Friday’s blue moon is last until 2018

That thing you've been waiting for "once in a blue moon" may happen early Friday.

Friday features the second full moon of July at 5:42 a.m. The last blue moon occurred in August 2012. The next won't appear until January 2018.

It's a twisted definition, but a blue moon is defined as the second full moon in the same month. Here's more on how the term blue moon came to pass from NASA:

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This definition of a Blue Moon is a recent thing.

If you told a person in Shakespeare's day that something happens "once in a Blue Moon" they would attach no astronomical meaning to the statement. Blue moon simply meant rare or absurd, like making a date for "the Twelfth of Never." Since then, however, its meaning has shifted.

The modern definition sprang up in the 1940s. In those days the Maine Farmer's Almanac offered a definition of Blue Moon so convoluted many astronomers struggled to understand it. It involved factors such as ecclesiastical dates of Easter and Lent, tropical years, and the timing of seasons according to the dynamical mean sun. Aiming to explain blue moons to the layman, Sky & Telescope published an article in 1946 entitled "Once in a Blue Moon." The author James Hugh Pruett (1886-1955) cited the 1937 Maine almanac and opined that the "second [full moon] in a month, so I interpret it, is called Blue Moon."

This was not correct, but at least it could be understood. And thus the modern Blue Moon was born.

Most Blue Moons look pale gray and white, just like the Moon you've seen on any other night.  Squeezing a second full Moon into a calendar month doesn't change its color.

Nevertheless, on rare occasions the Moon can turn blue.

A truly-blue Moon usually requires a volcanic eruption. Back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth's atmosphere, and the Moon … it turned blue!

Krakatoa's ash was the reason. Some of the plumes were filled with particles 1 micron wide, about the same as the wavelength of red light.  Particles of this special size strongly scatter red light, while allowing blue light to pass through. Krakatoa’s clouds thus acted like a blue filter.

People also saw blue-colored Moons in 1983 after the eruption of the El Chichon volcano in Mexico. And there are reports of blue Moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.

Forecast: Summer's finest

Does it get any better than this? Sunny skies, low humidity and highs in the 80s. Give the air conditioning unit a rest. Yes, weather fans, this could be the best weather of the summer. I'm not holding my breath on rain chances this weekend.

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Twin Cities NWS

Next week hints at a cooler Canadian air mass. A taste of September as we turn the corner into August? Can the Vikings first preseason game be far away?

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Weatherspark Euro output

Enjoy the summery weekend ahead!