Since I have never seen one, I understand why a true blue moon is considered to be rare. Although there are certain atmospheric events, like dust clouds from a volcano which can literally turn the moon blue, these are very, very rare. There are multiple meanings in common usage that are not quite so rare. We will cover the two most common.

Sometimes this refers to the fourth full moon in a calendar quarter. This happens because full moons occur every 29.5 days. The next one will be in December 2020. The moons for that quarter are Oct. 1, Oct. 31, Nov. 30, and Dec. 30; four full moons in a single quarter.

This also is the timing for an alternative definition: two full moons in one month. This happens in October 2020, so the blue moon will be Oct. 31. So, our next blue moon will be on Halloween.

Sadly, the observatory remains closed due to COVID-19.

Aaron Clevenson
Author: Aaron ClevensonEmail: This email address is being protected from spambots. You need JavaScript enabled to view it.
I am the observatory director at the Insperity Observatory in Humble ISD. I am also an adjunct astronomy professor at Lone Star College-Montgomery where I teach solar system astronomy and stars and galaxies astronomy. I am the author of the astronomy textbook, “Astronomy for Mere Mortals.” I am a past president of the North Houston Astronomy Club, and was the chair of Astronomy Day in Southeast Texas in 2015 and 2016. He is an observing program director with The Astronomical League, coordinates their Master Observer Progression Awards, and has authored six of their observing programs.

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