《天原發微》Vol. 7

This passage from Tianyuan Fawei primarily explains the moon's movement patterns and its dual Yin-Yang characteristics. In the Bagua (Eight Trigrams) system, the moon corresponds to the Kan trigram (representing water). According to the Hetu (an ancient astronomical diagram), the moon embodies the essence of Metal. The Kan trigram contains a single Yang line within, symbolizing inner brightness veiled by outer darkness—manifesting as the moon in heaven and water on earth.

The moon possesses dual Yin-Yang attributes: its Yang aspect drives perpetual motion, while its Yin aspect enables illumination in darkness. Ancient astronomical records describe its first western appearance as "tiao" and its final eastern emergence as "nü" (concealment). These phenomena correlate with human governance—"tiao" foreshadows unruly feudal lords, while "nü" signals a ruler's neglect of ministers. The moon's waxing and waning mirrors a female (Yin) following the male (Yang/sun), requiring calendrical adjustments based on actual observations.

The moon emits no intrinsic light, relying entirely on solar reflection. Its phases result from changing solar angles: full when facing the sun, dark when opposite. In the prenatal Bagua arrangement, the moon initiates its course from the Dui trigram (west) yet perpetually lags behind the sun's velocity. When solar motion accelerates relative to lunar pace, moon phases delay; the converse brings earlier phases.

Zhang Zai (Master Zhang) observed that though the moon embodies Yin's essence, it expresses Yang properties—receiving sunlight without absorbing solar Jing (essence). During full moons (wangri), eclipses may occur as Yin-Yang essences cannot coexist. Zhu Xi (Zhu Hanshang) interpreted this through the Tai hexagram: when heaven-earth forces commune, all things flourish. The Kan trigram's Yang (Qian) flows into Kun's Yin, making the moon appear retrograde while actually moving swiftly. The moon's borrowed radiance exemplifies Kan-Li (water-fire) interaction, mirroring the Yu hexagram's description of celestial bodies maintaining perfect orbital precision.

(Note: Technical terms like "Kan trigram," "Dui trigram," and "Tai hexagram" refer to Yijing symbols; "Hetu" denotes an ancient cosmological diagram; "tiao" and "nü" are archaic lunar phase terminology.)

This passage primarily explains the principles of solar and lunar movements, eclipses, and the mutual generation of Yin and Yang. It employs the hexagrams from the Book of Changes (Yijing) and natural phenomena to illustrate these astronomical occurrences.

The moon (represented by the Kan hexagram) and the sun (represented by the Li hexagram) converge in the northern direction. The phrase "the sun and moon attain heaven" refers to the transformation of the fourth yang line of the Qian hexagram into the Li and Kan hexagrams. The "moon nearing fullness" mentioned in hexagrams such as Xiaoxu, Zhongfu, and Guimei all describe the full moon phenomenon where the moon is in the east and the sun is in the west.

The principle of solar and lunar eclipses is as follows: When Kan (moon) and Li (sun) intersect, a solar eclipse occurs if the moon obscures the sun, while a lunar eclipse occurs if the sun obscures the moon. The Li hexagram (fire) relies on the Kan hexagram (water) to emit light, just as fire needs fuel to burn. Water gathers Jing (essence), and when essence accumulates, it gives rise to Shenming (spiritual illumination). Examples such as rotting wood emitting light at night or blood transforming into phosphorescent fire are all results of the interaction between Yin and Yang essences.

Zhang Zai (Master Hengqu) stated that the sun, moon, and five planets move in reverse directions, encompassing the earth. The moon belongs to Yin but moves in the opposite direction of Yang, hence its speed is faster. The sun is fundamentally Yin in nature, while the moon is fundamentally Yang. Therefore, when Yin and Yang essences intersect during new and full moons, eclipses occur. The waxing and waning of the moon are due to its proximity to humans, while the sun is distant, causing people to observe changes in the illuminated portion of the moon.

Metals and water can emit light internally because they receive the essences of Yin and Yang. The reason the sun and moon maintain their eternal forms lies in the mutual dependence and harmonious balance of Yin and Yang essences within them.

Finally, the passage quotes Master Cheng: The moon has a fixed form, while the sun is farther away than the moon. People observe the moon's phases because the moon itself does not emit light but merely reflects sunlight. Without the moon's waxing and waning, the calculation of years would be impossible. The degree of the moon's phases corresponds to the waning of its "Po" (the dark portion of the moon). During a lunar eclipse, the moon precisely blocks sunlight, a manifestation of excessive Yin suppressing Yang.

This passage explains the principles behind the moon's waxing and waning cycles and the ancient terminology for lunar phases. The moon itself does not emit light but shines by reflecting sunlight. When the moon is directly opposite the sun (on the full moon day, called wang), its entire face is illuminated; when it approaches the sun (on the new moon day, called shuo), we see only its dark side.

The moon's movement follows Yin-Yang hierarchy: when distant from the sun (during the waxing period from first quarter to full moon), its light progressively increases, termed "radiant fullness" (guang ying); when nearing the sun (during the waning period from full to last quarter), its light gradually diminishes, called "brightness waning" (ming que). Yang Xiong once described that before full moon (first quarter), the moon rises in the west with sunlight striking its right side; after full moon (last quarter), it rises in the east with sunlight shifting to its left. Zhu Xi later disputed Yang Xiong's interpretation.

Classical texts preserve specialized lunar terminology: "dead luminosity" (si po) refers to the completely dark state at new moon (the 1st day); "adjacent dead luminosity" (pang si po) denotes the 2nd day; "beginning brightness" (zai sheng ming) marks the 3rd day when the crescent first appears. The Book of Rites records the moon being "full at triple-five" (15th day) and "void at triple-five" (30th day). Scholar Cai Jiufeng's research correlates these terms with specific dates: "shuo" on the 1st marks first visibility, "sheng ming" on the 3rd signifies initial moonlight, "wang" on the 15th indicates peak illumination, "sheng po" from the 16th onward shows darkening light, and "hui" on the 30th means complete darkness.

Scholars debate the exact dating of these terms: Kong Yingda argued "sheng ming" occurs on the 1st, while others maintained "zai sheng po" refers to the 16th. In reality, the moon's cycle is continuous—even on the 1st there exists extremely faint light. As moonlight strengthens (ming sheng), the dark aspect (po) diminishes; conversely after full moon, the dark portion (po) gradually increases while the brightness (ming) fades.

Plain-Language Interpretation

1. The moon’s first light and its dark aspect must not be confused

When the moon first “gives birth to light” (begins to shine), we cannot simply say this must happen on the fourteenth, fifteenth, or sixteenth day. Nor should we insist that the sixteenth day is the day the moon “gives birth to its dark aspect.”

Zhu Xi (the Master of Kaoting) said that the lengths of lunar months are calculated from an average of about twenty-nine and a half days, using the ratio of twenty-nine to nine hundred and forty. The date depends on which day the conjunction of the sun and moon—the new moon—falls.

The moon “gives birth to light” before the first quarter. By the fifteenth day, at the full moon, its light is complete. The moon “gives birth to its dark aspect” after the last quarter. By the fifteenth day, at the dark moon at the end of the month, its light has disappeared.

The ancients said that the sun is the soul and the moon is the dark aspect. This aspect is the moon’s dark side; when it is exhausted, light is born again. Another explanation says that the soul rests upon the dark aspect, like a person riding in a cart: the soul attaches to the dark aspect, which carries it.

The moon receives sunlight and is always a complete sphere; it is only when people look up from below that they see its phases waxing and waning. Master Zhen of Xishan said that the moon comes into being through the union of Yin and Yang. The moon itself has no light; its waxing and waning depend on how much sunlight it receives.

Around the new moon, when the sun and moon meet, the moon begins to show light after about three days. On the eighth day comes the first quarter, when roughly half its face is lit. On the fifteenth day comes the full moon, when its light is complete. This is called “full after three times five.” After the full moon it gradually wanes. On the twenty-second day comes the last quarter, when roughly half its face is dark. On the thirtieth day comes the dark moon, when its light has disappeared. This is called “waning after three times five.” At the dark moon, Yin is pure: the moon’s dark aspect remains while its light vanishes. When the sun and moon meet again at the new moon, light is born once more.

2. Shao Yong on the moon’s body and its shadow

Shao Yong said that the moon’s body is originally black, belonging to Yin, and appears white when it receives sunlight. The moon is the sun’s shadow. He also said that the moon is Yin within Yang; because it belongs to the Yang category, it can be seen even during the day.

The moon’s body is black and belongs to Yin; when sunlight makes it appear white, it has received Qi from Yang. Zhu Xi said that the heavens surround the earth, which is small. When the sun is below the earth, the moon’s light is great: light shines back from all around the earth, while the earth blocks the sunlight. The shadow within the moon is the shadow of the mountains, rivers, and land.

Wang Pu said that the sun and moon in the sky are like two mirrors facing each other, with the earth between them and empty space all around, like water. The faintly dark areas on the moon are therefore the reflection of the earth in a mirror, not actual cassia trees or a toad and rabbit. This explanation is reasonable and can dispel a misconception that has persisted for ages. The dark areas on the moon are caused simply by a solid mass of land blocking the sunlight.

This passage from Tianyuan Fawei primarily explains the principles behind lunar phase changes. Song dynasty scholars Zhen Dexiu (Xishan Zhenshi) and Zhu Xi (Master Zhu) each proposed their own interpretations:

The moon itself is a spherical body, always presenting only one face toward the sun to receive light. When the sun is in the west (at the You position) and the moon directly opposite in the east (at the Mao position), the moon receives illumination most fully. The space between heaven and earth is filled with Qi (vital energy), through which the sun and moon move. Sometimes when the moon is at the celestial center while the sun lies below the horizon, moonlight reflects from all sides, with the central dark area actually being Earth's shadow.

Lunar phases change due to shifting relative positions of the sun and moon: after the full moon (Wangyue), the two gradually separate, causing the illuminated portion to appear misaligned. By the new moon (Shuori, the first day of the lunar month) when sun and moon completely overlap, a solar eclipse occurs—the moon may pass above or below the sun, not completely unilluminated but with only a tiny lit surface visible.

Zhu Xi particularly emphasized: The moon itself doesn't wax or wane—what we observe results from viewing angles. During the moonless day (Huirì, month's end), the moon overlaps with the sun, only beginning to separate slowly by the third day. Because the sun moves slowly while the moon moves quickly, observers on Earth see the crescent moon from the side. Each night as the moon shifts eastward in position, it appears to retreat.

When sun and moon conjoin, the sun sits above the moon, illuminating only its far side invisible from Earth. As they gradually separate, moonlight becomes visible; at full moon the entire face reflects light; afterward it gradually dims again. From last quarter to the moonless day, the moon catches up to the sun, making moonlight completely disappear (called "Po"). If the moon blocks the sun, a solar eclipse occurs; if it passes behind, no eclipse happens. Regarding Qu Yuan's question in Tianwen ("Why can the moon die and revive?"), traditional calendrics explained: Early in the month as the moon moves away from the sun, its dark portion expands (Po dies while brightness grows); after full moon as it approaches the sun, the dark portion shrinks (Po grows while brightness dies). But Zhu Xi pointed out this explanation was flawed—if true, moonlight should appear on the moon's eastern side before full moon and western side afterward, contradicting actual observations.

The moon does not emit light itself; it resembles a silver pellet that only shines when illuminated by the sun. When the moon first appears, the sun is beside it, so we only see a crescent-shaped bright portion. As the sun gradually moves away, its oblique rays make the moon appear increasingly full. This is like applying white powder to half a pellet: viewed from the side, only a hook-like powder trace is visible, while facing it directly reveals the complete circle.

Modern scholar Wang Pu further explains: At the beginning of the lunar month, only a crescent is visible, and only when standing between the sun and moon during the full moon on the fifteenth day can one see its complete form. In truth, the moon remains perfectly spherical at all times—its apparent waxing and waning results solely from our changing viewing angles. Just as the moon's essence remains unchanged between the moonless "dark day" (month's end) and the full "bright day" (fifteenth), though invisible to human eyes. As the quintessential yin essence, the moon's form is perfectly round and its substance transparently pure, appearing bright where sunlight touches and dark where it does not.

Regarding lunar eclipses: Even at its brightest, the moon contains extremely subtle shadows. When the moon aligns perfectly opposite the sun, these shadows may obscure it completely, causing an eclipse. While this phenomenon reflects yin energy at its zenith overwhelming yang, if yin power shows even slight retreat, perfect opposition cannot occur and no total eclipse forms.

Han dynasty scholar Zhang Heng proposed: Fire emits light outward, while water reflects illumination. The moon's radiance comes entirely from solar reflection, with darkness marking where sunlight cannot reach. The moon appears brightest when directly opposite the sun, yet becomes invisible when too close to it. Starlight requires refraction through aqueous qi—when stars oppose the sun, they disappear because Earth's shadow ("dark void") obscures them. This explains why the full moon shines like fire yet becomes invisible by day: brightness appears most vivid when viewed from darkness, while strong light overwhelms dimmer sources when observed from brilliance.

A Plain-Language Interpretation of Tianyuan Fawei

On “Dark Emptiness” and Lunar Eclipses

“Dark Emptiness” here means that Yin embraces Yang to form the Moon, which is symbolized by Kan. The single Yang line within Kan originally comes from the single Yin line within Li. When the Qi of Qian enters Kun, it forms the Moon; when the Qi of Kun enters Qian, it forms the Sun. On the day of the full moon, the Sun and Moon face one another and their true essences meet. Li, the Sun, has the form of Qian and seeks to draw in the single Yang line within the Moon. The Moon is therefore obscured, and a lunar eclipse occurs. “Dark Emptiness” refers to the part of the single Yang line within Li that has been taken away. This happens before sleep; yet no one, from ancient times to the present, has explained it this way.

The Moon’s Distance from the Sun and the Crescent and Full Moon

It also says: As the Moon moves closer to the Sun, a sliver of light first appears; as it gradually moves farther away, the illuminated portion grows. For example, when the Sun is at the position of Wu and the Moon at the position of You, the relation is “one near, three far,” forming a quarter moon. At the full moon, the Sun is in the west and the Moon in the east. Seen from below, the Moon appears fully illuminated.

The Moon’s Course in “A Brief Account of the Armillary Sphere”

A Brief Account of the Armillary Sphere records: The Moon moves quickly and the Sun slowly, so they take a little more than twenty-nine days to meet. Thus, in one year there are twelve meetings. Their meeting brings about the dark Moon; from darkness it is renewed, and brightness begins again—this is called the new Moon. The Moon moves quickly and gradually farther from the Sun. Measured around the celestial sphere, it is a little over 91 degrees from the Sun when near, and a little over 374 degrees when far away. This is what is meant by “one near, three far,” forming a quarter moon: the first quarter. When it reaches its farthest point and faces the Sun, it is a little over 182 degrees and 62 minutes from the Sun. This is called being opposite the Sun, dividing the sky in half and forming the full Moon—the Sun and Moon face one another. After passing this point, when it is a little over 274 degrees from the Sun, the relation is again “one near, three far,” forming a quarter moon: the last quarter. The first quarter usually falls on the eighth day, the last quarter on the twenty-second, and the full Moon on the fifteenth. These are the usual dates. The first quarter may fall early, on the seventh, or late, on the ninth; the last quarter may fall early, on the twenty-first, or late, on the twenty-third; and the full Moon may fall early, on the fourteenth, or late, on the sixteenth. These are the variations. At the first quarter, the Moon is half full, like the upper string of a bow; at the last quarter, it is half waned, like the lower string of a bow.

The Pre-Heaven Diagram and the Moon’s Monthly Pattern

It also says: The Pre-Heaven Diagram contains the pattern of one lunar month. From Fu through Zhen, the Moon first becomes bright on the third day. When it reaches Dui, it reaches its first quarter on the eighth day. When it reaches Qian, it is full on the fifteenth day.

When the moon reaches the Xun position (southeast) on the fifteenth day of the lunar month (full moon day), it begins to wane. By the eighteenth day at the Gen position (northeast), it enters the last quarter phase. On the twenty-third day at the Kun position (southwest), its light gradually fades, and by the thirtieth day, it becomes completely invisible (new moon day).

A single lunar cycle encompasses four distinct phases: the new moon (end of month), the waxing crescent (beginning of month), the first and last quarters (half-moon phases), and the full moon. The waxing moon represents the growth of Yang energy, descending from above, while the waning moon signifies the retreat of Yin energy, ascending from below. At full moon, both Yin and Yang energies are at their peak—the sun sinks below the earth while the moon shines brightly in the sky. During the transition between new and waxing crescent moons, the sun and moon align in the celestial vault, mirroring the alchemical principle of water extinguishing fire and metal returning to its original nature.

The celestial stems (Jia, Yi, Bing, Ding, Geng, Xin, etc.) describe the sun’s positions at dusk and dawn, not the divisions of the Bagua (Eight Trigrams). The moon’s phases—brightness, darkness, quarters, full moon, and new moon—are named according to its orientation toward or away from the sun.

From the new moon (first sliver of light) to the first quarter, the sun sets in the You position (west), and the moon begins to generate its "po" (Yin essence). Here, Yang emerges within Yin as the moon bows to greet the sun, its light gradually increasing from below until the balance of Yin and Yang forms the first quarter. After the full moon and through the last quarter, the sun rises in the Mao position (east), and the moon’s "hun" (Yang essence) starts to fade. Now, Yin arises within Yang as the moon tilts back toward the sun. The pre-full moon brightness transforms into Yin essence, with light receding from below until it vanishes, then rebalancing to form the last quarter.

The Zhen trigram (east) is numbered one, Dui (west) two, Qian (northwest) three, Xun (southeast) four, Gen (northeast) five, and Kun (southwest) six, with each shift occurring every five days. The dawn of the new moon (first day) marks the initial interaction of solar and lunar energies. From the twenty-sixth to the second day, the Kun phase dominates: when the sun moves to Zhen, the moon reveals one part of light; at Dui, two parts; and at Qian, full brightness. After the sixteenth day, the Xun phase begins as moonlight wanes, culminating in the sun-moon overlap at the new moon, where no light remains.

When the sun and moon conjoin during the new moon phase, the moon is eclipsed by the sun, revealing only its dark side—thus Yin essence fully manifests as the new moon, corresponding to the Kun trigram. Here, the moon aligns with the Yi position (east-southeast) where the sun also resides. Kun pairs with the Yi moon; by the new moon, the moon turns from east to north to meet the sun (east as Yi, north as Gui). Yi’s Yin energy exhausts itself in the Gui position, finally converging at the Qian trigram’s Ren position (north, where Yin gathers—Ren and Gui belong to the northern water phase).

When we return to the time of twelve conjunctions in a year, when all things flourish, the moon appears once again in the directions symbolized by the Zhen and Dui trigrams.

Only after Heaven and Earth join does Zhen arise. Zhen represents the first joining of Heaven and Earth, from which all things begin to sprout. The Kun trigram comes to an end at the positions of Yi and Gui, and the Qian trigram then takes up the image of Zhen at the Geng position. The waxing half-moon, full moon, dark moon, and new moon link end to end in an unceasing cycle.

First phase, the birth of light (days one through three): Within pure Yin, one Yang begins to join, giving rise to a slender line of light; the moon corresponds to the Zhen image. At this time, the moon appears at dusk in the west, at the Geng position. The sun has already sunk below the earth, hence the saying that Zhen “receives Geng.”

Second phase (days four through eight): Two Yang lines grow, and light fills half the moon. This is the waxing quarter of the moon’s dark body, and the moon takes the Dui image. At dusk it appears in the south, in the region of Xin.

Third phase (days nine through fifteen): On the fifteenth day, the moon reaches fullness, the height of pure Yang. It takes the Qian image and appears at dusk in the east, in the region of Jia. Its illuminated face is roughly level with the sun and the earth; this is the full moon.

Fourth phase (beginning on the sixteenth day): Qian begins to receive the next Yin line, becoming Xun and giving rise to the dark body. The moon takes the Xun image and sets at dawn in the west, in the region of Xin.

Fifth phase (around the twenty-third day): At the waning quarter, another Yin line arises in the middle of Gen. The moon takes the Gen image and sets at dawn in the south, in the region of Bing.

The sixth phase ends on the thirtieth day: The three Yang lines have all transformed; the light is gone, and the moon’s body lies hidden in the northwest. The month has six phases. When all six have run their course, the moon yields its place to the following month and is born anew from the Zhen trigram.

Chapter Fourteen of the Cantong qi says: “Gen stands at Bing; the waning quarter falls on the twenty-third. Kun, Yi, on the thirtieth day: its light is lost in the northeast. When the phases are complete, they yield their place in turn; the succeeding body takes over, and the Dragon is born anew.” Accordingly, in the Na Jia system, Qian is the father and receives Jia Zi and Ren Wu; Zhen is the eldest son, succeeding to the father’s body, and receives Geng Zi and Geng Wu—the Zi and Wu matching those of Qian. Kun is the mother and receives Yi Wei and Gui Chou; Xun is the eldest daughter, succeeding to the mother’s body, and receives Xin Chou and Xin Wei—the Chou and Wei matching those of Kun. Kan is the second son, with Wu Yin and Wu Shen; Gen is the youngest son, with Bing Xu and Bing Chen; Li is the second daughter, with Ji Mao and Ji You; Dui is the youngest daughter, with Ding Si and Ding Hai. Ren, Gui, Geng, Xin, Wu, Ji, Bing, Ding, Jia, and Yi are assigned from bottom to top. Qian and Kun each use two lines to enclose the six children’s trigrams within them, and thus transformation and generation can take place. Wu and Ji occupy the central Earth phase. Kan, Yang, receives Wu; Li, Yin, receives Ji. Each Yin and Yang receives its corresponding kind and occupies the center. Ren is Yang essence.

Jia is the first of Yang, Gui the end of Yin (the image of blood), and Yi the beginning of Yin; the images of the two “elders” are contained within them. Kan and Li occupy the center and govern its workings: they are precisely the pivot of Heaven and Earth’s creative transformation. The 384 lines correspond to the number of one complete circuit of Heaven: one line equals one day, while the four hexagrams’ twenty-four lines correspond to the twenty-four solar terms. A month has the first quarter, last quarter, full moon, dark moon, and new moon; a day has Zi, Wu, Mao, and You. If we take a day as our measure, the principles of a month and a year are all contained within it: from Zi to Si comes the “growth of light,” from the first quarter to the full moon; from Wu to Hai comes the “growth of shadow,” from the last quarter to the dark moon. The twelve two-hour periods correspond to the twelve sovereign hexagrams, and the seventy-two lines to the seventy-two pentads. Viewed over a month, the two quarters correspond to the Yin-Yang “two eights”; the 360 two-hour periods correspond to 360 lines, and also to 360 days. Viewed over a year, the six lines of Na Jia are assigned to the twelve sovereign hexagrams, with each used twice. When the year’s cycle is complete, the transformations of a whole year are condensed into a month. Observe the moon through the day, and the year through the month; find the hour within the day, and the quarter-hour within the hour. Within the work of a single quarter-hour, the year’s climate and transformations can be carefully inferred—and so on.

Shao Yong (Master Shao) said: “Yin embraces Yang to form the moon; the moon is Yang within Yin. As Yang wanes, Yin is born, so the sun sets in the west and the moon rises in the east. When Yin is at its height, it contends with Yang, so at the full moon the moon appears in the east. Heaven is the father and the sun the son, so Heaven revolves to the left. The sun is the husband and the moon the wife, so the sun rises in the east and the moon is born in the west. When the moon is far from the sun, its light grows and it moves slowly; when it nears the sun, its shadow grows and it moves quickly. This is the principle of ruler and minister.

“Though the sun moves to the right, it in fact turns to the left along with Heaven. Though the moon moves swiftly, whenever it meets the sun it is always behind it. The stars follow the moon and appear at night. According to the various calendrical accounts, the moon moves fastest from the first to the fourth day of the month, traveling a little more than fourteen degrees in a night. It is equally fast from the twenty-fourth day to the dark moon, covering the same distance. From the fifth to the eighth day it moves somewhat less quickly, traveling a little more than thirteen degrees in a night; from the twentieth to the twenty-third it again moves a little faster, covering the same distance. From the ninth to the nineteenth it moves slowly, traveling a little more than twelve degrees in a night. Averaged over a month, the moon travels thirteen and seven nineteenth parts of a degree per day.”

When the Moon is far from the Sun, its bright side gradually grows and it moves more slowly; when it is near the Sun, its dark side gradually grows and it moves more quickly. In this lies the meaning of the relations between ruler and subject, superior and subordinate. The Moon has no light of its own; it shines by borrowing the Sun’s light. When it reaches fullness and its greatest brightness, it contends with Yang Qi. Should a ruler not be careful?

The heavens revolve to the left, while the Sun travels to the right. The Sun rises in the east and the Moon appears in the west, symbolizing the proper order of father and son, husband and wife, and Yin and Yang. If the Sun and Moon both rose in the east and the Moon opposed the Yang, that would not be the ordinary course of the Dao. A mistaken account says that the heavens are greatest, the Sun comes next, and the Moon after that; therefore, the Sun cannot keep pace with the heavens, and the Moon cannot keep pace with the Sun. Each has its place, one honored and one humble, and this is precisely what Heaven makes clear. When King Wen elaborated the Changes, he used the word “repeated” for Kan and its image for the Moon: one Yang line lies at the center, encircled by two Yin lines. Over the course of a year, the Moon completes twelve conjunctions, always following the Sun and never daring to precede Yang. The meaning is profound. Yin assists Yang in bringing the work of the year to completion, so the Moon’s aid is indispensable; yet when Yin overreaches and encroaches upon Yang, a noble person must be ready to guard against it.

Thus, when Qian speaks of the Sun and Moon, it says they “shine together”; when sowing is discussed, it says the Sun and Moon must “not overstep”; and when constancy is discussed, it says the Sun and Moon “follow Heaven’s course.” The Sun and Moon are also spoken of together, but the Moon is never placed before the Sun. All of this conveys the principle of honoring Yang and restraining Yin. When the Moon alone is singled out as a warning, the message is especially urgent. In “Small Restraint,” the line says, “The Moon is nearly full; to set out brings misfortune.” The fourth line contains a hidden Kan, and Xun contains a hidden Zhen: the Moon is in the east, the Sun in the west. In “Small Restraint,” one Yin line overreaches the Yang of the top line, bringing misfortune. The fifth line of “Returning Maiden” also says, “The Moon is nearly full; good fortune.” Here the Moon in Kan is in the east and the Sun in Li is in the west. Yin and Yang occupy their proper places; though the Sun and Moon face one another and are almost full, neither overreaches the other, so the outcome is auspicious. The Moon may rule the night, but not the day. Such is the course of Heaven—how much more so in human affairs?

When the Sun faces the Moon, the Moon is eclipsed; when the Moon obscures the Sun, the Sun is eclipsed. Yin must not overreach Yang, a subject must not overreach a ruler, a wife must not overreach her husband, and a petty person must not overreach a noble one. Chengzi said: “A subject occupying the honored position—that is what Wang Mang and Dong Zhuo did; this can still be spoken of. A woman occupying the honored position—that is what the Lady Nüwa and Empress Wu did; these are extraordinary upheavals, which cannot be spoken of.”

The Treatise on Pitch-Pipes and the Calendar in the Book of Han says that the Moon turns from right to left, in accord with the Eight Trigrams of the Earlier Heaven diagram.

This passage explains the correspondence between the moon's cyclical movements and the transformations of Yin-Yang. Ancient observers used trigrams and timekeeping to meticulously track lunar phases.

The moon undergoes periodic changes: The last day of the lunar month is called "Hui" (obscurity), when the moon becomes completely invisible; the first day is called "Shuo" (new moon), marking the moon's reappearance. On the 7th-8th days, the half-moon phase is called "Shangxian" (first quarter); the full moon on the 15th day is called "Wang" (fullness); the 22nd-23rd days show another half-moon called "Xiaxian" (last quarter); by the 30th day it returns to "Hui."

Detailed observations reveal: During Hui, the moon corresponds to Kun trigram (pure Yin), with sun and moon conjoining at Zi hour (midnight), leaving no moonlight. As the moon emerges rightward, it aligns with Gen trigram (one Yang arising), appearing faintly between Xu and Hai hours (7-11 PM). At first quarter, the moon stands due west, matching Kan trigram (one Yang at center), with half its surface illuminated.

During full moon (Wang), the sun reaches due south (Wu position) while the moon stands due north (Zi position) - their opposition creates full illumination. Then the moon turns leftward; by Chen-Si hours (7-11 AM), shadows begin appearing. During last quarter, the moon in the east corresponds to Dui trigram (one Yin outside), showing darkening on its other half.

Finally, the moon progresses to Zhen trigram (two Yin growing stronger), its light diminishing until returning to Kun's complete darkness, completing the cycle. Ancient scholars arranged trigrams north-to-west to mirror the moon's retrograde motion, while line transformations from outer to inner positions represented sunlight progressively illuminating the moon. This observation method aligns with Han Dynasty calendrical theories and the innate Bagua system.

Notably, solar observations used Wu hour (noon), lunar observations Zi hour (midnight), and stellar observations dusk or dawn - methods documented in texts like the Cantongqi (Kinship of the Three).

Tianyuan Fawei, Scroll Seven (A Plain-Language Interpretation)

This section discusses methods such as “withdrawing and crossing over” and “receiving light from outside,” so it places the Kan and Li trigrams at the center of its argument.

It also calls Zhen “bringing forth brightness” and Dui “the first quarter of the moon in the west,” reversing the entire system of interpretation and opposing the established views of earlier Confucian scholars. The author even believes this has brought to light the principle of the “moon” in Laozi’s teaching, calling it a triumph for the ages.

In my view, this is simply one interpretation that can be drawn from the Yi. Moreover, Zhu Xi examined the Cantong qi with great care. The teachings passed down from Wei Boyang (Wei Ao) have been transmitted for more than a thousand years, since the Han dynasty, and none of the worthy scholars of earlier generations lightly rejected them. How, then, can anyone criticize them so casually?