On August 31, the moon will be full for the second time this month. Because of that, this one is called a blue moon—it almost never happens.
The tale of how it came to be called that arrives wrapped in one of the many misconceptions promulgated by the supposed discoverers of America. Originally that epithet was given purely to the third full moon of a season, if it was followed by a rogue fourth full moon, Space.com reports. Normally a season contains only three.
But an article in a 1946 issue of Sky and Telescope misinterpreted that definition to mean the second full moon in a calendar month. That definition is what made it, decades later, into the board game Trivial Pursuit, where it took firm root in the national consciousness, at least the part that pays attention to such things. By the time the misnomer was exposed, there was no going back.
The one thing the term “blue” does not refer to is the moon’s color, although certain atmospheric phenomena, such as volcano ash, can lend the moon varying hues, as NASA explains. Given that the moon takes 29.5 days to go from full to new to full, it’s inevitable that sometimes a month will have two full ones. August has been bookended by two of them, the first one on August 1 and the last tonight, August 31.
The twice-in-a-month type of blue moons happen about every three years, according to LiveScience. The last blue moon, LiveScience said, was on New Year’s Eve 2009. Even rarer is when two blue moons occur in the same calendar year. The last time that happened was in 1999, and it’s not due again until 2018, LiveScience said.
This blue moon will bloom at 9:58 a.m. Eastern Daylight Time, though does not rise till around 7:15 p.m. But any change in brilliance or fullness will be virtually indiscernible. After that you can go back to saying “once in a blue moon” in the comfort of knowing that that is a long way away.
It will be unique for another reason as well: It will shine over a private memorial service for astronaut Neil Armstrong, the first human to walk on the moon. He walked on last Saturday, August 25, at age 82. His family “has suggested paying tribute to him by looking at the moon and giving the astronaut a wink,” the Associated Press reported.