Setting the Record Straight: What is a Blue Moon?

I beg to differ with EarthSky.org’s article by Bruce McClure and Deborah Byrd of September 27, 2020 entitled "When is the Next Blue Moon?". We have a full Moon on the 1st of the month in October 2020 and again on the 31st of the month. The second full moon we will see in October, on Halloween by the way, is not a Blue Moon. We have not had a true Blue Moon in all of 2020 and will not see a true Blue Moon until August of 2021. Let me explain… 

As McClure and Byrd point out quite succinctly, the prevailing sentiment defines a blue moon as the second full moon that falls in any one given month. They also point out another less known formula for defining blue moons. Because we can sometimes experience 13 full moons in a year instead of 12, this means that there are times when one of the four yearly seasons will experience four full moons instead of the more common three. For academic conformity, the third full moon of the set of four is then determined to be the odd moon out, so to speak, and is called a Blue Moon. When you understand, that a moon cycle is usually shorter than a calendar month, (most February’s excluded) you can see how some years will end up with 13 full moons during the full year.

Both explanations have their historical pedigrees and both have their advocates and scientific foundations.

Unfortunately, both explanations are mistaken. 

The long lost original definition of a Blue Moon is neither the second full moon in a month, or the seasonally unique third moon of four, but defined through a formula based on a much more ancient system. 

This system is the system of Astrology. 

In order to explain, I must give a quick lesson in Astrological methodology. In Astrology, we map the planets and stars, and so on, against the Zodiac, a 360-degree coordinate system that is roughly equivalent to extending our equator out into space on all sides of the earth like a never-ending pie chart, with a line connecting the two poles of the earth running right through the center of the pie. Our western zodiac divides this pie into 12 equal parts of 30 degrees each. This adds up to the complete 360-degree circle, or Zodiacal band across the sky, that gives us our infamous Sun signs.

[In my case, I am using a Tropical Zodiac system. Many scientists argue that we should be using the Sidereal coordinate system, but that is a whole other article. Suffice it for now to say that either method is appropriate IF you are sure to use the proper methods appropriate to the proper system. Sort of like the difference between different types of psychology. Both are beneficial as long as you understand the methodology.]

Now, since we know the sun moves at almost one degree per day, and a zodiacal sign is thirty degrees in size, it can be seen that every thirty days, and every thirty degrees, the sun will change signs. In twelve months, the sun completes one full revolution around the Zodiac. The Moon moves almost thirteen times faster than the Sun: A full revolution of the Moon around the zodiac takes only about 28 and a half days. 

Now since the Sun takes 30 days to traverse any one zodiacal sun sign, and the Moon takes only 28 and a half, there are times, as we discussed above, that there will be 13 full moons in a year. This means that in certain years (approximately 1 out of every 3), there are two full moons in a row in the same astrological sign. THIS is the more ancient meaning of a Blue Moon: Two full moons in the same astrological sign: NOT in the same month. 

In Astrological parlance, this means that in certain years, one of the 12 zodiacal signs will be “highly activated” compared to the other 11. In the case of 2019, it was the Zodiacal sign of Libra with two full moons in Libra. The first on March 20th, 2019 at 0 degrees of Libra and the second, which became the “Blue Moon”, on April 19th, 2019 at 29 degrees of Libra.

We will not be seeing another true Blue Moon until August of 2021, when we see the first Full Moon at 1 ½ degrees of Aquarius in July of 2021 and the second Full Moon in the sign of Aquarius at 29 ½ degrees on August 22nd, 2021. We will have to wait until 2024 to see another true Blue Moon in Capricorn. I guess you could say that true Blue Moons truly do only come once in a Blue Moon.