中星者，所以验岁时之气候，每于平旦初昏，见于南方正午之位者是也，四时十二月以
次而转。第在尧时天心建子，甲辰冬至，日次虚鼠；汉太初冬至，日次牵牛；唐大衍冬至，日
次南斗；宋至今冬至，日次南箕。又尧时中星昏中昴，今则昏中近奎矣。古今不同如此，始
见岁差有度也。岁差者，以天有三百六十五度四分度之一，岁有三百六十五日四分日之
一，天度行四分之一而稍有余，日行四分之一而颇不足，故天度常舒，日度常缩，天渐差而
西，日渐差而东，此所以古今有导，自尧时至今已差五十余度。东晋虞喜谓约以五十年差
一度，何承天以为太过，乃倍其年而又觉不及，至隋刘焯取二家之说而折中之，谓七十五年
差一度，自后诸说不同，至元世祖用郭守敬所造授时历，谓六十六年有奇差一度者，似为
近之；然岁久时更，恐尚有未能必者。兹并录之，以见岁差之概。
五营运大论曰∶所谓戊己分者，奎壁角轸，则天地之门户也。夫奎壁临干，当戊土之
位；角轸临巽，当己土之位。遁甲经亦曰∶六戊为天门，六己为地户。然而曰门曰户，必有
所谓，先贤俱未详及。予尝考周天七政躔度，则春分二月中，日躔壁初，以次而南，三月入
奎类，四月入胃昴毕，五月入觜参，六月入井鬼，七月入柳星张；秋分八月中，日躔翼末，以
交于轸，循次而北，九月入角亢，十月入氐房心，十一月入尾箕，十二月入斗牛，正月入女虚
危，至二月复交于春分而入奎壁矣。是日之长也，时之暖也，万物之发生也，皆从奎壁始；
日之短也，时之寒也，万物之收藏也，皆从角轸始。故曰春分司启，秋分司闭。夫既司启
闭，要非门户而何？然自奎壁而南，日就阳道，故曰天门；角轸而北，日就阴道，故曰地户。
又如春分日躔壁初，故言奎壁；秋分日躔翼末，何以不言翼轸而言角轸？盖自角以后十四
宿，计一百七十三度四分度之一；自奎以后十四宿，计一百九十二度。度有不齐，此秋之所
以在翼末，而经言角轸者，正以翼度将完而角轸正当其令。且奎壁角轸为对待之宿，而奎
壁为西北之交，角轸为东南之交，故经云奎壁角轸，天地之门户也。是以伏羲六十四卦方
图，以干居西北，坤居东南，正合天门地户之义。凡候之所始，即道之所生，有不可不通也。
中星，是用来验证岁时气候的，即在黎明或黄昏时分，出现在南方正午位置的那些星宿，它们随着四季和十二个月依次轮转。然而在尧帝时代，天心指向子位，甲辰年冬至时，太阳位于虚宿（鼠）；汉代太初历冬至时，太阳位于牵牛宿；唐代大衍历冬至时，太阳位于南斗宿；宋代至今的冬至，太阳位于南箕宿。另外，尧帝时代黄昏时中天的星宿是昴宿，而现在则是奎宿附近。古今差异如此之大，这才显示出岁差的存在。所谓岁差，是因为天球有三百六十五又四分之一度，而回归年有三百六十五又四分之一日。天球运行四分之一度时略有盈余，太阳运行四分之一度时则明显不足，因此天球的度数常显舒展，太阳的度数常显收缩。天球逐渐向西偏移，太阳逐渐向东偏移，这就是为什么古今存在差异，从尧帝时代到现在已经相差五十多度。东晋虞喜认为大约五十年差一度，何承天认为这太快了，于是加倍年份却又觉得不够，到了隋朝刘焯综合两家之说取其中值，认为七十五年差一度。此后各种说法不一，直到元世祖采用郭守敬所编制的《授时历》，认为六十六年多差一度，似乎比较接近真相；然而岁月长久、时代更迭，恐怕仍有不能确定的地方。这里一并记录这些说法，以展示岁差的大致情况。
《五营运大论》说：所谓的“戊己分”，指的是奎壁二宿和角轸二宿，这是天地出入的门户。奎壁二宿对应乾卦方位，处于戊土之位；角轸二宿对应巽卦方位，处于己土之位。《遁甲经》也说：六戊为天门，六己为地户。然而既然称为门和户，必然有其具体所指，先贤们都未曾详细阐明。我曾经考察周天七政的躔度（日月五星在黄道上的运行轨迹），发现春分二月时，太阳运行到壁宿初始，依次向南移动，三月进入奎宿范围，四月进入胃、昴、毕宿，五月进入觜、参宿，六月进入井、鬼宿，七月进入柳、星、张宿；秋分八月时，太阳运行到翼宿末端，进而交接于轸宿，依次向北移动，九月进入角、亢宿，十月进入氐、房、心宿，十一月进入尾、箕宿，十二月进入斗、牛宿，正月进入女、虚、危宿，到二月再次交接于春分点并进入奎壁宿。这样看来，白昼变长、气候变暖、万物萌发生长，都从奎壁宿开始；白昼变短、气候变寒、万物收敛收藏，都从角轸宿开始。所以说春分主管开启，秋分主管闭合。既然主管开启与闭合，难道不是门户吗？然而从奎壁宿向南，太阳走向阳道，所以称为天门；从角轸宿向北，太阳走向阴道，所以称为地户。又如春分时太阳运行到壁宿初始，所以说奎壁；秋分时太阳运行到翼宿末端，为何不说翼轸而说角轸呢？这是因为自角宿以后的十四宿，共计一百七十三又四分之一度；自奎宿以后的十四宿，共计一百九十二度。度数不相等，这就是为什么秋分点在翼宿末端，而经典中提及角轸的原因，正是因为翼宿的度数即将结束，而角宿和轸宿正当其令。况且奎壁与角轸是对应的星宿群，奎壁位于西北交汇处，角轸位于东南交汇处，所以经典说“奎壁角轸，天地之门户”。因此伏羲六十四卦方图中，乾卦居于西北，坤卦居于东南，正好符合天门地户的含义。凡是候气之所起始，就是道之所产生，这是不可不通晓的道理。
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.