Ancient astronomical azimuth system
Ancient Chinese Methods of Determining Directions and CoordinatesThe ancients, whether conducting astronomical observations or going about daily life, always needed to distinguish directions. They treated directions as a form of reference, essentially establishing a coordinate system that made recording, navigation, and communication much easier.
Terrestrial Directions (Directions on the Ground)
The ancient dictionary Shuowen Jiezi records: “十 (Shí) is the tool of counting. One stroke represents East-West, another stroke represents South-North. Thus, the four cardinal directions and the center are complete.” Xu Shen explained that the character “十” depicts the intersection of the Zi-Wu (meridian) and Mao-You (parallel) lines on a map. Ordinary people considered the “four directions — East, West, South, and North” sufficient for daily use.The boundaries of farmland were called “sì zhì” (four extremities), the surrounding area of a house was called “sì lín” (four neighbors), a man’s ambitions were described as reaching the “sì fāng” (four directions), and China was said to lie within the “sì hǎi” (four seas). All these terms using “sì” (four) refer to the four cardinal directions: East, West, South, and North.Around any given location, people distinguished “front, back, left, right.” The ancients regarded the South as the most honorable direction because the sun travels in the southern sky. Therefore, people faced South with their back to the North. Houses were built with the principle of “sitting in the North, facing the South” (zuò běi cháo nán).When a person faces South, South is in front, North is behind, East is on the left, and West is on the right. The Shizi links the four directions with the four seasons. The Guanzi states: “Spring (East) gives birth on the left, Autumn (West) harvests on the right, Summer (South) grows in front, and Winter (North) stores behind.”Ancient maps hung on walls were also divided into “top, bottom, left, right.” The convention of the ancients was: top = South, bottom = North, left = East, right = West.The ancients believed Heaven is round like a jade bi disc, and Earth is square like a jade cong. Therefore, the Earth has eight directions — the four cardinal directions plus four corner directions (sì yú), commonly called “four sides and eight directions” (sì miàn bā fāng). Later, these were combined with the Eight Trigrams (Bagua), corresponding to the eight directional
positions.To
determine direction, the ancients observed the moment when the sun reached its highest point (true South). At that moment, the shadow of the gnomon (guī biǎo) on the ground is shortest. During the Spring and Autumn Equinoxes, the sunrise and sunset directions are exactly due East and due West.
Ancient Methods of Measuring Directions on the Ground
Here are several classic methods recorded in ancient texts:1. The “Zheng Zhao Xi” Method (Kaogong Ji)
On a flat piece of ground, draw a circle and erect a vertical gnomon at the center. Mark the positions of the gnomon’s shadow at sunrise and sunset. Draw lines from the center to these two points on the circle. Connect the two intersection points on the circle to form the true East-West line. The line perpendicular to it is the true North-South line.2. The “Li Zhao Xi” Method (Huainanzi)
On flat ground, erect a fixed gnomon (ding biǎo). Draw a circle with a radius of ten paces using the gnomon as center. Use a movable gnomon (yóu biǎo) placed on the circumference. At sunrise, align the fixed gnomon, movable gnomon, and the sun in a straight line and mark the position. Repeat at sunset. The line connecting these two marks is the true North-South line; the perpendicular line is true East-West.3. The Precise Shadow-Tracking Method (used by Guo Shoujing in the Yuan Dynasty)
Guo Shoujing used a square board with nineteen concentric circles and a gnomon at the center. By carefully tracking the tip of the gnomon’s shadow from sunrise to sunset as it moved across the circles, any two points where the shadow tip crossed the same circle could be connected to form the true East-West line.4. Nighttime Verification Using the North Star
The Kaogong Ji notes that while the North-South line is determined by the sun’s shadow during the day, it should be verified at night by observing the North Star (Polaris).5. Magnetic Compass Method
A magnetized needle suspended in the air, floated on a small piece of wood in water, or pivoted in a socket would naturally point North-South.6. South-Pointing Chariot
Legend says the Yellow Emperor invented the South-Pointing Chariot. A wooden figure on the chariot always pointed South no matter which way the vehicle turned. This is believed to have been a mechanical device using differential gears. Later dynasties repeatedly attempted to reconstruct it.
Four Seasons, Four Directions, and the Theory of Heavenly Stems, Earthly Branches, and Five Elements
Through observation, the ancients discovered climatic differences across regions and summarized that the North is cold and the South is warm. They believed all things in the universe are governed by the cycle of the Five Elements (Wuxing). The four directions and four seasons also correspond to these elements:
North (cold) → Water (Shui). Associated with stems Rén, Guǐ and branches Hài, Zǐ, Chǒu. Corresponds to Winter.
South (warm) → Fire (Huǒ). Associated with stems Bǐng, Dīng and branches Sì, Wǔ, Wèi. Corresponds to Summer.
East (growth) → Wood (Mù). Associated with stems Jiǎ, Yǐ and branches Yín, Mǎo, Chén. Corresponds to Spring.
West (harvest) → Metal (Jīn). Associated with stems Gēng, Xīn and branches Shēn, Yǒu, Xū. Corresponds to Autumn.
Center → Earth (Tǔ). Associated with stems Wù, Jǐ and branches Chén, Xū, Chǒu, Wèi (plus the last 18 days of each season).
Even within a single day, night (cold and dark) belongs to Water, daytime (bright and warm) to Fire, morning (cold to warm) to Wood, and dusk (warm to cold) to Metal. Combined with Yin-Yang and the Eight Trigrams, this formed the complete framework of Yin-Yang and Five Elements theory. In ancient times, practitioners of Yin-Yang were often also astronomers (the same was true in the West — Johannes Kepler earned a living through astrology).
Twenty-Four Mountains, Sixty Dragons, and Seventy-Two Dragons
According to the theory of a round Heaven and flat Earth, the ancients used mountain ranges visible on the horizon to indicate directions. The most widely used system was the Twenty-Four Mountains (Èrshí Sì Shān) — a combination of eight Heavenly Stems, four corner directions (Sì Wéi), and twelve Earthly Branches. The circle is divided into 24 equal parts.This system already existed by the Western Han dynasty. In the Song dynasty, the geomantic compass (luópán) further subdivided each mountain into ten smaller segments (totaling 240 divisions). The Ming dynasty armillary sphere at the Purple Mountain Observatory also uses this 24-mountain
system.In
the Tang dynasty, diviners created the Sixty Dragons (Liùshí Fēn Lóng) system by dividing the circle into 12 branches, then splitting each branch into five segments and pairing them with Yang or Yin Heavenly Stems.Later, to make the Twenty-Four Mountains divide evenly, the Seventy-Two Dragons (Qīshí Èr Lóng) or “Traversing Mountains Seventy-Two Dragons” system was developed (60 + 8 + 4 = 72). The saying “one mountain governs three dragons” refers to this division.(Note: The Sixty and Seventy-Two Dragon systems belong purely to yin-house feng shui and numerology; they are no longer directly related to astronomy.)
Measuring Altitude Above the Horizon
Ancient expressions such as “the sun is three zhang high,” “the sun is three rods high,” or “the moon rises above the willow tips” are common but vague. The Kaiyuan Zhanjing cites the Shishi Xingjing (Star Catalog of Shi Shen from the Spring and Autumn-Warring States period), showing that angular measurement instruments or methods already existed by then.Ancient maritime navigation had the saying “guiding the stars across the ocean.” Several methods were used to measure stellar altitude:
Qian Xing Chi (Star-Measuring Ruler): Measured in chi (feet), cun (inches), and fen. A vertical ruler was held with its base level with the distant sea horizon.
Qian Xing Ban (Star-Measuring Boards): Similar in principle to a modern sextant. A set of 12 square wooden boards of different sizes with a string through the center. The altitude was read by which “finger-width” (zhi) the star aligned with.
Hand Measurement (Shǒu Zhǎng): Using the outstretched right hand at arm’s length to estimate altitude in finger-widths and hand-widths.
Ecliptic Coordinates on the Celestial Sphere
While the ancients preferred equatorial coordinate instruments for observation, they found ecliptic coordinates more convenient when calculating the Sun’s apparent path (ri chán), the Moon’s path (yuè lí), eclipses, and the motion of the five planets.On the ecliptic, they divided the circle into the 12 zodiac signs (Shí Èr Gōng) and the 28 lunar mansions (Èr Shí Bā Xiù). These 28 mansions were grouped into the Four Images (Sì Xiàng / Four Symbols): Azure Dragon, Vermilion Bird, White Tiger, and Black Tortoise.Over time, due to the precession of the equinoxes, the 12 zodiac signs and 28 mansions gradually shifted relative to the solar terms. To avoid this complication, later scholars recommended using the solar terms themselves as fixed reference points rather than tying coordinates to specific constellations.The famous astronomer and philosopher Shao Yong summarized this idea: “The Winter Solstice lies at the center of Zi, and the heart of Heaven does not shift.”