Yin and Yang in Equal Measure: The Way of Central Harmony and the Equilibrium of Day and Night at the Spring Equinox
This article offers a multi-dimensional exploration of the Spring Equinox (Chunfen) among the Twenty-Four Solar Terms, drawing on pre-Qin Confucian and Daoist philosophy, the original meanings of Chinese characters, and astronomical phenomena. By analyzing the character 'fen' (divide/equalize), the sun's alignment with the equator, and seasonal signs such as the return of swallows and the onset of thunder, it reveals the profound wisdom of 'central harmony' and the unity of Heaven and humanity embedded in the equinox's equal days and nights.
Chapter One: The Original Meaning of Fen -- A Ruler That Divides the World in Two
I. Why Is Fen Called Fen$8
Before entering into specific discussion of the Spring Equinox, we must first contemplate the character fen itself. Why is this character used to name this solar term$9 What is its original meaning$10 How does it differ fundamentally from li ("establish") in Lichun (Start of Spring) or zhe ("hibernation") in Jingzhe (Awakening of Insects)$11
Master Xu Shen states in the Shuowen Jiezi (Explaining Graphs and Analyzing Characters): "Fen means 'to distinguish.' It is composed of ba (eight) and dao (knife). The knife is used to distinguish and separate things." The original meaning of fen is "to distinguish" -- to differentiate, to categorize, to divide. Its form incorporates ba, which the Shuowen explains as "to separate, having the form of things dividing and turning away from each other," an inherent meaning of splitting apart; and dao, the tool used to cut things apart. Together, the image of the character fen leaps vividly to life: a knife dividing a whole into two.
But there is a crucial detail here worth exploring: when a knife divides something, are the two resulting halves equal or unequal$12
Etymologically, what fen emphasizes is precisely that sense of "dividing evenly into two." When we say "divide things up" (fen dongxi), the most natural expectation is that the division be fair and even. "Equal division" (pingfen), "even distribution" (junfen), "allocation" (fenpei) -- all these words built around fen implicitly require "equality." A knife cutting straight through the center, dividing the whole into two perfectly equal halves -- this is the purest image of the character fen.
What, then, does the fen in "Spring Equinox" (chunfen) divide$13 And is the division equal$14
The answer is definitive: what the Spring Equinox "divides" is day and night, yin and yang, cold and heat. And this "division" is precisely an absolutely equal "bisection." On this day, the sun shines directly on the equator, and day and night are nearly equal in length across the globe; yin and yang are at this moment exactly half and half, neither more nor less. This is precisely what Master Dong Zhongshu described: "Yin and yang are in equal halves; therefore day and night are equal and cold and heat are balanced." Fen here is not a haphazard cutting, but the cosmic knife descending with precision from the exact midpoint of the year, bisecting day and night, yin and yang, cold and heat -- each receiving its proper half.
II. Why Is "Equal Division" Beautiful$15
Why did the ancients give a special name to this state of "equal division" and specifically commemorate it$16 This touches upon a deep philosophical question: why are equality and equal division worthy of being treasured$17
In many cultures, the "middle" is often regarded as mediocre -- neither the best nor the worst, merely an unremarkable middle state. But in Chinese thought, zhong ("center"), ping ("balance"), and jun ("evenness") are elevated to the highest status. Why this difference$18
To understand this, we must return to the ancients' profound wariness of "excess" and "deficiency." In their view, whenever anything deviates from its proper measure -- whether toward too much or too little -- it tends toward its opposite. When yang energy is too abundant, things become parched, overheated, scorched; when yin energy is too abundant, things grow cold, stagnant, lifeless. Only when yin and yang are in equal halves and each finds its balance can all things grow under the most suitable conditions. The Spring Equinox is precisely such a "just right" moment -- yang energy has returned sufficiently to dispel winter's bitter cold, yet has not grown so fierce as to scorch all things. Cold and heat are balanced, day and night are equal; this is one of the most "centered, correct, balanced, and harmonious" moments in the year.
The "Great Plan" (Hongfan) chapter of the Book of Documents records the great principles of governance that Jizi presented to King Wu of Zhou, among them the "Royal Extreme" (huangji): "Without bias, without inclination, follow the righteousness of the king; without favoritism, follow the way of the king; without malice, follow the path of the king. Without bias, without partisanship, the kingly way is broad and grand; without partisanship, without bias, the kingly way is level and smooth; without turning aside, without leaning, the kingly way is straight and true." The repeated appearance of "without bias," "without inclination," "level and smooth," and "straight and true" is a paean to "balance" and "centrality" as the highest political ideal. The Spring Equinox's "equal days and nights, balanced cold and heat" is precisely the perfect embodiment of this "without bias, without inclination" Way of Heaven in the natural order of time.
Why did the ancients connect the highest ideal of governance (the Royal Extreme, the Kingly Way) with the equilibrium of the Way of Heaven (the equinox's equal days and nights)$19 Because in their cosmology, the Way of Heaven and the Way of Humanity are interconnected and isomorphic. Since the Way of Heaven regards "balance" as beautiful, the Way of Humanity should likewise regard "balance" as beautiful. A ruler governing the realm should be like Heaven and Earth on the day of the Spring Equinox -- impartial and unbiased, fair and balanced, enabling all things to find their proper place. The Spring Equinox is therefore not merely an astronomical phenomenon but a political and moral metaphor -- it is a mirror that Heaven holds up for the human world, reflecting "what justice looks like."
III. Fen and Li: The Deep Difference Between Two Types of Solar Term Names
The naming of the Twenty-Four Solar Terms was by no means arbitrary. Close examination reveals that they fall roughly into several categories: some are named with li ("establish": Start of Spring, Start of Summer, Start of Autumn, Start of Winter); some with fen ("equinox") or zhi ("solstice": Spring Equinox, Autumn Equinox, Summer Solstice, Winter Solstice); some after phenological signs (Awakening of Insects, Clear and Bright, Grain in Ear, White Dew, Frost's Descent, etc.); and some after climatic conditions (Minor Heat, Major Heat, Minor Cold, Major Cold, etc.). These differences in naming reflect the ancients' different understandings of the nature of different solar terms.
Li means "to begin, to establish." Start of Spring and Start of Summer mark the "beginning" of a season -- they are starting points, thresholds, emphasizing "transition" and "inception." Fen and zhi, however, are entirely different. Zhi means "extreme, having reached the limit" -- at the Summer Solstice the sun's shadow is shortest and daylight longest; at the Winter Solstice the shadow is longest and daylight shortest. These are moments when yang or yin energy reaches its apex. Fen means "equal division" -- at the Spring and Autumn Equinoxes day and night are equal in length, moments when yin and yang are exactly half and half.
This reveals a profound structure: in the temporal order of the year, the "four establishments" (si li) mark the beginnings of the four seasons, the "two solstices" (er zhi) mark the two poles of yin and yang, and the "two equinoxes" (er fen) mark the equilibrium of yin and yang. Li, zhi, and fen -- these three types of nodes constitute three dimensions for understanding the temporal structure of the year: inception, apex, and equilibrium.
The Spring Equinox's fen (equilibrium) occupies an exceedingly subtle position within this structure. It is neither the inception (that is li) nor the apex (that is zhi), but the "midpoint" between the two poles -- from the Winter Solstice's yin apex, through the Spring Equinox's equal halves of yin and yang, toward the Summer Solstice's yang apex. The Spring Equinox falls precisely at that "exactly equal" instant along the path of waning yin and rising yang. This instant is fleeting -- after the equinox, yang surpasses yin, and day grows longer than night. Precisely because it is so fleeting, so evanescent, the ancients solemnly marked it, reminding people: take heed -- at this moment Heaven and Earth stand in perfect equilibrium; please also adjust your body and mind to accord with this balance.
This is the deep meaning carried by the name fen. Unlike li, which announces the arrival of a new season, or zhi, which marks the peak of an extreme, fen quietly points to a "point of balance" -- a state of equilibrium worthy of dwelling in, savoring, and emulating.
IV. From Fen to Zhong: A Philosophical Tradition Drawn from a Single Character
The "equality" and "equal halves" to which fen points share a blood kinship with one of the most central concepts in Chinese philosophy -- zhong ("the center, the mean").
What is zhong$20 The Shuowen Jiezi says: "Zhong means 'inner.'" But the deeper meaning of zhong is "impartial and unbiased," "exactly right." An arrow hitting the bullseye is zhong (in the fourth tone); handling a matter with perfect propriety is zhong (in the first tone). Zhong is both spatial centrality and evaluative appropriateness.
The Spring Equinox's "equal days and nights, balanced cold and heat" is precisely the manifestation of the Way of Heaven's zhong. Day is not longer than night, night is not longer than day -- exactly in the center; cold does not prevail over heat, heat does not prevail over cold -- exactly in balance. This state, in the words of the Doctrine of the Mean (Zhongyong), is zhong: "Before the feelings of pleasure, anger, sorrow, and joy are aroused, this is called zhong (the center). When they are aroused and all attain their due measure, this is called he (harmony). Zhong is the great root of all under Heaven; he is the universal Way of all under Heaven. When zhong and he are brought to their utmost, Heaven and Earth take their proper positions and all things are nourished."
This passage may be called the master key to understanding the philosophical import of the Spring Equinox; we shall devote a full chapter to it later. Here let us simply note: the reason the ancients gave the special name fen to the day of "equal days and nights, balanced cold and heat" is precisely that they saw in this astronomical phenomenon the embodiment of zhong -- that highest philosophical ideal. The Spring Equinox is nothing other than Heaven and Earth themselves demonstrating what zhong means, what "when zhong and he are brought to their utmost, Heaven and Earth take their proper positions and all things are nourished" means. When day and night are equal and cold and heat are balanced, Heaven and Earth stand in their most "centered and correct" position ("Heaven and Earth take their proper positions"), and all things flourish vigorously in this centered, harmonious environment ("all things are nourished").
From this we can see that a seemingly simple character, fen, in fact draws out the most profound thread of Chinese philosophy -- the veneration of zhong, he, ping, and jun. This thread runs through both the Confucian and Daoist traditions, traces back to Yao and Shun, and extends down through a hundred generations, constituting the distinctive spiritual coloring of Chinese civilization. And the Spring Equinox is the anchor point of this spiritual thread in astronomical calendrics.