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October 20, 2018 Sat UTC
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The concept of the second full moon in the month being the blue moon is actually a very new definition as a result of an older and more obscure practice from the Maine Farmers' Almanac. Due to a misinterpretation in 1946 by an author for Sky and Telescope, the premise that two full moons in the same calendar month evolved and morphed into a trivia question that compounded the mistake in the early 1980s. Link to information about the Blue Moon Mistake
The zodiacal blue moon is similar to the calendar blue moon in that the two Full Moons occur in similar time references. But the blue moon of the signs is actually more "different" than the same. The significant difference is that calendar blue moon can never occur in the same zodiac sign for that particular month. The blue moon of the signs is more closely associated with the seasonal blue moon as described in the old Maine Farmers' Almanac. The zodiac blue moons can occur in any month between the 18th to the 23rd day of the month.
The zodiacal blue moon may be associated with the past practice of Seasonal blue moons implemented by the Maine Farmer's Almanac that dates back to the early 1800s as a way to account for the extra Full Moon within a calendar year. If four Full Moons occurred within a season, then the third Full Moon in that season was deemed to be a blue moon. The only months that a Seasonal blue moon can occur in are February, May, August and November. The Maine Farmer's Almanac made it even more difficult to determine the periods of the seasonal blue moon because they used an obscure method for determining the seasons. Archived Page ~ Seasonal Blue Moon dates from Once in a Blue Moon [The updated page is Traditional Blue Moon.] While the zodiacal blue moons can, conceivably occur in any of the 12 signs, the zodiac blue moons only rarely occur during the time specified for the Seasonal blue moon. See the table below for an example.
On the other hand, seasonal blue moons and zodiacal blue moons routinely occur in the same year and season. But even that is not an absolute. Notice that the 2002 and 2008 seasonal and zodiac blue moons occur in two different seasons.
This seasonal blue moon practice was not common. In fact, it does not appear that any of the other popular almanacs in the United States used this blue moon calculation for the extra Full Moon contained within the season. In the year 2005, the zodiac sign blue moon (two full moons occurring in the same sign within a year) and the seasonal blue moon will occur a month apart, July 21 and August 19, respectively.
Here is a comparison between the Seasonal Blue Moon and the Zodiac Blue Moon:
|1991||November 21 (A)||October 23 (A)||Aries|
|1994||August 21 (Su)||July 22 (Su)||Capricorn|
|1997||May 22 (Sp)||June 20 (Sp)||Sagittarius|
|2000||February 19 (W)||March 20 (W)||Virgo|
|2002||November 20 (A)||August 22 (Su)||Aquarius|
|2005||August 19 (Su)||July 22 (Su)||Capricorn|
|2008||February 21 (W)||May 20 (Sp)||Scorpio|
|2010||November 21 (A)||October 23 (A)||Aries|
|2013||August 21 (Su)||August 21 (Su)||Aquarius|
|2016||May 21 (Sp)||June 20 (Sp)||Sagittarius|
|2019||February 19 (W)||April 19 (Sp)||Libra|
|2021||November 19 (A)||August 22 (Su)||Aquarius|
|2024||August 19 (Su)||July 21 (Su)||Capricorn|
|2027||February 20 (W)||May 20 (Sp)||Scorpio|
|2029||November 21 (A)||
September 22 (Su)|
Apprx 1 hr pre-Autumn
Using Northern Hemisphere EST Sp=Spring, Su=Summer, A=Autumn, W=Winter.
|Approx Calendar Date||March 21st||June 21st||September 21st||December 21st|
|Zodiac Sun Sign||Aries 0°||Cancer 0°||Libra 0°||Capricorn 0°|
*- The seasonal reference table is for the northern hemisphere, the seasons are reversed for the southern hemisphere.
© 2005, J McCaul - do not copy any portion
of this text without author's permission