So how was it a blue moon?
The answer comes from one of the earliest mentions of the term, “blue moon,” taken from the 1937 edition of the “Maine Farmer’s Almanac”. That book says that a blue moon occurs when “one of the four seasons would contain four full moons instead of the usual three.”
This only happens seven times in every 19 years. Older almanac makers (people have been making astronomy-based calendars for millennia) had trouble figuring out when the blue moons would occur, so the phrase “once in a blue moon” came to represent uncertainty and a rare event.
Why do many think that a blue moon is the second full moon of the month? That definition showed up as a mistake within a decade and caught on, being a simpler explanation. It doesn’t hurt that months with two full moons are rare — the lunar month of 29.5 days means that a full moon would have to happen on the 1st and then the 30th or 31st of a month to count.
Since 1980, the two-full-moons-in-a-month definition has been considered an official alternate meaning for “blue moon.”
Thus, Tuesday night’s blue moon is more accurately called a “seasonal blue moon.” It’s the third — of four — full moons occurring during the summer season of 2013. The final full moon of the season will occur just before autumn begins, occurring on Sept. 19.
If you want a moon to actually be blue in color, that usually requires major atmospheric pollution — smoke from large forest fires and ash from volcanic eruptions have been known to color the moon a bluish color.