There’s a full Moon tonight, but if you miss it, don’t worry. There’s another one coming. July 2015 has two full Moons. The first occurs on the night of July 1-2, followed by a second on July 31.

According to modern folklore the second full Moon in a calendar month is a “Blue Moon.” Don’t expect the Moon to actually turn blue, though. Squeezing a second full Moon into a calendar month does nothing to chnge its actual color.

Nevertheless, it can happen: 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. Volcanic exhaust 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.

Forest fires can do the same trick. A famous example is the giant muskeg fire of Sept. 1953 in Alberta, Canada. Clouds of smoke containing micron-sized oil droplets produced lavender suns and blue Moons all the way from North America to England.

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