Blue Moon: What Is It And When Does It Occur?

On the night of the 31st July a 'blue' moon will rise above skies across the world.

The phenomena occurs when two of the full natural satellites appear in one calendar month, and after tonight, it wont happen again until 2018.

Usually there are twelve full moons annually, but this year there are thirteen. The spectacle only happens about every two and a half years.

The blue moon won't be blue in colour tonight

Although the display is not technically blue, there are rare occasions when the natural satellite shines a shade of azure.

The unusual phenomenon can occur after volcanic eruptions, forest fires or dust storms when tiny dust particles enter the atmosphere and scatter red light while letting through the blue light.

Indonesians witnessed a full blue moon in 1883 after the Krakatoa volcano erupted.

Geoff Chester of the U.S. Naval Observatory told the Associated Press that the traditional definition of a blue moon is two full moons in a month.

Chester continued to explain that an older definition also exists. A Maine almanac calls a blue moon the fourth full moon in an astronomical season.

The last full moon occurred this month on the 2nd of July.

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