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Lunar Eclipse Tetrads


Currently the Lunar Sign is:

Moon Moon Signs in Virgo Virgo 27° 32'
September 20, 2017 Wed UTC

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Lunar Tetrad Series


When is the next Lunar Tetrad? 2032 - 2033




Although I love eclipses, my main focus has been on the solar ones (new moon eclipses). I am always inspired by the eclipses and how they tend to significantly impact our lives both in the physical realm, as well as the psychological. Some people save their money for years to chase down the "land fall" for the next total solar eclipse. On the other hand, and luckily. there are considerly more total lunar eclispes that we can observe by just stepping out onto our back porch, as long as the event occurs after dark with a clear sky in our areas.

These coppery colored blood moon lunar eclipses are routinely awe inspiring, and now, with the age of electronic devices, we can watch from our computers/mobile devices while the event happens ten-thousand miles away from our neighborhoods. Technology is simply amazing.

I recently learned a new term that is associated with total lunar eclipses. Tetrads were first noticed in the 1800s. The lunar tetrad, also refered to as lunar tetradia, was coined to refer to four total lunar eclipses that occur in a sequence. Each total lunar eclipse occurs approximately six months apart, over a two year period of time. Tetrads occur in groups over a period of about 500 years, with 16 to 18 tetrad groups occurring relatively close together; approximately every six to twenty years over a span of about 300 years. Then, the tetrad series stop, with none appearing for about 200 to 300 years.

Tetradia

Sometimes 4 total lunar eclipses occur in a row with intervals of 6 lunations (semester), and this is called a tetrad. Giovanni Schiaparelli noticed that there are eras when such tetrads occur comparatively frequently, interrupted by eras when they are rare. This variation takes about 6 centuries. Antonie Pannekoek (1951) offered an explanation for this phenomenon and found a period of 591 years. Van den Bergh (1954) from Theodor von Oppolzer's Canon der Finsternisse found a period of 586 years. This happens to be an eclipse cycle; see Meeus [I] (1997). Recently Tudor Hughes explained the variation from secular changes in the eccentricity of the Earth's orbit: the period for occurrence of tetrads is variable and currently is about 565 years; see Meeus III (2004) for a detailed discussion.
Reference: https://en.wikipedia.org/wiki/Eclipse_cycle and https://en.wikipedia.org/wiki/Tetrad_%28astronomy%29 (Sept. 2015)

While I used the NASA eclipse site to track the tetrad eclipse series, I note that there is considerable discussion concerning the third (April 2015) eclipse not being a true "total" eclipse in Sky and Telescope. In the discussion, Renz, Meeus and Espenak address the potential for the "different" calculations. However, as with all calculations, there will always be some debate and wiggle room. The discussion between such esteemed experts is fun to watch unfold. I am a lowly observer and simple admirer of the events. We will let them work it out.

I did go back through the NASA records to 500BC to look at the series of tetrads. They are interesting to explore, now that I even know what they are. It's funny how we can study something for years and have no clue about a small facet of a subject. That being said, I am including a list of the most recent and upcoming tetrads based on the NASA eclipse site. I broke the groups into three centuries.

When looking through the charts, below, you may notice that I included the saros numer as well as the nodal family. I observed that the same series would show up two or more times. For example, the same group showed up a total of four time between 1949 until 2004. I also noticed that some of them would roll into a new group. A part of the group from 1909 - 1910 shows again with the 2072 - 2073 group. The patterns are interesting to explore. If I find the time, I may include some of the previous tetrad series. Some people find religious and historical significance in these tetrads. I won't go down that path, just yet. For right now, I'm just looking for patterns.

Consider doing your own research, now that you know, to see what you find and observe. Feel free to contact me with your discoveries.


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