This article was originally published on 8/10/2020. It has been updated to reflect new information.
Skywatchers are in for a treat this Halloween: a rare blue moon will be visible across the entire United States for the first time in decades. The last time a full moon on All Hallows Eve occurred in all time zones was in 1944, according to the Farmer's Almanac. So you won't want to miss it!
What is a blue moon?
Now in a perfect world, a blue moon would most likely mean that the moon appears blue during the night. That’s usually not the case, unfortunately. A moon isn’t considered a “blue moon” because of its color, but rather when it occurs.
As you may already know, full moons typically occur once a month. When 13 fulls moons occur in one year instead of 12, that means you’ve got a blue moon (and its uncommon nature is the basis for the phrase "once in a blue moon.).
There are two types of blue moons: A full moon can be considered blue when it's the second of two full moons in a calendar month (aka a monthly blue moon), or when it’s the third of four full moons in a single season (aka a seasonal blue moon), according to Earth Sky.
While there’s a good chance October’s blue moon won’t actually appear blue for us Earth dwellers down below, it doesn’t mean it’s completely impossible. According to timeanddate, a moon can appear blue if the atmosphere is filled with dust or smoke particles of a certain size. Therefore, if a massive volcanic eruption occurred, that could give the moon a blueish tone. The 1980 eruption of Mt. St. Helens in the US, the 1983 eruption of Mt. Krakatoa in Indonesia, and the 1991 eruption of Mount Pinatubo in the Philippines have all made the moon look blue.
What type of blue moon will we see in 2020?
This year’s blue moon will be a monthly blue moon. There will be a full moon on October 1st and October 31, 2020. Because October has two full moons scheduled, the latter full moon is dubbed as the blue moon.
The blue moon will reach its peak on October 31 at 10:49 a.m., EDT, according to the Farmer's Almanac. We won't be able to catch another blue moon on Halloween night until 2039, so be sure to look up at the night sky. What better way to celebrate the spooky season than with a rare full moon?
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