Chapter Three: The Astronomical Foundation of Jingzhe -- The Sun Reaches 345 Degrees of Celestial Longitude

I. 345 Degrees Celestial Longitude: The Astronomical Position of Jingzhe

From the perspective of modern astronomy, the determination of Jingzhe has a precise standard: when the sun reaches 345 degrees of ecliptic longitude, it is Jingzhe.

What is ecliptic longitude$21 It is the longitude in the ecliptic coordinate system. The ecliptic is the apparent path traced by the sun across the celestial sphere over the course of a year, as seen from earth. Although the ancients did not know that the earth revolves around the sun, through long observation they accurately grasped the sun's annual cycle against the backdrop of the fixed stars and divided this path into 360 degrees. Taking the vernal equinox as zero degrees of ecliptic longitude, each 15-degree increment corresponds to one solar term. Thus the twenty-four solar terms evenly divide one complete circuit of the ecliptic (360 degrees).

In this system: the Vernal Equinox (Chunfen) is at zero degrees, Qingming at 15 degrees, Guyu at 30 degrees, and so forth. Jingzhe corresponds to 345 degrees -- that is, 15 degrees before the Vernal Equinox (360 degrees, i.e., the next zero). In other words, after Jingzhe, the sun travels another 15 degrees before reaching the equinoctial point. This precisely positions Jingzhe in mid-spring, just before the equinox.

It should be specially noted that pre-Qin peoples of course did not possess the precise concept of "345 degrees ecliptic longitude." The method of defining solar terms by the sun's ecliptic degree is a product of later refinement in astronomical calendrical science. However, the astronomical reality underlying this precise degree -- that the sun has progressed to mid-spring, that day and night are about to be equal, that yang qi is daily growing stronger -- was grasped by the ancients long ago through their own methods (shadow observations, stellar observations, phenological verification). The modern "345 degrees ecliptic longitude" merely restates, in more precise mathematical language, the celestial node that the ancients had already discerned.

II. Shadows and the Gnomon: How the Ancients Determined the Seasons

How did the ancients determine the specific dates of solar terms such as Jingzhe$22 The most fundamental method was the observation of shadows.

The Rites of Zhou (Zhouli), in the section on the Grand Minister of Education, records: "Using the method of the gnomon to measure the depth of the earth, to correct the sun's shadow, and thereby to find the center of the land." The gnomon (guibiao) was one of China's most ancient astronomical instruments. A vertical pole (the "biao") plus a horizontal measuring rod (the "gui") formed a complete system for astronomical observation. By measuring the length of the pole's shadow at noon, the ancients could precisely determine the sun's altitude in the sky and thereby fix the solar terms.

The higher the sun, the shorter the noon shadow; the lower the sun, the longer the noon shadow. At the winter solstice the sun is lowest and the shadow longest; at the summer solstice the sun is highest and the shadow shortest. Jingzhe falls between the solstices, near the equinox, and its shadow length lies between the two extremes -- and is shortening day by day. This signifies that the sun is rising day by day, daylight is lengthening, and yang qi is strengthening. Through years and decades of observing and recording the shadow cast by the gnomon, the ancients gradually mastered the laws governing shadow changes and were able to predict the arrival of each solar term with precision.

Here arises a question worthy of reflection: How did the ancients first conceive the idea of measuring shadows$23 The change in shadow length is exceedingly gradual, with only the slightest daily difference. What keenness and patience drove the ancients to undertake this observation spanning multiple generations$24

The answer may lie in those four characters, "respectfully deliver the seasons." And Jingzhe was precisely one of the solar terms for which this "delivery of seasons" was most urgently needed. For Jingzhe marked the beginning of spring plowing -- to miss the planting window around Jingzhe meant a year of poor harvests or even famine. In an age with no other standard of time, knowing precisely "when to begin spring plowing" was a matter of life and death for the entire community. It was this pressure of survival that drove the ancients with extraordinary focus and perseverance to observe, day after day, the shadow cast by that gnomon pole, all to capture the precise onset of Jingzhe, the starting point of the agricultural season.

III. The Northern Dipper Points to Mao: Observing the Heavens to Fix Mid-Spring

In addition to shadow observation, the ancients also determined the solar terms by observing the stars, one of the most important methods being the observation of the handle of the Northern Dipper (Big Dipper).

The Heguanzi contains a celebrated passage: "When the Dipper 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." The handle of the Northern Dipper, like an immense celestial pointer, rotates with the turning of the seasons. When the handle points east, it is spring throughout the world.

Within the more refined "Twelve Stations" (shi'er chen) system, the year is divided into twelve months, each corresponding to one of the twelve Earthly Branches (Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, Hai). As the Dipper handle completes one rotation per year, its pointing direction (the "jian") indicates the current month. The mid-spring second month in which Jingzhe falls finds the Dipper handle pointing to the Mao position -- hence the second month is also called the "Month of Mao." In the twelve Earthly Branches, Mao corresponds to due east and to mid-spring, fully consistent with "When the Dipper handle points east, all under heaven is spring."

Why did the ancients devote such attention to observing the Dipper handle's direction$25 Because the Northern Dipper held an exalted position in Chinese ancient astronomy. The Records of the Grand Historian (Shiji), in the "Treatise on the Celestial Offices," calls the Northern Dipper the "Emperor's Chariot" (di che), stating that it "moves through the center, commanding the four directions. It divides yin and yang, establishes the four seasons, harmonizes the five phases, shifts the intervals and measures, and fixes all reckonings -- all these are linked to the Dipper." The ancients did not see the Dipper handle as passively indicating the seasons but as actively "establishing" them: when the handle points to Mao, mid-spring arrives and Jingzhe comes. This mode of thought, which fuses stellar phenomena with seasonal order, astronomy with human affairs, is a typical expression of the Chinese cosmology of the unity of Heaven and humanity.

IV. The Mystery of the Character "Mao": Why Is the Second Month of Jingzhe Called the "Month of Mao"$26

The second month in which Jingzhe falls is called the "Month of Mao," and the Dipper handle points to Mao. The character "mao" itself contains meanings deeply consonant with the theme of Jingzhe and warrants specific exploration.

The twelve Earthly Branches are not merely calendrical symbols. In the exegetical tradition from the Han dynasty onward, each branch character was invested with meanings relating to the phenology and climate of its corresponding month. Xu Shen's Shuowen Jiezi offers explanations for these branch characters, often associating them with the state of growth of all things. Regarding "mao," the traditional interpretation holds that "mao" carries the sense of "mao" (to sprout forth) -- all things push through the earth and emerge. By the second month, the creatures roused by spring thunder push up through the ground and poke their heads out; this image of "sprouting forth from the earth" is precisely what the character "mao" seeks to capture.

There is an even more vivid explanation suggesting that the shape of the character "mao" resembles two doors thrown open -- the upper portions parting to either side, as though a portal has been flung wide. The second month is precisely the season when the gates of heaven and earth are thrown open and all things pour forth from their concealment. This image of "the portal thrown open" resonates perfectly with the "qi" (opening) in Jingzhe's original name "Qizhe," and with the Monthly Ordinances' designation of the door-spirit (hu) as the object of worship in mid-spring -- three elements in a trinity, echoing across each other. They all point to the same core image: opening, emerging, moving from interior to exterior, from hiding to action.

Thus the name "Month of Mao" is by no means an arbitrary calendrical symbol. Like "Qizhe," "Jingzhe," and "the sacrifice to the door-spirit," it crystallizes the ancients' grasp of the central theme of the second month of mid-spring: this is a month of "opening," a month when all things push through the earth and surge into action. Whether it is the "Mao" indicated by the Dipper handle, the solar term name "Qizhe/Jingzhe," or the door-spirit worshipped in the Monthly Ordinances, all are speaking the same truth in different languages: the gate of heaven and earth, closed for a whole winter, is in this month thrown open at last.