The August 20 blue moon will not fit the common definition of its title.
The moon will not have a blue appearance, and is not the second full moon this calendar month, EarthSky reported.
This "blue moon" will be the third of four full moons that will rise in the same season. This definition was actually put in place before the commonly recognized meaning of the term.
Using the common definition of "blue moon" is actually an error made in the 1940s and publicized in the 1980s. A radio program used a 1946 inaccurate article from Sky & Telescope Magazine as a source,the Weather Channel reported.
In both cases the full moon is an abnormality in the "19-year Metonic cycle," according to EarthSky.
"There are 235 full moons in 19 calendar years, but only 228 calendar months (or 76 three-month seasons). Therefore, it's inevitable that 7 out of 19 years will feature two full moons in one calendar month. And it's also inevitable that 7 out of 19 years will have four full moons in one season," EarthSky reported.
"There was a time, not long ago, when people saw blue moons almost every night. Full moons, half-moons, crescent moons - they were all blue, except some nights when they were green," Tony Phillips wrote for NASA's Solar System Exploration, the Weather Channel reported. "The year was 1883, the year an Indonesian volcano named Krakatoa exploded."
Ash from the volcano caused the blue hue. Particles from the eruption scattered red light, contributing to the color.
"White moonbeams shining through the clouds emerged blue and sometimes green," Phillips wrote.
The moon can be viewed at "9:45 p.m. EDT, 8:45 p.m. CDT, 7:45 p.m. MDT or 6:45 p.m. PDT," in the U.S., EarthSky reported.
The phenomenon really does happen "once in a blue moon." There won't be another one until May 21, 2016, the Weather Channel reported.
