Chapter 2: The Astronomical Foundations of Lichun: How Did the Ancients Determine the Year's Beginning$19

I. 315° of Ecliptic Longitude: Lichun's Precise Position on the Celestial Sphere

In the language of modern astronomy, the definition of Lichun is extremely precise: the moment when the sun reaches 315° of ecliptic longitude is precisely Lichun. To understand this statement, we must first understand what "ecliptic longitude" means.

Over the course of a year, the sun appears to trace a fixed path against the background of the stars, slowly completing one full circuit—this path is the "ecliptic." Although the ancients had no modern terminology such as "ecliptic" or "ecliptic longitude," through prolonged observation they had already grasped the pattern of the sun's annual movement among the stars. Later generations divided the ecliptic's circuit into 360°, calculated from the Spring Equinox point as 0°. Thus, the Twenty-Four Solar Terms correspond precisely to twenty-four evenly distributed points along the ecliptic, one every 15°: Spring Equinox at 0°, Qingming at 15°, Guyu at 30°, and so on in sequence—Lichun corresponds precisely to 315°.

Here lies a phenomenon that appears strange at first glance but is in fact deeply meaningful: Lichun is the solar term of the "year's beginning," and by rights ought logically to correspond to 0°—why, then, does it correspond to 315°, with the Spring Equinox instead occupying the position of 0°$20

This apparent "contradiction" precisely preserves the historical trace of how the system of solar terms evolved. The first solar terms the ancients precisely determined were the "Two Equinoxes and Two Solstices"—the Spring and Autumn Equinoxes, and the Summer and Winter Solstices. Among these, the equinoxes have day and night of equal length, while the solstices have the shortest and longest shadows of the year; these four points bear the most distinct and most easily observed markers among celestial phenomena. Later generations took the Spring Equinox point as the 0° starting point for ecliptic longitude precisely because of this foundational status of the equinox in astronomical measurement. Lichun, as one of the "Four Beginnings," was supplemented into the framework only after the "Two Equinoxes and Two Solstices" had already been established—its position on the ecliptic (315°) is obtained by tracing back 45° (that is, three solar terms) from the Spring Equinox point. Thus an intriguing configuration emerges: the "starting point of astronomical measurement" is the Spring Equinox (0°), while the "starting point of the year's human affairs" is Lichun. One is "the beginning of Heaven," the other "the beginning of the year"—this discrepancy between the two precisely reflects the subtle reconciliation the ancients achieved between "pure celestial phenomena" and "the calendar of human affairs."

II. Measuring Shadows with Gnomon and Scale: The Oldest Astronomical Instrument and the Shadow of Lichun

How did the ancients, lacking modern instruments, determine the precise moment of Lichun$21 The most basic and most ancient method was to observe the sun's shadow.

The Rites of Zhou (Zhouli), in the "Office of Earth, Great Minister of Education" (地官·大司徒), records: "By the method of the earth-gnomon, one measures the depth of the earth, rectifies the sun's shadow, and thereby seeks the center of the land" (以土圭之法测土深，正日景（影），以求地中). The gnomon-and-scale apparatus (gui biao) is one of the oldest astronomical observation instruments in China. A vertically erected "gnomon" (biao, a pole) together with a horizontally placed "scale" (gui, a ruler) together constitute a complete system of astronomical observation. By measuring the length of the gnomon's shadow at noon, the ancients could precisely determine the sun's altitude in the sky, and thereby establish the solar term.

The underlying principle is this: the higher the sun, the shorter the noon shadow; the lower the sun, the longer the shadow. On the Winter Solstice, the sun's position is lowest, and the noon shadow is longest—this is the longest shadow of the year; on the Summer Solstice, the sun's position is highest, and the shadow is shortest. Lichun falls between the Winter Solstice and the Spring Equinox; its shadow length has already noticeably shortened compared to the Winter Solstice, but has not yet shortened to the degree of the Spring Equinox. By recording, day after day and year after year, the length of the gnomon's noon shadow, the ancients were able, once the shadow shortened to a specific measurement, to declare with precision: "Lichun has arrived."

Here lies a question worth pondering deeply: from the Winter Solstice to Lichun, the shortening of the sun's shadow is extremely gradual—the daily change is minute, almost imperceptible to the naked eye. What manner of acuity and patience drove the ancients to measure, day after day, year after year, the shadow cast by that gnomon pole, and to extract from this exceedingly subtle change the pattern of the solar terms$22

The answer still lies within the four characters "respectfully bestowing the seasons." For the ancients, whose state was founded upon agriculture, determining the season of spring plowing was a matter of the first importance, bearing upon the survival of the entire people. Lichun, as the "starting gun" of a year's agricultural labor, had to be determined with precision, for this bore directly upon whether or not the farming season would be missed. It was precisely this pressure of survival that drove the ancients, with extraordinary focus and perseverance, to hone the art of shadow measurement to an extremely high degree of precision. In an age with no other standard of time whatsoever, the shadow cast by that gnomon pole was the ancients' sole "calendar."

III. The Dipper's Handle Points to Yin: The Northern Dipper as Heaven's Clock

Besides measuring shadows with gnomon and scale, the ancients had another, more intuitive and more poetic method of determining the time—observing the direction in which the handle of the Northern Dipper pointed.

The Northern Dipper (Ursa Major) rotates around the north celestial pole throughout the year, and its handle (斗柄) functions like an enormous pointer, indicating different directions as the seasons change. The Heguanzi, in its chapter "Cyclical Flow" (环流), contains a saying renowned through the ages: "When the Dipper's handle points east, all under Heaven is spring; when it points south, all under Heaven is summer; when it points west, all under Heaven is autumn; when it points north, all under Heaven is winter" (斗柄东指，天下皆春；斗柄南指，天下皆夏；斗柄西指，天下皆秋；斗柄北指，天下皆冬). In extremely concise language, this passage articulates the function of the Northern Dipper as "Heaven's clock"—one need only gaze up at the Dipper at dusk, observe which direction its handle points, and thereby know the season.

The first month, in which Lichun falls, is also called the "Yin Month" (寅月). Within the system correlating the Twelve Earthly Branches with the twelve directions, "yin" (寅) corresponds to the direction slightly north of due east. The ancients spoke of "Dipper establishment" (斗建)—the direction in which the Dipper's handle pointed at early dusk—which shifted by one position (chen) each month. In the first month, the Dipper's handle points to "yin," called "Dipper establishment at yin" (斗建在寅); hence the first month is also called "the month of establishing yin" (建寅之月). When the ancients, at dusk in early spring, saw the Dipper's handle pointing precisely toward the northeastern "yin" position, they knew that Lichun, the season marking the year's beginning, had arrived.

Why "yin"$23 This touches upon a long-debated question in the history of the calendar. The years' beginnings differed across the Xia, Shang, and Zhou dynasties: the Xia calendar "established yin" (took the first month as the year's beginning), the Yin (Shang) calendar "established chou" (took the twelfth month as the year's beginning), and the Zhou calendar "established zi" (took the eleventh month, the month of the Winter Solstice, as the year's beginning). This threefold difference is known to later generations as "the Three Correct Beginnings" (三正). The calendar we follow today—which takes as the year's beginning the first month in which Lichun falls—continues precisely the tradition of "the Xia correct beginning, establishing yin." The Master (Confucius), in the Analects, "Wei Ling Gong" chapter, in answering Yan Yuan's question about governing a state, explicitly advocated "following the seasons of the Xia" (行夏之时)—adopting the calendar of the Xia dynasty. Why did the Master alone hold the Xia calendar in such high regard$24 Precisely because the Xia calendar's establishing of yin, taking spring as the beginning, most closely accords with the natural rhythm of Heaven and Earth's generation and growth—the year beginning at the moment vital qi first sprouts is precisely the correct way of Heaven and humanity resonating with one another.

IV. From the "Two Equinoxes and Two Solstices" to the "Four Beginnings": The Expansion of the Solar-Term System and the Birth of Lichun

As mentioned above, in the earliest system of solar terms, there existed only the "Two Equinoxes and Two Solstices." These four solar terms were the easiest to determine through astronomical observation, but they could only divide the year roughly into four segments—far from sufficient for fine-grained agricultural production. The ancients therefore supplemented the "Two Equinoxes and Two Solstices" with the "Four Beginnings"—Lichun, Lixia, Liqiu, and Lidong—subdividing the year from four segments into eight, the so-called "Eight Nodes" (八节).

The establishment of the "Four Beginnings" marked a leap forward in the ancients' understanding of the seasons. The "Two Equinoxes and Two Solstices" focus on the "extremes" of celestial phenomena—the longest, shortest, and equal divisions of day and night; the "Four Beginnings," by contrast, focus on the "commencement" of seasonal transitions—from what moment does the qi of Heaven and Earth undergo its fundamental shift from one season to the next$25 This question is more subtle than the former, because seasonal transitions, unlike solstices and equinoxes, have no distinct astronomical marker, and require a comprehensive consideration of sunlight, temperature, phenological signs, and other factors.

And among the "Four Beginnings," the birth of Lichun carries a special weight. It must mark not only "the beginning of spring," but also bear the responsibility of "the beginning of the year." This means that determining Lichun is not merely an astronomical problem, but a fundamental problem concerning where the entire calendrical system should "begin." When the ancients finally settled upon the Yin month, upon the moment when yang qi first stirs beneath the earth, as the solar term marking the year's beginning, they were in effect declaring, in the most solemn manner, that the beginning of the year ought to coincide with the beginning of Heaven and Earth's generation and growth. This is the most concentrated expression, within the calendar, of the cosmology of "the unity of Heaven and humanity"—human time must be rooted in Heaven's time.

V. "Following the Seasons of the Xia": The Philosophy of Heaven and Humanity behind the Dispute over the Year's Beginning

As mentioned above, the Master advocated "following the seasons of the Xia," holding in exclusive esteem the Xia calendar, which took as the year's beginning the first month in which Lichun falls. This dispute over "the Three Correct Beginnings"—Xia establishing yin, Yin (Shang) establishing chou, Zhou establishing zi—was by no means a purely technical matter of calendrical calculation; concealed behind it lies a philosophical dispute over "upon what should human time be founded," a question well worth pursuing in its own right.

Why did the Xia, Shang (Yin), and Zhou dynasties each have a different year's beginning$26 The Zhou dynasty established zi, taking the eleventh month, in which the Winter Solstice falls, as the year's beginning—this was a choice of "astronomical primacy": the Winter Solstice, the point at which the single yang line returns, at which the sun's shadow is longest and the sun turns back from its southernmost point, is the most distinct astronomical extreme; taking this most distinct astronomical node as the year's beginning is, logically, the most "pure" choice. The Shang (Yin) dynasty established chou, taking the twelfth month as the year's beginning—a middle position. The Xia dynasty established yin, taking the first month (the month in which Lichun falls, in which vital qi first sprouts) as the year's beginning—this was a choice of "the primacy of vital qi": it takes as its beginning not an astronomical extreme, but the commencement of the generation and growth of all things.

Why did the Master choose, exclusively, to "follow the seasons of the Xia," rather than the astronomically more "pure" Zhou calendar establishing zi$27 This choice reveals the deepest value orientation of Confucian thought. Although the Winter Solstice is an astronomical extreme, at that time all things remain sealed and hidden, their vitality still latent beneath the earth—taking "storage" as the year's beginning does not accord with human affairs, with agriculture, with the spirit of life's "generation and growth." Lichun, though not an astronomical extreme (it occupies 315° on the ecliptic, not 0°), is the season at which vital qi truly begins to sprout, at which agricultural planning truly begins—taking "generation" as the year's beginning accords with the fundamental principle that "the great virtue of Heaven and Earth is called generation" (天地之大德曰生), and accords with the reality that human beings take farming as their foundation and the nurturing of life as their priority. In other words, the Master's choice to "follow the seasons of the Xia" was, between "the purity of astronomy" and "the urgency of life," a resolute choice of the latter—the year's beginning he sought was not an abstract astronomical coordinate, but a living "beginning," brimming with vitality, bearing upon the people's livelihood, and according with "the virtue of generating and regenerating" (生生之德).

This choice is precisely the concentrated expression of the spirit of the Chinese conception of time—"centered on humanity, valuing generation and growth." It tells us: the human calendar, at its root, serves the purpose of human life, of the nurturing and growth of all under Heaven; the reason the year's beginning is set at Lichun is precisely because Lichun is the beginning of "generation," the beginning of "agriculture," the beginning of "hope." That we today still take Lichun as the year's beginning around the time of the Spring Festival, still bid farewell to the old year and welcome the new around the time of Lichun, is, at root, precisely the continuation of this great tradition—one originating in the Xia dynasty and solemnly honored by the Master through his advocacy of "following the seasons of the Xia"—a tradition, full of humanistic warmth, that reveres "the sprouting of life" rather than "the astronomical extreme" as the beginning of the year.

VI. Cross-Verification of Celestial Phenomena and Phenological Signs: The Ancients' Spirit of "Empirical Verification"

The Lost Book of Zhou (Yi Zhou Shu), in its chapter "Explanation of the Seasonal Instructions" (时训解), gives a detailed phenological description for each solar term. Regarding Lichun, it records: "On the day of Lichun, the east wind melts the ice. Five days later, hibernating insects begin to stir. Five days after that, fish rise to the ice" (立春之日，东风解冻。又五日，蛰虫始振。又五日，鱼上冰). The east wind blowing, the solid ice melting, hibernating insects awakening, fish rising close to the ice—these three phenological phenomena were established by the ancients as the markers of Lichun's arrival. We will elaborate on the profound meaning of these Three Pentads one by one in a dedicated chapter below. What must be noted here first is this: the ancients never judged the arrival of Lichun by the single dimension of the sun's shadow alone, but cross-verified "celestial phenomena" (gnomon-and-scale shadow measurement, the Dipper's handle pointing to yin) against "phenological signs" (the east wind, the melting of ice, the stirring of insects, fish rising). Only when the sun in the heavens, the Dipper in the heavens, the wind upon the earth, the ice upon the earth, and the insects beneath the earth all simultaneously sent forth the signal "spring has come" would the ancients solemnly declare: Lichun has arrived, a new year has begun.

This method of dual verification through "celestial phenomena" and "phenological signs" embodies a plain yet rigorous spirit of "empirical verification" among the ancients. The measurement of the sun's shadow is the dimension of "Heaven"—abstract, requiring instruments and calculation; the observation of phenological signs is the dimension of "Earth"—concrete, perceptible to the ear and eye. The ancients did not sever these two, but let them mutually verify and calibrate one another—if the sun's shadow had already reached the degree of Lichun, and the signs of the east wind, the melting of ice, the stirring of insects, and fish rising also arrived on schedule, then the judgment of Lichun received the dual confirmation of both "Heaven" and "Earth." Behind this lies the ancients' firm conviction in "the resonance of Heaven and humanity" and "the unity of Heaven and Earth"—they believed that the position of the sun in the heavens and the condition of all things upon the earth must necessarily be coordinated and mutually responsive; if the two showed serious discrepancy (the sun's shadow having arrived while the phenological signs had not, what the "Explanation of the Seasonal Instructions" calls "phenological signs out of order"), this was a warning that the qi of Heaven and Earth had fallen into disorder. This dual grasp—"celestial phenomena fixing the measure, phenological signs verifying the reality"—is precisely the methodological foundation upon which China's solar-term culture has remained precise and unerring across several millennia.