Blue Moon on July 31, 2015

For the second time this month, the Moon is about to become full, but it most likely won't turn blue, like it's name suggests. According to modern folklore, whenever there are two full Moons in a calendar month, the second one is "blue." 

Most Blue Moons look pale gray and white, just like the Moon you've seen on any other night. 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! 

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 September 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.  At this time of year, summer wildfires often produce smoke with an abundance of micron-sized particles – just the right size to turn the Moon truly blue.

On the other hand, maybe it will turn red.  Often, when the Moon is hanging low, it looks red for the same reason that sunsets are red.  The atmosphere is full of aerosols much smaller than the ones injected by volcanoes.  These aerosols scatter blue light, while leaving the red behind.

For this reason, red Blue Moons are far more common than blue Blue Moons.

Step outside at sunset on July 31st, look east, and see what color presents itself.

Video courtesy Science@NASA

Featured image: NASA.