Posted on: August 17, 2020 at 4:56 pm
Last updated: October 14, 2020 at 5:59 pm

We’ve all probably used the saying “once in a blue moon”, and while we know it means a rare occurrence, it’s hard to imagine that it’s coined from the appearance of a rare blue-tinged moon that comes up once in many decades. Maybe the saying should be, “Once in a blue moon… literally.” 

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Well, a Blue Moon actually refers to a celestial phenomenon that occurs when there is an extra full moon within a given period. The phrase “Blue Moon” has nothing to do with that actual color of the moon, although under certain rare atmospheric conditions, a moon may appear with a slight bluish tinge. 

The concept of a Blue moon often strikes up a lot of confusion for many people. There is the seasonal Blue Moon and the monthly Blue Moon.  Originally, the seasonal Blue Moon was regarded as the true definition of a Blue Moon. It’s the third of four full moons in an astronomical season – between each solstice and equinox. This older but still accepted definition is attributed to the issue of the Maine Farmer’s Almanac published in August 1937 [1]. On the other hand, the monthly Blue Moon is influenced by the days in a calendar month. There are 29.5 days between two full moons (a lunar month). However, most months (except February) are longer by a day or two. 

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This means that over time, the lunar cycle falls short of the expected duration, and thus, a 13th full moon is seen in one calendar year. This is referred to as a monthly Blue Moon [2]. Both definitions of the Blue Moon are acceptable. You just have to pick one – or both. It’s entirely up to you. 

The 2020 Blue Moon

This is a rather special one because of the period it is expected to happen. This year, the Blue Moon is coming up on October 31st, 2020. That’s right, on Halloween.

The last Blue Moons (yes, plural – Blue Moons) were seen in January and March of 2018. The appearance of two Blue Moons in one year is referred to as a double Blue Moon and is a very rare occurrence. The next one is likely to happen in 2037 [4]. 

This October, the first full moon will appear on the 1st and is dubbed the Harvest Moon (the full moon closest to the autumn equinox) [5]. The second one is expected to appear on the 31st and is regarded as the Blue Moon itself. This is because, for over half a century, whenever two full moons are seen in a month, the second takes on the title of the Blue Moon [6]. 

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Blue Moons typically occur every two and a half years. However, the next time a blue moon will coincide with Halloween will be in 2039. According to meteorologist Brian Lada, “After the blue Moon on October 31, 2020, trick-or-treaters will need to wait until 2039 to see the next Blue Moon on Halloween,” [7].

The seasonal blue moon

This particular phenomenon is just as incredible as the other one. It will not be appearing in 2020 but is expected to show up on August 22, 2021. If you are unable to catch the Blue Moon this Halloween, you might still experience one next year [8]. 

Is there such a thing as an actual blue-colored moon? 

The moon has always been and will always be silver-colored. Interestingly, there have been times when people reported looking up to the sky to find an actual blue-colored moon. Unfortunately, they’re usually caused by dust particles and smoke. One such faux blue-colored moon occurred as a result of a volcanic eruption that was so powerful the moon appeared blue for almost 2 years. 

In 1883, the Krakatoa volcano in Indonesia exploded as violently and as far-reaching as a nuclear bomb. The thick plumes of ash from the rupture created ash-clouds filled with particles about a micron wide each. These particles went ahead to scatter red light while allowing other colors to be filtered through. Eventually, white light from the moon shone through the clouds and was decoded by the human eye to be blue [9]. 

  1. Tim Sharp. What Is a Blue Moon? Space. https://www.space.com/15455-blue-moon.html. Retrieved 13-08-2020.
  2. Vigdis Hocken. When Is the Next Blue Moon? Time and Date. https://www.timeanddate.com/astronomy/moon/blue-moon.html. Retrieved 13-08-2020.
  3. Kelly Vaughan. A Rare Halloween Blue Moon Will Be Seen This Year. BHG. https://www.space.com/40134-blue-moon-2018-last-one-until-2020.html. Retrieved 13-08-2020
  4. Deborah Byrd. All you need to know: 2020’s Harvest Moon. Earth Sky. https://earthsky.org/?p=2467. Retrieved 13-08-2020.
  5. Kelly Vaughan. A Rare Halloween Blue Moon Will Be Seen This Year. BHG. https://www.bhg.com/syndication/rare-blue-full-moon-halloween-2020/ Retrieved 13-08-2020.
  6. 2020 Will Have a Rare “Halloween Blue Moon” Which Won’t Occur Again Till 2039. Lessons Learned in Life. https://daily.lessonslearnedinlife.com/13-full-moons-to-appear-in-2020-halloween-blue-moon-next-one-in-2039. Retrieved 13-08-2020.
  7. Claudia Harmata. Halloween Blues! A Rare Blue Moon Will Light Up the Night Sky on October 31 This Year. People. https://people.com/home/halloween-blues-a-rare-blue-moon-will-light-up-the-night-sky-this-october/. Retrieved 13-08-2020.
  8. Blue Moon. Nasa.gov. https://www.nasa.gov/vision/universe/watchtheskies/07jul_bluemoon.html. Retrieved 13-08-2020.
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Penelope Wilson
Team Writer
Penelope is a writer and health enthusiast with a B.Arts in Language Studies. She is a deeply spiritual person, a relationship expert, a nutrition freak, and a skin-care maverick.

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