TL;DR: In the I Ching, the Li trigram (fire) is taken as the image of the sun and the Kan trigram (water) as the image of the moon; together they form the Sun and Moon pair, also called Kan Li. This pair is one of the foundational image pairs of Yixue, used to speak of light and dark, movement and stillness, fullness and emptiness. This is a reading within the image and number school, not a scientific claim about astronomy.
What are the Sun and Moon in the I Ching?
The Sun and Moon in the I Ching is the name for the image pair of the Li and Kan trigrams: Li figures the sun, Kan figures the moon. This pair is also called Kan Li, and in many divination books it is used as a pair of root categories parallel to Yin and Yang. This assignment belongs to the image and number school, based on the imagery of the two trigrams rather than on astronomical observation.
One point must be stated at once: this is a reading within a system of symbols. It does not say that the Kan trigram "is" the moon in a physical sense, but that within the frame of reference of the Yi, Kan carries enough qualities to play the role of the moon. The student needs to distinguish these two levels before going into detail.
Why is Li assigned to the sun and Kan to the moon?
The reason lies in the line imagery. The Li trigram ☲ consists of two yang lines outside and one yin line in the middle: bright outside, empty within, like a flame and the sun, which radiate light outward. The Kan trigram ☵ consists of two yin lines outside and one yang line in the middle: dark outside, solid within, like a stream and the moon, which hold the light inside.
The assignment has a textual basis in the Discussion of the Trigrams (Thuyết quái truyện): 「離為火、為日」 (Li is fire, is the sun), and the list of Kan's images includes 「為月」 (is the moon)[1]. The explanation by line structure has a named commentator: Zhu Xi (Chu Hy), in the Original Meaning of the Zhouyi (Chu Dịch bản nghĩa), describes Kan as 「陽陷隂中，外虛而中實」 (yang sunk within yin, yielding outside and solid within)[4], and Li as 「隂麗於陽，其象為火」 (yin clinging to yang, its image is fire)[5].
How do the Li and Kan trigrams differ?
The table below compares the two trigrams by criteria commonly used in Yixue.
| Criterion | Li trigram | Kan trigram |
|---|---|---|
| Line imagery | Yang, yin, yang (bright outside, empty within) | Yin, yang, yin (dark outside, solid within) |
| Main image | Fire, sun, light, manifestation | Water, moon, darkness, concealment |
| Five Phases | Fire | Water |
| Later Heaven direction | Due South | Due North |
| Qualities | Bright, hot, exposed, radiating | Still, cold, soft, deep |
| Relation | Kan controls Li by the Five Phases | Li is checked by Kan, but Li can also dry up Kan |
This polarization of qualities appears in divination materials: the Kan trigram is described as preferring stillness, cold, softness, suited to women and the night, to yin stems and branches and even numbers. This is a convention of the discipline, not a natural property of water.
What does the phrase "the way of the sun and moon" in the Great Treatise mean?
The second part of the Great Treatise (Hệ từ hạ truyện) reads: 「天地之道，貞觀者也；日月之道，貞明者也；天下之動，貞夫一者也」, roughly: the way of heaven and earth is to be correct and so be seen, the way of the sun and moon is to be correct and so give light, all movement under heaven is correct in being one[2]. Tradition attributes the Great Treatise to Confucius; modern scholarship does not regard that as proven and treats the text as a layer formed over several generations.
The same part also says 「日往則月來，月往則日來，日月相推而明生焉」: when the sun goes the moon comes, when the moon goes the sun comes, and sun and moon pushing on each other bring forth light[3].
The core of "the way of the sun and moon" is not what the sun and moon are, but that their movement has rhythm. The sun rises and sets, the moon waxes and wanes, and this repeated rhythm is the model for every cycle in the Yi: when things reach an extreme they change, when they change they flow, when they flow they endure.
Why is the Kan Li pair considered foundational?
Because these two trigrams combine into a complete yin yang pair at the level of imagery. Li is yang outside, Kan is yang inside. One radiates light outward, one keeps light within. Read this way, Sun and Moon are not two separate things but two phases of a single movement.
This understanding explains why in divination Kan and Li are often used as a corresponding pair to speak of male and female, day and night, manifest and hidden, movement and stillness. The book Thuật số Lạc Thư (Kỳ Môn Độn Giáp nguyên thủy), when discussing the nine "Su" of the Lac Thu chart, assigns to the Li trigram the usage type Cảnh, meaning bright, and to the Kan trigram Hưu, meaning restful. These two words capture the opposition exactly: one blazes up, one settles down.
How do books differ on these two trigrams?
There are points on which books do not agree, and this should be stated clearly.
- On direction: according to the Later Heaven Eight Trigrams, Li is Due South and Kan is Due North. This is an almost universal convention, little disputed.
- On the Five Phases and mutual control: the book Phong thủy cổ đại Trung Quốc: lý luận và thực tiễn (volume 2) presents a method of judging good and bad directions by changing lines and then examining the mutual generation and control of the Five Phases. By that method, when the Qian trigram changes its middle line into the Li trigram, Fire controls Metal, so the Li direction is considered unfavorable. This reading differs from the reading of Li as open and upright found in many other books. This article notes both, and does not choose for the reader.
How do Kan and Li enter divination disciplines?
In Liu Yao, Kan and Li are used at the palace level and the line level. Materials on the Eight Trigrams and palace assignment state that in the Kan palace, the first hexagram is Bát Thuần Khảm, the seventh is the Du Hồn hexagram and the eighth is the Quy Hồn hexagram; a hexagram belonging to a palace carries the Five Phases of that palace. Thus the Kan hexagram carries the Water phase, while the Five Phases of each line depend on the stem and branch placed in the line.
The placement of stems and branches also follows its own rules. Materials on stem placement state that the Li trigram takes the stem Kỷ and the Kan trigram takes the stem Mậu. On branch placement, the yang trigrams (Qian, Kan, Gen, Zhen) run through the branches in forward order and the yin trigrams (Xun, Li, Kun, Dui) in reverse order, each starting from its own branch: the six lines of the doubled Kan hexagram, from the bottom up, take Dần, Thìn, Ngọ, Thân, Tuất, Tý; those of the doubled Li hexagram take Mão, Sửu, Hợi, Dậu, Mùi, Tỵ. Thus Kan and Li stand on the two sides of forward and reverse, matching their opposed positions.
In feng shui, the Kan image also appears when assessing terrain. A document on loan đầu cites the case of Kan above and Dui below, the Tiết hexagram; the book treats Dui (the marsh) here as water too, reads the hexagram as governing lowness, and concludes that the grave site is rather low, a place where water easily gathers. This is an example showing that hexagram images are used as a tool to describe terrain, not only to divine affairs.
Are Li and Kan the sun and moon in an astronomical sense?
No. This is the most easily misunderstood point. The I Ching is not an astronomical system, and the Li trigram was not constructed from observation of the sun's orbit. Assigning Li to the sun and Kan to the moon is a symbolic operation: choosing the two largest images in nature to represent the two poles of light and dark.
Some hold that this assignment reflects a lost ancient astronomical knowledge, but no research in the available sources proves this.
What is the use of reading this Sun and Moon pair?
Gợi ý của Nhật Nguyệt: the practical value of the Kan Li pair lies in giving a framework for seeing two sides of the same matter. When Li appears, the question to ask is whether this matter needs to be brought into the light or is being burned beyond measure. When Kan appears, the question to ask is whether this matter needs to settle down or is sinking too deep.
This is the site's interpretation, not a teaching of the hexagram. The original text of the I Ching does not speak of work management skills or personal decision making. Borrowing hexagram images for self questioning is a later use, and the user should know what they are doing.
Conclusion
The Sun and Moon pair in the I Ching is the image pair Li and Kan: Li figures the sun, Kan figures the moon, forming an opposing pair that complements each other. The phrase "the way of the sun and moon" in the Great Treatise speaks of the lawful rhythm of the universe's movement, and tradition attributes that text to Confucius while modern scholarship sees it as a textual layer formed over many generations. This assignment belongs to the image and number school, valuable as a symbolic system for thought, not a scientific claim about the sun and moon.
