There’s no better day for a full moon than Halloween, as it always feels fortuitous when the spookiest lunar phase and spookiest holiday align. In addition to the chance of a werewolf sighting, a full moon means trick-or-treaters can navigate their annual routes by moonlight late into the night.
But Halloween 2020 is getting something even rarer than a full moon: a blue moon. And no, the moon won’t actually be blue, but it still earns the moniker given to the second full moon of the month.
You might be wondering how the name “blue moon” came about if the moon doesn’t actually appear blue. An old NASA article has the answer. The name originated in 1883 after the eruption of a volcano on Krakatoa, an Indonesian island. It spewed so much dust into the atmosphere that the moon appeared blue, which was such a rare sight that the term “once in a blue moon” was coined.
The connection of the term “blue moon” with rarity was transferred to its current astronomical definition. And with every month’s full moon having a unique name (e.g. Corn Moon, Pink Moon) it was useful to have a name for the second full moon of a month, no matter which one it was.
And blue moons are rare. The last one occurred on March 31, 2018. There are about seven blue moons every 19 years, an observation first articulated by astronomer James Hugh Pruett in 2946.
So while Halloween is going to feel different this year as the very real and actually scary threat of COVID-19 hangs over the festivities, looking up at the last blue moon until August 31, 2023 and the last Halloween blue moon until 2039.