The Six Jia

At the summer solstice the sun rises at Yin and sets at Xu; the season is hot, with long days and short nights, so Yin-Wu-Xu correspond to Jia-Yin, Jia-Wu, and Jia-Xu. At the winter solstice the sun rises at Chen and sets at Shen; the season is cold, with short days and long nights, so Shen-Zi-Chen correspond to Jia-Zi, Jia-Wu, and Jia-Shen. Zi and Wu are the two functions of Water and Fire; the Six Jia image the transformations of yin and yang. Hence the winter solstice opens from the First Ju because Zi is put to use, and the summer solstice opens from the Ninth Ju because Wu is put to use. From Zi to Si the yang qi gradually rises; Jia is placed over Zi, Chou, Yin, Mao, Chen, and Si, rotating leftward, so the Six Yi are distributed in forward order. After Wu the qi gradually recedes, so Ding, Bing, and Yi are used to image the retreat of qi. The reverse is likewise true.