You’ve likely heard the term, “Once in a blue moon,” but what you may not know is that a Blue Moon is a genuine astronomical event. And you can catch a true Blue Moon tomorrow night, on Sunday, August 22.
Confusingly, there are two definitions of a Blue Moon. Informally, the term has come to mean a second full moon in a single calendar month. But the original definition, also known as a seasonal Blue Moon or a true Blue Moon, is the third full moon over a season. In this context, a season means the period of time between a solstice and an equinox, which this year is between June 2021 and September 2021. Tomorrow’s event is a true Blue Moon, which happens only around once every three years.
According to Sky and Telescope, the two different definitions of the Blue Moon originated with a mix-up in that magazine in 1946. The new definition spread through common usage, and Sky and Telescope addressed its “Blue Moon blooper” in 1999. The mystery was unwound by historian Donald W. Olson and research librarian Margaret Vaverek who were tracing where the two contradictory definitions originated from.
So is a Blue Moon when you can see the moon colored blue? Not exactly. The moon sometimes appears to be tinted blue because of particles in the atmosphere, usually from dust or smoke. This happens most often in the vicinity of a volcano or wildfire, as these put a lot of ash up into the atmosphere. That can coincide with a seasonal Blue Moon, but it doesn’t necessarily.
Even if it won’t be literally blue, tomorrow night will still be a special opportunity to see the big, beautiful full moon. Like other full moons, the Blue Moon will rise around sunset. So take a look into the sky and see if you can spot the moon, as the next time you’ll be able to observe a true Blue Moon won’t be until August 2024.