Tonight’s “alternative” Blue Moon

The moon, not quite full, on August 18 (Kevin Wolf via Flickr)

The moon, not quite full, on August 18 (Kevin Wolf via Flickr)

Usually, when we talk about a Blue Moon, we’re referring to the relatively rare occurrence of a second full moon in a single month.

But just like months sometimes squeeze in an extra full moon, so do seasons.  Instead of having just three full moons, seasons – from time to time – tack on a fourth.

So there’s an alternative definition of Blue Moon: the third full moon in a four moon season.  As tonight’s full moon is the third of four this summer, it’s a Blue Moon by this standard.

“[T]his definition actually preceded the more modern definition of a Blue Moon as being the second of two full moons to occur in one calendar month,” writes Earth Sky.

A Blue Moon is equally rare whichever definition you prefer.

“[I]t’s inevitable that 7 out of 19 years will feature two full moons in one calendar month,” Earth Sky notes. “And it’s also inevitable that 7 out of 19 years will have four full moons in one season.”

“Once in a Blue Moon,” as the saying goes.

Related: Blue moon: one small mistake, giant folklore for the sky | Blue moon and Sturgeon moon in August 2012

Of course, under either definition, the term “Blue Moon” is a misnomer, as the moon is not actually the color blue.

In Washington, tonight’s Blue Moon rises at 7:27 p.m. in the eastern sky and sets Wednesday morning at 7:05 a.m.  Thus, from dusk to dawn, the Blue Moon will glow.  It is officially 100 percent full at 9:45 p.m.

Tonight’s full moon is also known as the Sturgeon Moon. According to the Old Farmer’s Almanac, fishing tribes near the Great Lakes are credited with naming this moon – identifying the season when ample sturgeon were caught.

Also on Capital Weather Gang

Top 10 wettest summer in reach for D.C.; Wettest on record for Southeast cities