Story highlights Blue moon will come Friday

Another one will not appear until 2018

Particles of smoke and dust can affect the hue of the moon

Will you be watching the blue moon Friday? Share your photos with CNN iReport.

(CNN) If anything unusual happens to you Friday, you'll be right to say it was "once in a blue moon."

The July 31 appearance of the month's second full moon will be the first such occurrence in the Americas since August 2012. Every month has a full moon, but because the lunar cycle and the calendar year aren't perfectly synched, about every three years we wind up with two in the same calendar month.

But Earth's satellite will most likely not appear blue at all.

Typically, when a moon does take on a bluish hue, it is because of smoke or dust particles in the atmosphere, such as during a cataclysmic volcanic eruption.

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One example of this happened in 1883, when the Indonesian volcano Krakatoa erupted, spewing so much ash into the atmosphere that the moon took on a cerulean tinge for years, night after night. After the massive explosion, which scientists believe rivaled a 100-megaton nuclear bomb, the volcanic debris caused vibrant red sunsets and the moon to have a bluish tint.

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