Chapter Two: The Astronomical Foundations of White Dew — The Sun at Ecliptic Longitude 165 Degrees

I. Ecliptic Longitude 165°: White Dew's Coordinates on the Celestial Sphere

How is White Dew precisely determined$17 In the framework of modern astronomy, White Dew corresponds to the moment when the sun reaches an ecliptic longitude of 165°. Behind this seemingly abstract number lies the wisdom crystallized from millennia of the ancients' observation of the heavens and measurement of shadows.

The "ecliptic longitude" is the positional coordinate of the sun along the ecliptic (the projection of the earth's orbital plane onto the celestial sphere). Taking the vernal equinox point as ecliptic longitude 0°, each 15° the sun travels corresponds to one solar term. Vernal Equinox is 0°, Clear and Bright 15°, Grain Rain 30°... and so on, with the Beginning of Autumn at 135°, End of Heat at 150°, White Dew at 165°, and Autumnal Equinox at 180°. When the sun reaches ecliptic longitude 165°, it is only 15° — that is, half a month — from the Autumnal Equinox (180°). This positioning is deeply meaningful: though White Dew belongs to mid-autumn's node, it already approaches the equinox when day and night are equal, making it a pivotal moment when yin and yang are about to reach equilibrium while yin qi is still in the ascendant.

II. The Gnomon and Shadow: White Dew's Shadow Position

The ancients lacked the concept of "ecliptic longitude." How then did they determine White Dew$18 The most fundamental, most ancient method was observing the sun's shadow.

The Rites of Zhou (Zhouli, "Officers of Earth: Grand Minister of Education") records: "Using the method of the earth gnomon to measure the depth of the soil, correct the sun's shadow, and thereby find the center of the earth." The gnomon — a vertical post (the "biao") plus a horizontal ruler (the "gui") — constituted one of China's oldest astronomical observation systems. By measuring the length of the shadow at noon, the ancients could precisely determine the sun's altitude in the sky, thereby fixing the solar terms.

At the summer solstice, the noon shadow is shortest because the sun sits highest; at the winter solstice, the noon shadow is longest because the sun sits lowest. From summer to winter solstice, the shadow grows longer daily. White Dew falls after the summer solstice and before the autumnal equinox, so its noon shadow length lies between those two — longer than the solstice shadow, slightly shorter than the equinox shadow. When the ancients measured the noon shadow reaching a particular length, they knew White Dew had arrived — a length established through generations of astronomical officials' long-term observation and repeated calibration.

Here lies a thought-provoking question: the most prominent feature of White Dew is the morning dew, which appears between night and dawn, seemingly irrelevant to the noon shadow. Why didn't the ancients simply observe the dew directly instead of going through the roundabout of measuring the noon shadow$19

The answer reveals the profundity of the ancients' astronomical thinking. The appearance of dew is influenced by many contingent factors such as the day's weather, cloud cover, and wind speed, making it unreliable — one day may be dewless due to overcast skies, another heavily dewed due to clear skies. If one relied solely on dew to determine the solar term, the result would be contingent and imprecise. The noon shadow, by contrast, depends only on the sun's position — it is a stable measure of Heaven's Way, undisturbed by daily weather. The ancients' wisdom lay in this: they saw through the surface, contingent phenomenon of dew to find its stable, necessary astronomical basis — the sun's arrival at ecliptic longitude 165°. The shadow is the "cause," the dew the "effect"; measuring the shadow grasps the solar term at its root rather than being led about by surface phenomena. This is the spirit of "investigating things to extend knowledge" (gewu zhizhi) — not resting content with appearances, but pursuing the heavenly principle behind them.

III. Star Observations and White Dew: Guidance from the Celestial River

Besides the sun's shadow, the ancients also determined the seasons by observing stellar lodges. The Book of Documents ("Canon of Yao") records: "When the night sky's central star is Xu, thereby to regulate mid-autumn" (xiao zhong xing xu, yi yin zhongqiu) — at mid-autumn (around the autumnal equinox), the marker constellation appearing due south in the evening sky is "Xu" (Emptiness). White Dew falls at the beginning of mid-autumn, close to the equinox, so the evening sky's southern meridian was already approaching this configuration.

Even more noteworthy is that during White Dew, one of the most magnificent spectacles of the night sky is the Milky Way (called "Tian Han" or "Yun Han" in ancient times) stretching across the heavens. The Classic of Poetry (Xiaoya, "Great East") has the line: "Heaven has its Han River, that shines and casts its light" (wei tian you Han, jian yi you guang). In the clear, high autumn nights, the Milky Way shines with special brilliance. The ancients gazed upward at the celestial river and downward at the white dew — the star-river above and the dew-pearls below echoed each other across the distance — what a poetic dialogue between heaven and humanity!

Why was stellar observation so important for determining solar terms$20 Because the sun's shadow can only be measured during clear daytime hours, while star patterns provide complementary nocturnal coordinates. The shadow measures the sun; the stars measure the stellar background — the two cross-reference and verify each other, constituting a three-dimensional, redundant astronomical observation system. With this system, the ancients firmly "pinned" the invisible passage of time to coordinates on the celestial sphere, making the determination of solar terms both precise and stable.

IV. From the "Four Beginnings, Two Equinoxes, and Two Solstices" to the Twenty-Four Solar Terms: White Dew's Position

In the earliest solar-term system, there were only the "two equinoxes and two solstices" — Vernal Equinox, Autumnal Equinox, Summer Solstice, Winter Solstice. Later the "four beginnings" were added — Beginning of Spring, Summer, Autumn, and Winter — forming the "eight nodes." Subsequently, the ancients further subdivided the year into twenty-four solar terms, of which White Dew is one.

White Dew occupies a distinctive position among the twenty-four solar terms — it is the third of autumn's six terms (Beginning of Autumn, End of Heat, White Dew, Autumnal Equinox, Cold Dew, Frost's Descent), situated just before the midpoint of autumn, in "mid-autumn." It follows End of Heat (where summer's remaining heat finally ceases) and precedes Autumnal Equinox (where day and night reach equilibrium). This position bestows upon White Dew a dual character: on one hand, it has completely bid farewell to summer's swelter (End of Heat has already passed), and coolness deepens; on the other, it has not yet reached the equinox's balance of yin and yang — yin is heavy but yang has not yet been exhausted, producing a distinctive pattern of "cold nights and warm days."

It is precisely this dramatic temperature differential between night and day that creates White Dew's most characteristic phenological feature: during the day, yang qi still dominates, and surface water vapor steams upward; at night, yin qi surges, temperatures plummet, and the vapor condenses into dew. The greater the temperature swing, the heavier the dew. White Dew's dew is the product of summer and autumn, yang and yin, violently alternating within a single day-and-night cycle. The ancients placed this solar term at the onset of mid-autumn precisely because they captured this period of maximum temperature contrast and most typical dew formation.