When someone says “Once in a Blue Moon,” you know what they mean: Rare, seldom, even absurd.
This year it means August 31st.
For the second time this month, the Moon is about to become full. There was one full Moon on August 1st/2nd, and now a second is coming on August 31st. According to modern folklore, whenever there are two full Moons in a calendar month, the second one is “blue.”
But will the moody Moon of August 31st actually turn blue? Probably not.
Most Blue Moons look pale gray and white, indistinguishable from any other Moon you’ve ever seen. Squeezing a second full Moon into a calendar month doesn’t change the physical properties of the Moon itself, so its color remains the same.
With that caveat in mind, however, be aware that on rare occasions it can happen.
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.