type occurs about once every 2 to 2 1/2 years.

When you have a look at the full moon on these dates, it isn't really any different from any other full moon, so don't expect it to appear any "bluer" than usual. There is a very rare physical effect that can make the moon appear blue, but this is unrelated to "blue moons" that show up as artifacts of the difference between the lengths of a lunar cycle and our calendar months.

In the unusual event that a volcanic eruption or forest fire produces dust and/or smoke plumes having particles of nearly uniform size, with those particles a little under a micron in diameter, the plumes can scatter long wavelengths of light (reds, oranges) more than the short wavelengths, allowing the bluer ones to pass though and giving the moon a blue hue. However, it is much more common for smoke and dust to be generated with a large range of sizes, most of them a good deal smaller than a micron. In these more common cases, the smoke or dust has the opposite effect, scattering blue and green light out of the beam more efficiently and leaving the moon with a yellow, orange or reddish cast. Some of you may have seen this happen a few weeks ago, when smoke from forest fires in western Canada was transported our way and gave us a couple of nights with "orange moons."

This week's blue (but not really) moon will be setting in the western sky a little after 6:30 in the morning on Friday, and will rise again Friday evening a little before 8:30. It'd be a great time to listen to a little doo-wop from the Marcels ("Blue Moon") or maybe "New Blue Moon" by the Traveling Wilburys.


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  • Mark Adcox Jul 27, 2015
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    ... and none realized that the picture of the moon was placed incorrectly.

  • Brian White Jul 27, 2015
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    It took seven people to write this?