Blue Moon and Lunar Cycles: Pearly-Gray or Wildfire-Red?

A blue moon isn't really blue, in most cases. The one that appears on July 31 this year will not be blue-it will be milky gray, like most moons.

Still, the moon will be the second full moon in the month of July, which makes it somewhat unusual. July won't have two entire moons in its monthly cycle again until 2018, NASA noted here.

Because a lunar period is 29 and a half days, you can wind up having a second full moon in a calendar month, according to NASA's website.

Basically the definition of a blue moon as the "second (full moon) in a month" arose from a 1946 article in Sky & Telescope magazine.

In some cases, such as soon after a volcano erupts, a moon will actually appear to be blue. This happened in 1883, when the Indonesian volcano Krakatoa exploded. The ash clouds of somewhat larger-than-usual particles scattered red light, allowing other colors to shine. Moonbeams passing through clouds shone blue and sometimes green, NASA's website notes.

In the case of the forest fires currently burning in the Western part of North America, the particles thrown by the fires will likely bring out red light, making the moon shine red, according to NASA's website.

In any case, if you step outside at sunset on July 31, you'll see the second full moon of the month. 

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