Introduction: Why Does the Moment of Supreme Culmination Mark the Beginning of Reversal$1

Among the twenty-four solar terms, the Summer Solstice (Xiazhi, 夏至) occupies a singularly remarkable position. It does not mark the "beginning" of anything — the terms Beginning of Spring, Beginning of Summer, Beginning of Autumn, and Beginning of Winter all bear the character li (立, "to establish"), signifying the founding of a new season. Nor does it mark a "midpoint" — the Spring and Autumn Equinoxes take their names from the equal division of day and night. What the Summer Solstice marks is a state of culmination — the critical juncture at which the sun has traveled to its northernmost point, daylight has stretched to its longest, and the gnomon's shadow has shrunk to its shortest. It is the apex of the year, the moment when yang energy is most replete, most blazing, most full.

And yet it is precisely upon this summit that a strange reversal begins. The ancients declared with the utmost solemnity: on the day of the Summer Solstice, "one yin is born" (yiyin sheng, 一阴生). At the very instant yang energy reaches its absolute zenith, the yin force that will eventually supplant it — the force that will draw the entire cosmos toward cool autumn and frigid winter — has already begun to stir in the deepest recesses below. What a profound, what a breathtaking insight! It means that culmination is itself reversal; the peak is the very inception of decline; within the most resplendent light, the seed of darkness already lies concealed.

Why should this be so$2 Why does the Way of Heaven ordain reversal at the very moment of fullness$3 Why can yang energy not simply continue to grow, rather than halting abruptly at the Summer Solstice and turning downward$4 This question is the greatest riddle the Summer Solstice poses to us, and it is one of the core propositions of the entire Chinese cosmological tradition. It draws upon a thread of thought running through the pre-Qin masters and stretching back to the Three Dynasties — the principle that the extreme of fullness inevitably reverses, that the apex of any force marks its turning. To understand the Summer Solstice is to understand the fundamental way the Chinese perceive prosperity and decline, gain and loss, life and death, rise and fall.

The Book of Documents (Shangshu, "Canon of Yao") states: "He then commanded Xi and He, in reverent accordance with august Heaven, to calculate and delineate the sun, moon, and stars, and so respectfully bestow the seasons upon the people." In that passage's record — "the day at its longest, the star Huo (Antares), thereby fixing midsummer" — the ancestors had already used the most basic astronomical observation to identify the Summer Solstice: the day when daylight is longest. But they were not content merely to "know which day has the longest daylight." They pressed further: What does this day mean$5 What position does it occupy in the great cycle of Heaven and Earth's transformations$6 And what spirit, what conduct, should a person bring to such a moment$7

The Commentary on the Appended Phrases (Xici zhuan) of the Book of Changes (Yijing) says: "As the sun departs, the moon arrives; as the moon departs, the sun arrives — sun and moon push each other forward, and thus light is born. As cold departs, heat arrives; as heat departs, cold arrives — cold and heat push each other forward, and thus the year is completed." The two characters xiang tui (相推, "push each other forward") reveal the essential nature of the Way of Heaven: no side can dominate forever. At the extreme of yang, yin must arise to push it onward; at the extreme of fullness, decline must follow. The Summer Solstice is precisely the most critical pivot in this process of mutual pushing. On this day, "heat" reaches the critical threshold of its transformation into "cold"; yang reaches the summit from which it must yield to yin.

This essay will proceed from the core ideas of the pre-Qin Confucian and Daoist traditions, reaching back to the mythology and ritual institutions of the Three Dynasties and high antiquity, to offer as thorough an interpretation of the Summer Solstice as possible. We shall contemplate the original meaning of the character zhi (至, "to arrive; the ultimate"), and interrogate the astronomical truths behind "the sun reaching its northernmost point, daylight reaching its longest, and the shadow reaching its shortest." We shall anatomize the core cosmological proposition "one yin is born at the Summer Solstice," examining how the Confucians read from it a counsel of vigilance at the peak, and how the Daoists read from it the principle of "reversal is the movement of the Way" and the wisdom of retiring when one's work is done. We shall investigate the messages of yin and yang encoded in the three pentads — the shedding of deer antlers, the first singing of cicadas, and the sprouting of banxia (Pinellia) — and we shall explore why the ancient rite of sacrificing to the Earth Spirit at the Square Mound at the Summer Solstice forms a perfect symmetry with the sacrifice to Heaven at the Round Mound at the Winter Solstice. Through these layers of inquiry, perhaps we may touch that ancient wisdom of our forebears — "fullness invites diminishment; humility receives increase" — a lantern that illuminates our lives to this day.