On October 31, the night sky will be illuminated thanks to this celestial event.

Halloween is always full of spooky surprises, but this year will have one spectacular sight that has nothing to do with jack-o-lanterns or trick-or-treating. A blue moon is expected to make an appearance on October 31, 2020. The name "blue moon" doesn't indicate that the moon will be a bewitching shade of indigo, nor does it reference the Belgian-style ale; instead, it explains when the full moon will occur. Full moons generally rise in the sky once a month, but October 2020 will have two—once on October 1 and the other on October 31. This rarity is what coined the phrase "once in a blue moon" back in 1821, according to the Farmer's Almanac, but the official explanation of a blue moon wasn't defined until August 1937.

Blue Moon
Credit: PhotoAlto/Frederic Cirou/Getty Images

For more than 50 years, whenever two full moons appear in the same month (approximately every two-and-a-half to three years), the second occurrence is called a "Blue Moon." There are two types of blue moons. The one we'll experience in October refers to the second of two full moons in a calendar month. However, a blue moon can also be the third or fourth full moon in a single season, according to Earth Sky.

Sadly, it's unlikely that this year's full moon will look unique to the naked eye. "Most Blue Moons look pale gray and white, indistinguishable from any other moon you've ever seen," according to NASA. "Squeezing a second full moon into a calendar month doesn't change the physical properties of the moon itself, so the color remains the same."

However, the moon did appear to have a blue tint after the volcano Krakatoa erupted in Indonesia in 1883. A moon can appear blue if the atmosphere is filled with dust or smoke particles of a certain size, which can be the result of a volcanic eruption, according to timeanddate. While the event is unlikely to cause a true blue hue, history proves that it's possible.


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