Zhang Heng

• Born: 78 AD
• Birthplace: Nanyang, China
• Died: 139 AD
• Place of death: Luoyang, China

Background

Zhang Heng (or Chang Heng) was a Chinese scientist, geographer, poet, and literary scholar of the Eastern Han Dynasty (206 BCE–220 CE). Zhang is one of the few Chinese intellectuals whose work survived the changing of dynasties following the Eastern (or Later) Han. Zhang's best known innovations include the seismometer, the odometer, and a model of the celestial sphere.

Zhang was born in 78 CE, in Xie County of Nayang (modern Nanyang County in Henan Province). He was born into an intellectual family and received an education in literature and philosophy.

Around 94 CE, Zhang moved to the capital of Chang'an, located northwest of modern Shangxi Province, to seek higher education. Chang'an was the hub of China's silk and textile trade, and provided Zhang with exposure to literature and learning from all over China. He then moved to Luoyang, a city in Henan Province, where he attended Luoyang Taixue, one of China's first institutions of higher education. Zhang studied literature, poetry and philosophy, and first attained fame as a literary figure during his time there.

Zhang entered the service of the imperial government in 111, and worked as an assistant to the court historian. Records from this era describe Zhang as lacking ambition, having refused advancement on several occasions to remain in lower posts, where he had greater freedom to study philosophy and science. He spent more than a decade working as an aide to the court astrologer, Tai Shi-ling, where he learned about stellar navigation and the design of meteorological instruments.

Zhang was eventually promoted to chief astronomer and minister to the Emperor An-ti. During his tenure with the Imperial Chancellery of Astronomy, Zhang revolutionized the study of the cosmos and redesigned the calendar to conform to current scientific knowledge. Some records indicate that Zhang was also active in the legal field as a specialist in Confucian morals and ethics.

When Zhang died in 139 at the age of sixty-one, he was considered China's foremost scientist. In 2003, the Chinese Academy of Sciences announced its design for the Zhang Heng Hubble Telescope—like NASA's Hubble Space Telescope—scheduled to begin operation in 2008.

Literature, Mathematics, and Cosmology

Zhang Heng's first accomplishments were in the fields of literature and art. Many of his poems were collected in the Wen Xuan, the oldest surviving anthology of Chinese literature, originally compiled by the historian Xiao Tong. Among his most famous works are a series of lyric poems called the "rhapsodies" which focused on political and social events. Among his most famous poems are the "Si Chou Shi" ("Four Chapters of Distressed Poems") and his "Four Stanzas of Sorrow."

During the course of his career, Zhang composed literary works ranging from history to erotica. Many of Zhang's writings are accompanied by paintings, and Zhang is considered one of the greatest painters of the Eastern Han Dynasty.

As a mathematician, Zhang's primary interests were geometry and number theory. Zhang studied magic squares, a type of recreational mathematics believed to relate to cosmological phenomena.

Zhang is noted for his attempts to calculate π as the square root of 10, or approximately 3.162. Zhang's calculation is not particularly accurate, but it is unique in that he was the first Chinese mathematician to base his calculations on theoretical rather than practical measurement, making Zhang a forerunner of theoretical mathematics.

Zhang used his understanding of mathematics in the invention of practical devices. He invented the first odometer, a device that records distance. His odometer was a small wooden device that could be mounted on a carriage. The machine used a movable human figure bearing a mallet to strike a wooden drum with the passage of each li (0.5 kilometers).

Many of Zhang's most notable contributions were in the fields of cosmology and astronomy, and are collected in his famous text, the "Lin Xian." Zhang's theories suggest the idea that the universe is infinite in time and space. He proposed the theory that both the earth and the universe were spherical. Zhang also theorized that the shadow of the earth was responsible for phenomena such as the lunar eclipse, and that the rotation of the planets was related to climate change and meteorological phenomena.

Zhang drew China's first star chart, which included more than 100 constellations and over 2,500 stars. Most of Zhang's astronomical theories were based on his invention of the first rotating celestial globe. Zhang's globe was nearly five meters in circumference and rotated using water power. The rotations of the globe were used to measure the movements of celestial bodies. In his book "The Chart and Interpretation of the Armillary Sphere," Zhang used his devices to accurately calculate the length of the year as 365 days.

Seismology

Zhang is believed to have invented the first seismograph in 132. However, modern scientists now understand that Zhang's invention was more accurately termed a seismometer, a device that measures the occurrence and relative location of seismological phenomena but does not measure the amplitude.

Zhang's seismometer was called the huo feng di dong yi (or di dong yi), meaning "instrument for inquiring into the wind and shaking of the earth." It was a bronze urn roughly two meters in diameter. The interior of the machine housed a series of gears and an inverted pendulum that translated subterranean vibrations into mechanical force.

The rim of the di dong yi was lined with eight dragon heads, each of which held a bronze ball in its mouth. Around the base, directly under each dragon head, was a bronze frog. Upon sensing a seismological vibration, the pendulum would strike the side of the urn causing one of the bronze balls to fall into the mouth of a frog. The resulting sound served as an alarm, while the location gave a rough estimate of the location of the event.

Historical records indicate that, in early 138, Zhang's device detected an earthquake which was confirmed several days later by a messenger from Gansu Province, 400 miles from Chang-an. The di dong yi remained in use until the thirteenth century, when the original and all copies of the device were destroyed during the Mongolian invasion.

In the first century, some Chinese scientists attributed earthquakes to subterranean phenomena, while others believed they were caused by cosmic objects, like meteors, impacting the earth. Zhang designed a device to settle this debate, using a water-powered engine. When a seismic event occurred, red ink would spill from several small containers. If the vibration originated from a meteor impact, then only the ink closest to the event would be discharged, whereas if the disturbance occurred beneath the machine, all of the ink would be discharged simultaneously.

Seismologists have long debated the design and function of Zhang's seismometer, and partial, but nonfunctional, reconstructions were designed in Japan in 1875 and in China in 1951. In 2005, researchers from the Chinese Academy of Sciences announced the first working reproduction of the di dong yi. Preliminary testing indicated that the machine functioned as reported in historical texts.

Modern seismographs can also measure the amplitude of a seismic disturbance, and are used to study earthquakes, the movement of tectonic plates, and as an early warning device for detecting nuclear weapons detonation.

Bibliography

Clark, Anthony. Ban Gu's History of Early China, Cambria Press, 2008. eBook Collection (EBSCOhost), search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=498610&site=ehost-live&scope=site&ebv=EB&ppid=pp‗180. Accessed 30 Oct. 2020.

Cullen, Christopher, author. “Restoration and Re-Creation in the Eastern Han.” Heavenly Numbers: Astronomy and Authority in Early Imperial China, 2017, doi:10.1093/oso/9780198733119.003.0007. Accessed 30 Oct. 2020.

De Crespigny, Rafe. “Zhang Heng.” A Biographical Dictionary of Later Han to the Three Kingdoms (23–220 AD), Brill, 2007, pp. 1049–1051.eBook Collection (EBSCOhost), search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=232253&site=ehost-live&scope=site. Accessed 30 Oct. 2020.

Lien, Y. Edmund. “Zhang Heng’s Huntian Yi Zhu Revisited.” T’oung Pao, vol. 98, no. 1–3, 2012, pp. 31–64, doi:10.1163/156853212X634644.

Robinson, Andrew.“Use a Toad to Catch a Snake.” New Scientist, vol. 232, no. 3102, 2016, p. 42, doi:10.1016/S0262-4079(16)32238-2. Accessed 30 Oct. 2020.