True Blue Moon?

True Blue Moon?

Was Tuesday night's full Moon a "Blue Moon?" Some observers say "yes," but not because the Moon turned blue. Behold this picture of last night's moonrise over Volterraio Castle on the Island of Elba, Italy, then scroll down for further discussion:

 

"The Full Moon of Aug. 20-21 is a 'seasonal Blue Moon,'" explains photographer Stefano De Rosa, "because it is the third of four full moons in a single season."

But wait--isn't a Blue Moon the second full Moon in a calendar month? That would be the modern definition, which became popular in the late 20th century. De Rosa's definition is an older and, some would say, truer definition of "Blue Moon."

Which definition is correct? Both and neither. It's all folklore! The only true-blue Moon is a Moon that actually turns blue. And, yes, that can happen. Under certain circumstances volcanic dust and ash from forest fires can scatter the reds out of moonlight, leaving only a blue-cratered disk behind.