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Zhongxing (Central Stars) are used to verify the climatic conditions of seasons and years. These are the stars visible in the southern meridian position at dawn or dusk, rotating sequentially through the four seasons and twelve months. However, during the time of Emperor Yao, the celestial heart pointed to the Zi position; on the winter solstice of the Jia-Chen year, the sun was located at Xu (Rat); during the Taichu calendar of the Han Dynasty, the sun was at Qian Niu; during the Dayan calendar of the Tang Dynasty, the sun was at Nan Dou; from the Song Dynasty to the present, the sun is at Nan Ji. Furthermore, during Emperor Yao's time, the star culminating at dusk was Mao, whereas now it is near Kui. Such significant differences between ancient and modern times reveal the existence of axial precession. Axial precession arises because the celestial sphere comprises 365.25 degrees, while a tropical year consists of 365.25 days. The celestial sphere moves slightly more than a quarter degree, while the sun moves slightly less than a quarter day. Consequently, celestial degrees appear expansive, while solar degrees appear contracted. The celestial sphere gradually shifts westward, and the sun gradually shifts eastward. This explains why there are discrepancies between ancient and modern eras, with a cumulative difference of over 50 degrees since the time of Emperor Yao. Yu Xi of the Eastern Jin Dynasty proposed that the shift is approximately one degree every fifty years. He Chentian considered this rate too fast and doubled the period but found it insufficient. Liu Zhuo of the Sui Dynasty synthesized these views, proposing a shift of one degree every seventy-five years. Subsequent theories varied until the Yuan Shizu era, when Guo Shoujing's Shoushi Calendar suggested a shift of slightly more than one degree every sixty-six years, which seems closer to reality. However, given the passage of long ages and changing times, uncertainties may still exist. These various accounts are recorded here to illustrate the general concept of axial precession.

The "Wu Yun Da Lun" states: The so-called "Wu-Ji Division" refers to Kui-Bi and Jiao-Zhen, which serve as the gates of heaven and earth. Kui and Bi correspond to the Qian trigram position, aligning with the Wu Earth position; Jiao and Zhen correspond to the Xun trigram position, aligning with the Ji Earth position. The "Dunjia Jing" also states: The six Wus constitute the Gate of Heaven, and the six Jis constitute the Door of Earth. However, since they are termed gates and doors, there must be specific referents, yet ancient sages did not elaborate on this. I have examined the ecliptic longitudes of the Seven Luminaries and found that during the Spring Equinox in February, the sun enters the initial part of Bi, moving southward; in March, it enters Kui; in April, Wei, Mao, and Bi; in May, Zi and Shen; in June, Jing and Gui; in July, Liu, Xing, and Zhang. During the Autumn Equinox in August, the sun reaches the end of Yi, transitioning into Zhen, and moves northward; in September, Jiao and Kang; in October, Di, Fang, and Xin; in November, Wei and Ji; in December, Dou and Niu; in January, Nu, Xu, and Wei; returning to the Spring Equinox point in February, entering Kui and Bi again. Thus, the lengthening of days, warming of climate, and germination of all things begin from Kui and Bi; the shortening of days, cooling of climate, and storage of all things begin from Jiao and Zhen. Therefore, it is said that the Spring Equinox governs opening, and the Autumn Equinox governs closing. Since they govern opening and closing, are they not gates? Moving south from Kui and Bi, the sun follows the Yang path, hence called the Gate of Heaven; moving north from Jiao and Zhen, the sun follows the Yin path, hence called the Door of Earth. For instance, at the Spring Equinox, the sun is at the beginning of Bi, hence mentioning Kui and Bi. At the Autumn Equinox, the sun is at the end of Yi; why mention Jiao and Zhen instead of Yi and Zhen? This is because the fourteen mansions after Jiao total 173.25 degrees, while those after Kui total 192 degrees. Due to this inequality, the Autumn Equinox falls at the end of Yi, but the classics mention Jiao and Zhen because Yi's degrees are nearly exhausted, and Jiao and Zhen are currently dominant. Moreover, Kui-Bi and Jiao-Zhen are opposing mansion groups; Kui-Bi is at the northwest intersection, and Jiao-Zhen is at the southeast intersection. Hence, the classics state "Kui-Bi Jiao-Zhen, the gates of heaven and earth." Accordingly, in Fuxi's Square Diagram of the Sixty-Four Hexagrams, Qian is placed in the northwest and Kun in the southeast, perfectly aligning with the meaning of the Gate of Heaven and Door of Earth. Wherever qi observation begins, the Dao originates; this is a principle that must be thoroughly understood.