Seasonal Blue Moon to Light Up the Sky for First Time in Nearly Three Years

A blue moon is scheduled to take place this week, though not in the terms that most understand it -- it's not blue and it's not the second full moon to take place during the month of August. Rather, the event will mark a "seasonal blue moon," or the third of fourth full moons to take place in a season as defined by the period between a solstice and equinox, or vice versa.

That the term "blue moon" is used in regards to two different lunar events dates back to 1980 when a radio program used a 1946 article from the magazine Sky & Telescope that inaccurately defined a blue moon as the second full moon in a single month. The definition stuck, even surpassing the correct definition in terms of use and understanding.

According to EarthSky, the last seasonal blue moon took place almost three years ago, with the next one slotted to take place May 21, 2016.

Officially, this week's blue moon will begin on Aug. 21 at 1:45 UTC, which translates to Aug. 20 at 9:45 p.m. EDT. And while it will not technically appear blue in the sky, there have been times throughout history in which the lunar surface has taken on a blue hue.

In 1883 an Indonesian volcano named Krakatoa exploded with a force so extreme that people as far as 600 kilometers away heard the noise as loud as a cannon shot. As the resulting plumes of ash rose through the ranks of Earth's atmosphere, the moon turned blue.

Less violent explosions, including the 1983 El Chichon volcano in Mexico and Mt. St. Helens in 1980, have reportedly caused the moon to appear blue as well. According to NASA, the key to a blue moon is in the presence of particles slightly wider than the wavelength of red light -- and no others.

Forest fires as well contribute from time to time to this rare phenomenon. In 1950, for example, a series of fires in Alberta, Canada resulted in people as far away as England reporting an indigo-colored moon.