六气终始早晏，五运太少盈虚，原之以至理，考之以至数，而垂万古无有差异也。经曰五日一候应之，应
五行也。故三候成一气，即十五日也。三气成一节，节谓立春、春分、立夏、夏至、立秋、秋分、
立冬、冬至，此八节也。四方二十四气而分主四时，一岁成矣。春秋言分者，以月气言之，则二
月半初气终，而交二之气；八月半四气尽，而交五之气。若以四时之气言之，则阴阳寒暄之气，
到此可分之时也。昼夜分五十刻，亦阴阳之中分也。故经曰分则气异是也。冬夏言至者，以六气
言之，则五月半司天之气至其所在，十一月半在泉之气至其所在。以四时之令言之，则阴阳之气
至此极至之时也。夏至日长不过六十刻，阳至此而极；冬至日短不过四十刻，阴至此而极，皆天
候之未变。故经曰至则气同是也。
天自西而东转，其日月五星循天从东而西转。故《白虎通》曰∶天左旋，日月星辰右行。又
曰∶日为阳，月为阴。行有分纪，周有道理。日则昼夜行天之一度，一度有百刻，即一日；月则
昼夜行天之十三度有奇者，谓复行一度之中，作十九分，分之得七。（一度有百刻，作十九分，
分之得七。每一分该五刻强，五七三十五刻强。是月昼夜行天十三度零三十五刻强。）大率月行
疾速，终以二十七日月行一周天。是将十三度及十九分分之七数，总之则二十九日，计行天三百
八十七度有奇。计月行疾之数比日行迟之数，则二十九日日方行天二十九度，月已先行一周天，
三百六十五度，外又行天之二十二度，反少七度而不及日也。阴阳家说谓日月之行，自有前后，
迟速不等，固无常准，则有大小月尽之异也。本三百六十五日，四分度之一，即二十五刻，（是
日行三百六十五日零二十五刻，当为一岁矣。）当为一岁。自除岁外之余，即有三百六十日，又
除小月所少之日六日，止有三百五十四日而成一岁，通少十一日二十五刻，乃盈闰为十二月之制，
则有立首之气。气乃三候之至，月半示斗建之方，乃十二辰之方也。闰月之纪，则无立气建方，
皆他气，但根据历以八节见之，推其所余，乃成闰，天度毕矣。故经曰∶立端于始，表正于中。推
余于终，而天度毕矣者，此之谓也。
观天之杳冥，岂复有度乎？乃日月行一日之处，指二十八宿为证，而记之曰度。故经曰∶星
辰者，所以制日月之行也。制谓制度也。天亦无候，以风雨霜露草木之类应期，可验而测之曰候。
言一候之日，亦五运之气相生而值之，即五日也。如环无端，周而复始。《书》曰∶期三百六旬
有六日，以闰月定四时成岁，即其义也。医工之流不可不知。经曰∶不知年之所加，气之盛衰，
虚实之所起，不可以为工矣。
六气（风、寒、暑、湿、燥、火六种气候因素）的结束与开始有早有晚，五运（木、火、土、金、水五种运行之气）的强弱、盈亏和虚实，追溯其根本在于至理，考证其依据在于精确的数理，从而流传万古而没有任何差异。医经说：五天为一候，对应五行（木、火、土、金、水）。所以三候构成一气，即十五天。三气构成一个节气，所谓“节”，指的是立春、春分、立夏、夏至、立秋、秋分、立冬、冬至这八个节气。四方分布的二十四节气分别主宰四季，一年便形成了。《春秋》中提到的“分”（春分、秋分），从月令之气来说，二月过半时初之气结束，交接二之气；八月过半时四之气结束，交接五之气。如果从四季之气来说，是指阴阳寒暖之气到了此时可以平分的时候。昼夜各五十刻，也是阴阳的中分。所以医经说：“分则气异”。冬夏提到的“至”（夏至、冬至），从六气来说，五月过半时司天之气到达其所在位置，十一月过半时在泉之气到达其所在位置。从四季时令来说，是指阴阳之气到了此时达到极点的时候。夏至白天最长不超过六十刻，阳气在此达到极盛；冬至白天最短不超过四十刻，阴气在此达到极盛，这都是天候尚未变化的表现。所以医经说：“至则气同”。
天体自西向东旋转，太阳、月亮和金木水火土五星沿着天体从东向西运行。所以《白虎通义》说：天向左旋转，日月星辰向右运行。又说：太阳属阳，月亮属阴。它们的运行有分界和记录，周天运行有其道理。太阳每天昼夜在天行一度，一度有一百刻，即一天；月亮每天昼夜在天行十三度多一点，即在重新运行一度的过程中，分为十九份，每份得七。（一度有一百刻，分为十九份，每份得七。每一份包含五刻多，五乘以七是三十五刻多。因此月亮昼夜在天行十三度零三十五刻多。）大致上月亮运行疾速，经过二十七天后月亮运行一周天。将这十三度及十九份中的七数总计，则是二十九天，计算月亮运行天体三百八十七度多一点。计算月亮运行疾速的数量比太阳运行迟缓的数量，那么二十九天里太阳才运行天体二十九度，月亮已经先行了一周天，三百六十五度，另外又运行了天体的二十二度，反而比太阳少七度而没有追上太阳。阴阳家说法认为日月之行，自有前后，快慢不等，本来没有固定的标准，因此有大月和小月结束的差异。本应是三百六十五日四分之一，即二十五刻，（这是太阳运行三百六十五日零二十五刻，应当是一年。）当为一年。除去整年之外的余数，就是三百六十日，再减去小月所少的六天，只剩下三百五十四日成为一年，总共少了十一日二十五刻，于是通过设置闰月来制定十二个月的制度，这就有了立首之气。气是三候的终结，半月示出斗柄指向的方向，也就是十二辰的方向。闰月的纪年，就没有立气和建方的标志，都是其他的气，只是根据历法以八个节气显现出来，推算剩余的余数，就成为了闰月，天体的度数也就完备了。所以医经说：“在开始确立端点，在中间树立正准，在末尾推算余数，而天体的度数就完备了”，说的就是这个意思。
观察天空的幽深昏暗，难道还有度数吗？那是以太阳和月亮运行一天的位置，指着二十八宿作为证据，并记录为度。所以医经说：“星辰是用来制约日月运行的。”制约意思是制度、规范。天也没有候，以风雨霜露草木之类应合时节，可以验证和推测它叫做候。说一候的日子，也是五运之气相生而值临的，即五天。像圆环一样没有开端，周而复始。《尚书》说：“周期三百六十六天，用闰月确定四季形成年份”，就是这个含义。医生这一类人不可不知道。医经说：“不知道年份所加的气运，气的盛衰，虚实的起因，不可以做医生。”
The beginning and ending of the Six Qi (the six climatic factors: wind, cold, summer heat, dampness, dryness, and fire) may be early or late; the deficiency and excess of the Five Yuns (the five phases of movement: wood, fire, earth, metal, and water) are traced to ultimate principles and examined by precise numbers, thereby remaining unchanged for ten thousand generations. The Classics state: "Five days constitute one Hou (a period of time), corresponding to the Five Elements." Therefore, three Hou constitute one Qi (climatic phase), which is fifteen days. Three Qis constitute one Jie (solar term). The term "Jie" refers to the eight solar terms: Lichun (Start of Spring), Chunfen (Spring Equinox), Lixia (Start of Summer), Xiazhi (Summer Solstice), Liqiu (Start of Autumn), Qiufen (Autumn Equinox), Lidong (Start of Winter), and Dongzhi (Winter Solstice). The twenty-four solar terms distributed in the four directions govern the four seasons, completing one year. When the *Spring and Autumn Annals* mentions "Fen" (Equinoxes), regarding the qi of the months, at the midpoint of the second month, the first Qi ends and transitions into the second Qi; at the midpoint of the eighth month, the fourth Qi ends and transitions into the fifth Qi. However, regarding the qi of the four seasons, it signifies that the qi of yin-yang, cold and warmth, reaches a point where they can be divided equally. Day and night each consist of fifty ke (time units), representing the equal division of yin and yang. Thus, the Classics say: "At the Equinox, the qi differs." When winter and summer mention "Zhi" (Solstices), regarding the Six Qi, at the midpoint of the fifth month, the Si Tian (ruling qi above) reaches its position; at the midpoint of the eleventh month, the Zai Quan (governing qi below) reaches its position. Regarding the commands of the four seasons, it signifies that the qi of yin and yang reaches its extreme limit at this time. At the Summer Solstice, the day does not exceed sixty ke, as yang qi reaches its extremity here; at the Winter Solstice, the day does not fall short of forty ke, as yin qi reaches its extremity here. These represent the state before the heavenly climate changes. Thus, the Classics say: "At the Solstice, the qi is identical."
The Heaven rotates from west to east, while the sun, moon, and five planets revolve around the heaven from east to west. Therefore, the *Baihu Tongyi* states: "Heaven rotates to the left, while the sun, moon, and stars move to the right." It further states: "The sun is yang, and the moon is yin." Their movements have divisions and records, and their cycles follow specific principles. The sun travels one degree across the sky per day and night; one degree consists of one hundred ke, constituting one day. The moon travels slightly more than thirteen degrees across the sky per day and night. This means that within the course of one degree, it is divided into nineteen parts, with each part yielding seven. (One degree has one hundred ke, divided into nineteen parts, each yielding seven. Each part contains slightly more than five ke; five times seven is slightly more than thirty-five ke. Thus, the moon travels slightly more than thirteen degrees and thirty-five ke across the sky per day and night.) Generally, the moon moves swiftly, completing one revolution around the sky in twenty-seven days. Summing up the thirteen degrees and the seven parts of nineteen, it totals twenty-nine days, during which the moon travels slightly more than three hundred eighty-seven degrees across the sky. Comparing the swift motion of the moon to the slow motion of the sun: in twenty-nine days, the sun has only traveled twenty-nine degrees across the sky, whereas the moon has already completed one full revolution, traveling three hundred sixty-five degrees, plus an additional twenty-two degrees across the sky. Consequently, the moon lags behind the sun by seven degrees and has not yet caught up. Proponents of Yin-Yang astrology argue that the movements of the sun and moon naturally have front and back positions, with unequal speeds, lacking a fixed standard, hence the differences between large months and small months. The base should be three hundred sixty-five and one-quarter days, i.e., twenty-five ke. (This implies the sun travels three hundred sixty-five days and twenty-five ke, which constitutes one year.) Excluding the whole year, the remainder is three hundred sixty days; subtracting the six days missing from small months, only three hundred fifty-four days remain to form a year. This results in a total deficit of eleven days and twenty-five ke. Therefore, the system of twelve months is established through intercalation (leap months), giving rise to the Qi of starting the year. A Qi is the culmination of three Hou. At the half-month mark, the direction of the Dipper's handle indicates the direction of the Twelve Earthly Branches. For leap months, there is no establishment of Qi or directional markers; they belong to other Qis but are manifested through the eight solar terms according to the calendar. By calculating the remainder, the leap month is formed, and the degrees of the heaven are completed. Thus, the Classics say: "Establish the origin at the beginning, set the standard in the middle, calculate the remainder at the end, and the degrees of the heaven are complete." This is what is meant.
Observing the profound darkness of the sky, are there still degrees? They use the daily position of the sun and moon, pointing to the Twenty-Eight Mansions (Lunar Lodges) as evidence, and record them as degrees. Thus, the Classics say: "Stars and constellations are used to regulate the movement of the sun and moon." "Regulate" means to establish a system or standard. Heaven also has no "Hou," but uses phenomena such as wind, rain, frost, dew, and vegetation responding to their seasons, which can be verified and predicted, called "Hou." One Hou lasts five days, during which the Five Yuns generate each other and take effect. Like a circle without an end, it cycles repeatedly. The *Book of Documents* states: "A cycle of three hundred sixty-six days, using leap months to determine the four seasons and form the year," which embodies this principle. Physicians must not fail to understand this. The Classics say: "If one does not know the influence added by the year, the flourishing and declining states of the qi, and the origins of deficiency and excess, one cannot serve as a physician."