Rotting Grass Transforms into Fireflies: The Philosophy of Metamorphosis and the Earth Virtue of Late Summer in the Great Heat
This article offers a deep reading of Great Heat (Dashu), the twelfth of the twenty-four solar terms, through the lenses of pre-Qin Confucian and Daoist philosophy, classical etymology, and astronomical phenology. It reveals the cosmic significance of the sun reaching ecliptic longitude 120°, when summer heat attains its zenith, and explores the Earth virtue of late summer, the metamorphosis of rotting grass into fireflies, and the hexagram Dun with its two rising yin lines — illuminating the ancient wisdom of reversal at the extreme and counterbalancing excess.
Chapter Nine: The Phenological World of Great Heat — A Detailed Exegesis of the Three Pentads
I. Phenology: The Language of Heaven and Earth
The Lost Zhou Documents and the Monthly Ordinances both record the phenological signs of Great Heat. In the ancients' understanding, phenological signs were not isolated natural phenomena but the outward signs of heaven-earth energy in motion — the language through which heaven and earth "speak."
The Master said: "Does Heaven speak$6 The four seasons proceed, the hundred things are born — does Heaven speak$7" (Lunyu, "Yang Huo"). Heaven does not use words; it "speaks" through the progression of the four seasons and the changes of the ten thousand things.
The three pentads of Great Heat are: first, rotting grass transforms into fireflies; second, the earth is moist and the heat sultry; third, great rains descend in their season.
II. First Pentad: Rotting Grass Transforms into Fireflies — The Omen of Faint Light
As a phenological indicator, the appearance of fireflies signals that the humid, hot, decomposing environment of Great Heat has fully formed. The firefly's emergence requires moisture, warmth, and decomposing organic matter — precisely the conditions produced by Great Heat's "moist earth and sultry heat."
Philosophically, as we discussed in Chapter Five, "rotting grass transforms into fireflies" reveals the principle of "the rotten becoming miraculous" and Fire virtue's passage from fierce to gentle.
III. Second Pentad: Moist Earth and Sultry Heat — Steaming Humidity and Earth's Transformative Nurture
The second pentad, "the earth is moist and the heat sultry" (tu run ru shu), is the most essentially characteristic of the three and the richest in philosophical implication.
Ru (溽) means moist, humid-hot. "The earth is moist and the heat sultry" describes the ground's dampness and the oppressive, steaming quality of the heat — precisely Great Heat's characteristic climate of combined humidity and heat.
Why is Great Heat's heat specifically ru (humid) rather than dry$8 This returns us to "late summer Central Earth." In the Five-Phase system, Earth governs moisture (shi). The Yellow Emperor's Inner Canon says: "The center engenders moisture; moisture engenders earth." Since late summer is the time when Central Earth presides and Earth energy governs moisture, the heat of Great Heat must necessarily be "sultry heat" — humid heat.
Most profoundly, the two characters "moist earth" (tu run) directly indicate that Great Heat is the time when Earth virtue is most active and transformative power strongest. The moist, warm earth provides the optimal conditions for plant roots to absorb nutrients, for organic matter to decompose and convert — for what the Monthly Ordinances describes as "one may manure the fields, one may enrich the soil."
IV. Third Pentad: Great Rains Descend in Their Season — Fire's Extreme Generating Water, and Counterbalancing Excess
The third pentad, "great rains descend in their season," pushes Great Heat's paradox of the Way to its peak — at the very hottest moment of the year (Fire virtue at its zenith), great rains descend and Water manifests repeatedly.
Three layers of meaning unfold:
First — Fire drives the water cycle. Intense solar radiation (Fire) evaporates vast quantities of moisture, which rise to become clouds, which condense into rain. Fire's very intensity powers the water cycle; the stronger the fire, the more violent the resulting storms. Fire is not water's enemy — fire is the engine of the water cycle.
Second — Counterbalancing excess. Fire's extreme excess triggers the self-regulating mechanism of the Way: Water rises to restrain overweening Fire, pulling it back toward balance.
Third — Water and Fire in equilibrium, and all things come to fruition. The combination of Great Heat's "fire" (sweltering heat) and "water" (downpours) constitutes the auspicious image of Jiji (Already Across, ䷾) in the Book of Changes — Water above Fire below, yin and yang in harmonious union, all things achieving completion. Heat promotes growth; rain nourishes roots. What appears contradictory is in fact complementary.
Taken together, the three pentads present a complete picture of the Way of Heaven at Great Heat: rotting grass transforms into fireflies (Fire virtue turning toward decline, light born from decay) → moist earth and sultry heat (Earth virtue's transformative nurture, humid heat bringing things to fruition) → great rains descend (Fire at its extreme generating Water, counterbalancing excess, Water and Fire in equilibrium). From "Fire's turning," to "Earth's nurture," to "Water's restraint" — these three pentads are a vivid demonstration of Five-Phase regulation and yin-yang flow.