The claim: There will be a blue moon on Oct. 31, the first in 76 years
A social media post claims spookiness will reach its peak this Halloween with the special occurrence of a rare moon sighting.
"There's going to be a full blue moon on 31 October 2021," reads an Oct. 18 Facebook post. "That is Halloween for the first time in 76 years."
Similar posts on Facebook also claim it will be 19 years until the next blue moon will be visible in the night sky.
These posts have amassed hundreds of likes, comments and interactions since being shared on Facebook. USA TODAY has reached out to the users who posted the claim for comment.
A blue moon is a real phenomenon, albeit one that has nothing to do with a new hue.
But it's not happening this Halloween.
Blue moons are an extra full moon
To understand what exactly a blue moon is, we need to understand how lunar cycles compare to our solar-based calendar.
It takes the moon about 29.5 days to complete all eight phases we see in the night sky throughout a month: a new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter and finally waning crescent. These phases are defined by how much sunlight is illuminating the moon, according to NASA.
Because a lunar cycle takes about as long as a 30- or 31-day month in our calendar, we typically see one full moon monthly, or 12 full moons in a year.
But sometimes the periods don't exactly match up. It takes only about 354 days to have 12 full lunar cycles, which falls short of the 365 days (or 366, if you consider leap years) in our calendar – based on Earth's rotation around the sun.
So every two to three years (sometimes less), we will see two full moons in a month or 13 full moons in a calendar year. This extra lunar sighting is called a "blue moon," according to the Royal Museums Greenwich, home to the Royal Observatory in the U.K.
Sometimes a blue moon can fall on Halloween, as the post indicates. But the post is wrong about which Halloween.
The last blue moon to coincide with the spooky holiday was on Oct. 31, 2020, which was the first in 76 years, NASA spokesperson Lance D. Davis said in an email to USA TODAY. Davis confirmed there won't be a blue moon this Halloween since we already had one on Aug. 22.
"The moon is projected to be in the waning crescent phase for Halloween 2021," he said.
The next blue moon is forecasted for August 2023, much sooner than the 19 years one Facebook post claimed.
And contrary to the name, blue moons aren't actually blue. For the most part, the moon appears in its usual bone-white or gray color, but depending on particulates in the air, which act as a color filter, it can appear in other hues.
If water droplets, clouds or particles are thrown up into the atmosphere from a natural catastrophe like a volcanic explosion, the moon may appear blue, according to NASA. However, it can take on other colors like red if a blue moon coincides with a lunar eclipse.
Those phenomena could alter the moon's color at any point, though, not just during the blue moon.
Our rating: False
Based on our research, we rate FALSE the claim there will be a blue moon on Oct. 31, the first in 76 years. Blue moons are a phenomenon where there's an extra full moon in a calendar year outside of the usual 12. The claim is off by a year. The last blue moon to fall on Halloween was on Oct. 30, 2020, which was the first in 76 years. The next blue moon is not expected until 2023.
Our fact-check sources:
NASA, March 3, Earth's Moon
NASA, accessed Oct. 27, Moon in Motion
Royal Museum Greenwich, accessed Oct. 26, What is a blue moon?
The Old Farmer's Almanac, Aug. 5, Blue Moon in 2021: When is the Next Blue Moon?
Lance D. Davis, Oct. 27, Email exchange with USA TODAY
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This article originally appeared on USA TODAY: Fact check: No blue moon for Halloween this year