4 9 2
3 5 7
8 1 6
Earth Chart represents the distribution of Earth qi. It is unchanging but it is also changing in cycles. What is unchanging is the nature of qi. What is changing is the timeliness of qi.
Heaven Chart is changing with a different number placed at the center and the numbers fly forward in the Luoshu path. The central number is the "Ruling Star" - Ling Xing 令星.
What is wrong with Tan Yang Wu's treatment is that he mixes Earth qi and Heaven qi.
Next, we examine the Xian Tian Ba Gua with Hou Tian numbers:
Then we draw the Hou Tian Ba Gua together with the numbers from the above chart:
This is the Earth Chart but the numbers are from the Xian Tian Ba Gua.
In Xuan Kong Da Gua, 180 years are divided into the Upper Cycle with Periods 1, 2, 3, and 4; and the Lower Cycle with Periods 6, 7, 8 and 9. Note that the Upper Cycle consists of female trigrams while the Upper Cycle consists of male trigrams.
The Periods, however, are to be applied to the timeliness of the 64 Hexagrams for Zheng Shen and Ling Shen purposes.
In Xuan Kong Liu Fa, the eight directions are divided into two parts: SE, S, SW and W is one part while NW, N, NE and E is another part. The two parts are to represent the mountain and water locations for the Upper and Lower Cycles.
Upper Cycle: SE, S, SW, W for water; NW, N, NE, E for mountain.
Lower Cycle: NW, N, NE, E for water; SE, S, SW, W for mountain.
Feixing and Dagua are the two branches of Xuan Kong. As Feixing derives the complete charting based on 3 Yuan 9 Yun, it emphasized on Heaven qi. Dagua, on the other hand, is based on 2 Yuan 8 Yun and therefore talks about Earth qi. The former is the application while the latter is the body.
Xuan Kong Liu Fa is another method that claims to be the authentic Xuan Kong School. I will not comment on its validity here.
It is unfortunate that some people actually mix up 3 Yuan 9 Yun and 2 Yuan 8 Yun resulting in teaching that resembles an animal that is neither a donkey nor a horse 非驢非馬.