Oct 31, 2020

Halloween 2020's Unusual Full Moon

All manner of spooky imagery represents Halloween, or "All Hallow's Eve." Blood-sucking vampires, snarling werewolves, wicked witches, and zombies from the grave - just to name a few - are featured in masks, costumes, and decorations. 

Jack-o-lanterns, another familiar sight, light up porches and windows.

Halloween is also associated with the full moon, and tonight's moon will live up to the imagery and then some.  This evening, the moon will be full, a "Hunter's Moon," and a "Blue Moon." 

The Rare Halloween Full Moon

Despite the full moon's association with Halloween, a full moon on Halloween is actually uncommon. The last time the moon was full on Halloween was in 2001, 19 years ago. The moon will not be full again on Halloween until 2039. From that point, there will only be three more Halloween full moons before 2100 - 2058, 2077, and 2096 - in predictable 19-year intervals. This 19-year chronological cycle is sometimes referred to as the moon's "Metonic cycle."

What is a "Hunter's Moon?"

In addition to being a full moon, 2020's Halloween moon will also be a "Hunter's Moon." The "Hunter's Moon" is mentioned in several Anglo-Saxon sources as simply the name for October's full moon. However, astronomers have a slightly more precise definition that comes in handy this year. To understand precisely what defines a "Hunter's Moon," it helps to first know when the "Harvest Moon" occurs. 

The "Harvest Moon" is always the full moon closest to the autumnal equinox, which fell on September 22nd. In 2020, the "Harvest Moon" occurred on October 1st, which is itself something of rarity. The "Harvest Moon" is the source of much myth and folklore.

A "Hunter's Moon" is the full moon that follows after the "Harvest Moon." Thus, the old Anglo-Saxon reference is correct because the "Hunter's Moon" would always be October's full moon. Because the "Harvest Moon" in 2020 fell on October 1st, however, October gets two full moon cycles instead of just one. The second, full - "Hunter's Moon" - lands on Halloween.

Is a "Blue Moon" Really Blue?

In short, no. A "Blue Moon" is not really blue. However, the phrase is part of our lunar lexicon. Like the "Harvest Moon," the "Blue Moon" was even popularized in the 1960's by a doo-wop group called The Marcels.

What is a "Blue Moon?" A "Blue Moon" occurs when a calendar month has two full moon cycles instead of one. The second full moon is referred to as a "Blue Moon." 

While not rare, "Blue Moons" do not happen frequently. Before 2020, the last "Blue Moon" occurred in March 2018. Before that, January 2018 displayed a "Blue Moon." However, before that, no "Blue Moon" had lit up the night sky since July 2015. This sporadic phenomenon gives rise to the adage, "Once in a Blue Moon," referring to something that does not occur often.

For skywatchers in general, or those interested in the moon in particular, Halloween 2020 offers up a special viewing treat. Go outside and enjoy the full, Hunter's, Blue Moon of 2020.

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