Zhang Heng

Zhang Heng was a scholar-official who served the Eastern Han dynasty, which ruled China from its capital at Luoyang between 25 and 220 CE. Today he is remembered above all as a scientist and inventor. In his own time and for centuries afterwards he was just as famous as a poet.

He is often called a polymath, a person with expert knowledge in many fields. He worked on astronomy, mathematics, calendars, mechanical engineering, geography and map-making, and he wrote long rhapsodies (fu) and short lyric poems that were collected in the most important anthology of early Chinese literature.

His best-known invention, presented to the court in 132 CE, was a seismoscope: an instrument that reacted to earthquakes and showed their direction. Historians generally regard it as the earliest known device of its kind in the world.

Zhang Heng lived at almost the same time as the great Greek astronomer Ptolemy in Roman Egypt. The two never knew of each other, but modern historians sometimes compare them as the leading astronomers of the two great empires at either end of Eurasia.

Zhang Heng was born in 78 CE in Xi'e, a town in Nanyang Commandery in what is now Henan Province. His family was respected but not rich; his grandfather had been a commandery governor who supported the restoration of the Han dynasty under Emperor Guangwu. His father died when he was about ten, and he was raised by his mother and grandmother.

In 95, aged about seventeen, he left home to study in the two great cities of Chang'an and Luoyang, where he attended the Imperial Academy (Taixue). He became friends with notable scholars such as the mathematician and calligrapher Cui Yuan, the classicist Ma Rong and the philosopher Wang Fu. He was offered official posts but at first turned them down.

Around 100 he returned to Nanyang as an Officer of Merit, writing official documents for the local governor. There he spent years composing his rhapsodies on the capitals, and around the age of thirty he began serious study of astronomy and mathematics.

In 112 Emperor An summoned him to Luoyang because of his reputation in mathematics. He served twice as Chief Astronomer (115–120 and 126–132), then rose to become Palace Attendant, a close adviser to Emperor Shun. After clashing with the powerful palace eunuchs, he left the capital in about 136 to serve as chancellor of the kingdom of Hejian in the north. He was recalled to Luoyang in 138 and died there in 139.

Earthquakes were a serious concern for the Han state. They destroyed towns, and they were also read as signs from Heaven that something was wrong with the government. Knowing quickly where an earthquake had struck helped the court send relief and respond to the omen.

In 132 Zhang Heng presented a device called the houfeng didong yi (候風地動儀), often translated as "instrument for measuring the seasonal winds and the movements of the earth". According to the Book of the Later Han, it was a large bronze vessel shaped like a wine jar, about two metres across, with a domed lid. Eight dragon heads faced outward around it, one for each main direction, each holding a bronze ball. Below each dragon crouched a bronze toad with its mouth open.

Inside was a central column (duzhu) connected to eight arms or tracks. When the ground shook, the mechanism caused one dragon's mouth to open and drop its ball into the toad below with a loud sound. The direction of that dragon indicated where the earthquake had come from.

The same history tells the famous story that one day a ball fell although no one in Luoyang felt anything. Officials doubted the machine until, several days later, a messenger arrived reporting an earthquake in Longxi, in modern Gansu, some hundreds of kilometres to the northwest. After that, the story says, the court recorded the direction of earthquakes using the instrument.

As Chief Astronomer, Zhang Heng was responsible for observing the sky, recording omens, preparing the calendar and advising which days were lucky or unlucky. In Han China astronomy was both a science and a part of government: the movements of the heavens were believed to reflect and respond to events on earth.

In his treatise Spiritual Constitution of the Universe (Lingxian, 靈憲), written around 120, he described the cosmos according to the huntian ("enveloping heaven") theory: the heavens are like a hen's egg, round as a crossbow pellet, and the earth sits inside like the yolk. This competed with the older gaitian theory, in which heaven was a canopy above a flat earth.

Zhang wrote that the Moon, like water, has no light of its own and shines only by reflecting the Sun, so that the part facing the Sun is bright and the other part is dark. He explained lunar eclipses as the Moon passing into a region where the Sun's light is blocked. He also compiled a star catalogue recording about 2,500 stars in 124 constellations, and noted that there were thousands of fainter ones.

He took part in court debates on the calendar. In 123 he opposed a proposal to reform the calendar according to prophetic "apocryphal" texts (chenwei), arguing that they were unreliable, and he even proposed that such texts be banned. The calendar was not changed, but his proposal to ban the texts was rejected, and his outspoken views made him enemies.

An armillary sphere is a model of the sky made of rings, each representing a great circle such as the celestial equator or the path of the Sun (the ecliptic). Chinese astronomers had added an equatorial ring by 52 BCE and an ecliptic ring by 84 CE. Zhang Heng completed the instrument by adding horizon and meridian rings.

His most remarkable step was to make it move by itself. Using water flowing from a water clock and a waterwheel with gears, his sphere turned slowly so that it matched the rotation of the real sky. An observer indoors could call out which stars were rising or setting, and it would agree with those watching outside. Zhang is the first person known to have used water power to drive an astronomical instrument.

He also improved the inflow water clock (clepsydra). In a simple clock, water drips from a tank into a receiving vessel with a floating rod that marks the time. As the upper tank empties, the water pressure falls and the clock slows. In writings from around 117, Zhang described adding an extra compensating tank between the reservoir and the receiver to keep the flow steady.

These ideas had a long future. Later Chinese astronomers built ever more elaborate water-driven instruments, leading to Yi Xing's escapement mechanism in the 8th century and to Su Song's great astronomical clock tower in the 11th century, which explicitly named Zhang's armillary sphere as a model.

Zhang Heng worked on the value of pi, the ratio of a circle's circumference to its diameter. For a long time Chinese mathematicians had simply used 3. Working with the volumes of a cube and an inscribed sphere, Zhang arrived at the square root of 10, about 3.162. He also used a value of about 3.17 in astronomical calculations. Later mathematicians such as Liu Hui (3rd century) and Zu Chongzhi (5th century) reached far more accurate values.

He was also a map-maker. Sources record that he presented a topographical map to the court in 116. Some historians, including Rafe de Crespigny, credit him with introducing a rectangular grid for Chinese maps, an idea later described fully by the cartographer Pei Xiu in the 3rd century. The evidence is thin, however, and other scholars are more cautious.

Later texts also credit Zhang Heng with a south-pointing chariot, a two-wheeled cart whose gears kept a figure pointing south, and with an odometer cart that struck a drum after every li travelled. These attributions are uncertain. The south-pointing chariot was certainly built by Ma Jun in the 3rd century, and the odometer seems to have developed gradually among court craftsmen.

It is also likely that Zhang designed rather than personally cast his instruments. The historian Anthony Barbieri-Low argues that an official of his rank would have worked with skilled artisans and moulders in the imperial workshops, who deserve part of the credit.

To his contemporaries, Zhang Heng was first of all a writer. His paired rhapsodies on the Western Capital (Xijing fu) and Eastern Capital (Dongjing fu), modelled on Ban Gu's earlier "Two Capitals" rhapsodies, describe Chang'an and Luoyang in rich detail: palaces, markets, hunting parks, festivals and entertainments. Unlike Ban Gu, Zhang used the splendour of the old capital as a warning: luxury and decadence had ruined the Western Han and could ruin the Eastern Han too.

His Rhapsody on the Southern Capital celebrated his home region of Nanyang. In the Rhapsody on Contemplating the Mystery (Sixuan fu), written when he was under attack at court, he imagines a spiritual journey through the cosmos, echoing the ancient poem Li Sao, and asks whether a good man should withdraw from a corrupt world or stay and remain upright.

Two shorter works were especially influential. Returning to the Fields (Guitian fu) is a brief rhapsody expressing the wish to leave office for a simple life in nature, a theme that became central to later Chinese poetry. The Four Sorrows (Sichou shi) are among the earliest Chinese poems with seven characters per line; on the surface love poems, they were read as laments about being kept apart from the ruler by petty men.

Several of his works were preserved in the Selections of Refined Literature (Wen xuan), compiled by the Liang prince Xiao Tong in the 6th century, which guaranteed that later generations would read them. The poet Tao Qian praised Zhang Heng for restraint and simplicity of language.

Zhang Heng's official career shows the tensions of the Eastern Han court. Emperors often came to the throne young, and power shifted between the families of empresses, palace eunuchs and scholar-officials. Zhang belonged firmly to the scholar-official camp, and his principled positions cost him promotion.

Because of his views on the apocryphal texts and his opinion that the short-lived Gengshi Emperor deserved more credit in the history of the Han restoration, he was excluded from the committee writing the official history of the dynasty, and never achieved the prestigious post of court historian he seems to have wanted.

After an earthquake in 133 he submitted a memorial criticising new rules for recruiting officials. Soon afterwards he was made Palace Attendant, and he used his closeness to Emperor Shun to warn him against the eunuchs. The eunuchs slandered him, and he responded in literature, including the Response to Criticism (Yingjian), in which an imaginary visitor urges him to seek power and he replies that a gentleman values virtue above office.

Zhang Heng was honoured soon after his death. His friend Cui Yuan wrote the inscription for his funeral stele, praising calculations that "exhausted the heavens and the earth" and inventions that rivalled the work of the gods. In the 3rd century the official Xiahou Zhan wrote another inscription for his tomb, and the writer Fu Xuan complained that talents like Zhang Heng and the engineer Ma Jun had never been given posts in the Ministry of Works, where their skill could have benefited everyone.

His instruments had long afterlives. After the fall of the Western Jin, his old armillary instruments were recovered when Liu Yu, the future founder of the Liu Song dynasty, captured Chang'an in the 410s, and in 436 the astronomer Qian Lezhi was ordered to build a new water-powered celestial globe on Zhang's model. Craftsmen of the Northern Qi and Sui dynasties built new seismoscopes inspired by his.

Outside China, the seismologist John Milne, one of the founders of modern seismology in Japan in the 1870s and 1880s, wrote about Zhang Heng's instrument, and it became a standard example in histories of science. Today his name is carried by a lunar crater, the asteroid 1802 Zhang Heng, the mineral zhanghengite, and China's first seismo-electromagnetic satellite, Zhangheng-1, launched in 2018.

The only detailed description of the seismoscope is a short passage in Zhang Heng's biography in the Book of the Later Han, compiled by Fan Ye more than three centuries after his death. The instrument itself was lost; by the time of the Yuan dynasty, the scholar Zhou Mi noted that none of the seismoscopes built by later imitators survived either.

The central problem is the "central column" (duzhu). Most reconstructions take it to be a pendulum, but of which kind? The seismologist John Milne and, in 1936, the Chinese scholar Wang Zhenduo proposed a hanging pendulum that would swing toward one of the eight tracks. In 1939 the Japanese seismologists Akitsune Imamura and Hagiwara instead proposed an inverted pendulum, a standing column that falls in the direction of the first shock, arguing that a swinging pendulum would knock out more than one ball. Wang Zhenduo's later model also used a standing column. In 2005 a team of Chinese seismologists announced that they had built a working replica.

Engineers such as Hong-sen Yan have noted that no modern replica has achieved the sensitivity described in the historical account, which implies the device could detect a distant earthquake that people nearby could not feel. The Longxi story has also been questioned: some writers link it to a large Gansu earthquake recorded in the 130s or 140s, but its date and details cannot be confirmed from independent records.

Most historians therefore accept that Zhang Heng built a real instrument that responded to ground motion and indicated a direction, while treating the claims about its sensitivity and accuracy with caution. It is best described as a seismoscope, which shows that shaking happened, rather than a seismometer or seismograph, which measures and records it.

Zhang Heng is sometimes presented in popular works as a "Chinese Leonardo" or an ancient modern scientist. Historians caution against this. His astronomy was inseparable from omen interpretation: he read comets, eclipses and planetary movements as guidance for the conduct of government, and he accepted the traditional idea that natural disasters could be Heaven's response to misrule. His theory of earthquakes drew on ideas about qi and the "oracles of the winds", not on geology.

Comparisons with Ptolemy illustrate the point. The historians Jin Guantao, Fan Hongye and Liu Qingfeng have argued that Zhang's armillary sphere was physically very close to a geocentric model, but that he did not build a geometric, axiomatic theory like Ptolemy's; Chinese astronomy focused on arithmetic cycles and calendrical prediction rather than geometric models. Others reply that this reflects a different but highly effective tradition rather than a failure.

Joseph Needham's Science and Civilisation in China placed Zhang Heng at the start of a long Chinese tradition of mechanical clockwork, leading through Yi Xing to Su Song. Needham's framework, which asked why modern science arose in Europe rather than China, has itself been widely debated, but his volumes remain a basic reference for Zhang's instruments.

Finally, Zhang Heng's reputation has a history of its own. In the 20th century he became a national symbol of Chinese scientific achievement, pictured on postage stamps, featured in textbooks, and honoured with the name of an earthquake-monitoring satellite launched in 2018. Separating the historical Zhang Heng from these modern images, while recognising his genuine brilliance, is part of the historian's task.