Chapter Two: The Astronomical Foundation of Great Heat — The Sun at Ecliptic Longitude 120°

I. What Does Ecliptic Longitude 120° Signify$19

To locate Great Heat precisely, we must enter the domain of astronomy. In modern astronomical parlance, Great Heat is the moment when the sun reaches an ecliptic longitude of 120°. What does this "120°" actually mean, and what observational practice of the ancients does it correspond to$20

"Ecliptic longitude" refers to the sun's angular coordinate on the ecliptic — the projection of the Earth's orbital plane onto the celestial sphere, or, as seen from the ground, the path the sun traces against the background of fixed stars over the course of a year. Beginning from the vernal equinox point at 0°, the sun traverses 15° for each solar term. Vernal Equinox 0°, Clear and Bright 15°, Grain Rain 30°, Start of Summer 45°, Grain Full 60°, Grain in Ear 75°, Summer Solstice 90°, Lesser Heat 105°, Great Heat 120° — by this reckoning, Great Heat marks the point where the sun, setting out from the vernal equinox, has traveled 120° along the ecliptic.

This number already reveals Great Heat's position within the year. The Summer Solstice stands at 90°, the moment when the sun reaches its northernmost point and daylight is longest. At 120°, Great Heat is a full 30° — two solar terms — past the Solstice. What does this imply$21 It means that at Great Heat, the sun's point of direct incidence has already begun retreating southward from the Tropic of Cancer, daylight has begun to shorten, and the noon shadow has begun to lengthen.

Here emerges a deeply significant paradox — the key to understanding Great Heat: why is the hottest time of the year (Great Heat) not the time when the sun is highest and daylight longest (Summer Solstice), but rather some month after$22

II. Why Does "Peak Heat" Lag Behind "Solar Maximum"$23

This question touches upon an extraordinarily refined recognition in the ancients' astronomical observations — the accumulation and release of heat is subject to a lag.

At the Summer Solstice the sun is at its highest and daylight at its longest, but the heat accumulated by the earth has not yet reached its peak. It is like boiling water: at the moment the flame is strongest, the water does not instantly boil; only after heat has accumulated over a period of time does the water temperature reach its maximum. By the same principle, during the month from the Summer Solstice to Great Heat, although daylight is gradually shortening and the sun gradually moving south, each day the ground still absorbs more heat than it radiates away. Thus heat continues to accumulate, and around Great Heat the total stored heat reaches its peak — this is the hottest time of the year.

Though the ancients lacked modern physical concepts such as "heat capacity" or "thermal lag," through long experience and observation they keenly grasped this pattern. They discovered that "solar maximum" (the sun reaching its northernmost point) and "peak heat" (the hottest weather) did not fall on the same day; there was an appreciable time gap between them. This discovery was remarkable — it showed the ancients had recognized that the extremes of celestial phenomena and climatic phenomena do not coincide, that between heaven and earth there exists a kind of "lag in response."

This recognition of "lag" holds far-reaching philosophical significance. It tells us that cause and effect do not always correspond instantaneously. The strongest "cause" (the Summer Solstice's maximum sunlight) does not necessarily produce the strongest "effect" (the greatest heat) at once; only after a period of accumulation does the effect manifest somewhat later. This understanding — that there is a time gap between cause and effect — permeates many layers of pre-Qin thought. For instance, the Wenyan commentary on the Kun (Earth) hexagram of the Book of Changes states: "When a minister assassinates his lord or a son his father, the cause does not lie in a single day; its origins have been gradual." The eruption of great evil is not a matter of a single day but the result of long, gradual accumulation. This wisdom of "the gradual" is structurally identical to the way Great Heat's warmth accumulates gradually from the Solstice.

III. Gnomon and Shadow: The Sun's Shadow at Great Heat

By what method did the ancients determine solar terms such as Great Heat$24 The most fundamental method remained the observation of the sun's shadow.

The Rites of Zhou (Zhouli), in the section on the Grand Minister of Education, records: "By the method of the earth-measuring gnomon, one measures the depth of the land, corrects the sun's shadow, and thereby determines the center of the earth." The gnomon (guibiao) is one of China's most ancient astronomical instruments. A vertical pole (the "gnomon-post") combined with a horizontal measuring stick (the "gnomon-rule"): by measuring the length of the post's shadow at noon, the ancients could precisely determine the sun's altitude in the sky and thereby fix the solar terms.

On the Summer Solstice the noon shadow is shortest, because the sun stands at its highest point in the sky. On the Winter Solstice the noon shadow is longest, because the sun is at its lowest. At Great Heat, the shadow length has already grown slightly longer than at the Solstice — for the sun has begun to descend from its apex. By observing the minute changes in the noon shadow, the ancients could know: the sun has passed the Solstice, it is turning back, and Great Heat draws near.

Here lies a deeply suggestive contrast: Great Heat is the hottest time of the year, yet its sun-shadow is already lengthening — this is the astronomical manifestation of the divergence between "shadow" and "swelter." When the ancients saw the gnomon shadow lengthen slightly day by day yet felt the weather grow ever more scorching, they must have experienced acutely the subtlety and irony of the Way of Heaven: the "yang" above (as marked by the shadow) was already in retreat, while the "heat" below (as marked by sensation) still advanced. This contrast of "heaven retreating, earth advancing" is the most vivid portrait of Great Heat as the turning point from flourishing to decline.

IV. Star Patterns and Late Summer: The Wisdom of Observing the Heavens

Beyond gnomon-shadow observation, the ancients also determined the season by observing the positions of stellar lodges. The Monthly Ordinances of the Book of Rites (Liji) describes the month of late summer (the sixth month, in which Great Heat falls): "In the month of late summer, the sun is in the lodge Liu (Willow); at dusk the lodge Huo (Fire) culminates; at dawn the lodge Kui culminates."

These three phrases respectively indicate the stellar lodge occupied by the sun, the lodge culminating in the southern sky at dusk, and the lodge culminating at dawn. "The sun is in Liu" — the sun has moved into the Willow lodge, one of the seven lodges of the Southern Vermilion Bird. "At dusk Huo culminates" — at twilight, "Fire" (that is, the Heart lodge, home of the great star Antares) stands at the zenith of the southern sky. "At dawn Kui culminates" — at dawn, the Legs lodge stands at the southern meridian.

Particularly noteworthy is "at dusk Huo culminates." In earlier discussions of the Start of Summer and the Summer Solstice, we mentioned Antares (Xin Su Er, the Heart lodge's alpha star, Scorpii α) — this scarlet star holds an extraordinarily special position in pre-Qin astronomy. The "Canon of Yao" in the Classic of Documents states: "When the day is longest and the star is Fire, thereby one fixes midsummer." By the month of late summer (in which Great Heat falls), Antares at dusk has reached "culmination" — the highest point in the southern sky.

What exquisite resonance between heaven and humanity! Antares, the very symbol of "fire," scarlet and blazing; and the season in which it rises to the meridian at dusk is precisely the hottest period, when the Fire virtue reaches its zenith — the season of Great Heat. The reddest star in the sky has ascended to its highest point; the heat on earth has reached its peak — celestial phenomenon and climate, in this moment, achieve perfect synchrony. When the ancients gazed up at Antares enthroned in the midheaven, they knew: the moment of the year's most intense fire-energy had arrived.

But here another turning is foreshadowed. The Book of Odes (Shijing), in the "Seventh Month" poem of the Airs of Bin, opens: "In the seventh month the Fire star descends; in the ninth month garments are bestowed." The "descending Fire" (liu huo) refers precisely to Antares beginning to sink westward from the meridian at dusk — signaling that the summer heat will retreat and autumn's coolness approach. At Great Heat, Antares "culminates at dusk," poised at the very apex from which it is about to "descend." The fire star's passage from zenith toward decline mirrors Great Heat's passage from the zenith of swelter toward recession. Through observing the rise and fall of Antares, the ancients traced the entire arc of summer's Fire energy with perfect clarity — from the Solstice's "Fire visible in the south," to Great Heat's "Fire culminating at dusk," to the seventh month's "descending Fire" — a single star's trajectory is the rise and fall of a season's Fire virtue.