Saturday night's full moon won't look much different than any other, but it will still be called a "blue moon."

The full moon on Saturday, July 31, is the second full moon of the month, at least in some time zones. Under the modern definition that makes it a blue moon.

But while it sounds like an ancient bit of folklore, this definition of "blue moon" was actually coined in the mid-20th century.

In the 1930s, the Maine Farmers' Almanac said if one of the seasons had four full moons in it instead of three, the third full moon is called a "blue moon." (It's been suggested that this "extra" full moon was considered unlucky and might be the origin of 13 as an unlucky number.)

That definition appeared to originate in the 19th century, which is also when the phrase "once in a blue moon," meaning rarely, came into use. That was also the first time in recent history when the moon actually appeared blue, because of dust and ash from the eruption of the volcano Krakatoa in 1883.

But the phrase "blue moon" has been around even longer than that, for more than 400 years. Back then it meant something absurd, as in, "You may as well say the moon is blue."

But the modern definition, the second full moon of the month, first appears in an article in Sky and Telescope in 1946. The writer misinterpreted the definition written in the Maine Farmers' Almanac.

That definition was popularized by syndicated radio programs in the late 1970s and early 1980s, as a piece of "old folklore."