Skywatchers will see a blue moon, the second full moon in a given month on Friday, July 31. Contrary to its name, the moon will not actually change in color. This astronomical event occurs once every 2.7 years. The first full moon of July 2015 occurred on Thursday, July 2.
“Most blue moons appear pale gray and white, just like the moon you’ve seen on any other night,” stated NASA in a video. “Squeezing a second blue moon in a calendar month does not change its color. Nevertheless, the moon can turn blue. A truly blue moon usually requires a volcanic eruption.”
In history, people really did see a blue moon when there were volcanic eruptions or forest fires. It happened when Krakatoa erupted in Lampung Province, Indonesia in 1883 and also when Mt. Pinatubo erupted on Luzon Island, Philippines in 1991. A forest fire in Alberta, Canada in 1953 also made the moon appear blue.
The ash from the volcanic eruptions and the smoke from the fire served as a blue filter. Plumbs of smoke or ash sometimes contain particles that are one micron wide, which according to NASA is “about the same wavelength as red light.” The particles scattered red light, which allowed “blue light to pass through.”