Zhang Heng

Zhang Heng was a Chinese scholar-official of the Eastern Han dynasty who excelled as an astronomer, mathematician, engineer and poet. Serving at the imperial court in Luoyang in the early 2nd century, he built a water-powered armillary sphere, catalogued some 2,500 stars and in 132 presented the first known seismoscope, a device to detect distant earthquakes. His rhapsodies on the Han capitals were admired as literature for centuries.

Zhang Heng was born in 78 at Xi'e, a town in Nanyang Commandery, in present-day Henan. His family was distinguished but not wealthy; his grandfather had been a commandery governor who supported Emperor Guangwu's restoration of the Han dynasty. His father died when he was ten, and he was raised by his mother and grandmother.

In 95 he left home to study in the two great cities of the empire, Chang'an and Luoyang. At the imperial academy in Luoyang he became learned in the classics, and he modestly declined offers of official posts. At 23 he returned home as an officer under the governor of Nanyang, drafting documents and inscriptions while composing long rhapsodies about the capitals.

In 112 Emperor An, who had heard of his mathematical skill, summoned him to court. Zhang became Chief Astronomer, serving from 115 to 120 and again from 126 to 132. The post involved recording celestial events and omens and preparing the calendar. His outspoken views on apocryphal texts and on the dynasty's early history kept him from the prestigious office of court historian.

Zhang was the first person known to drive an armillary sphere, a model of the celestial sphere, by water power, using the flow of a water clock to turn it. He also improved the inflow water clock by adding a compensating tank to keep the flow steady. In his treatise The Spiritual Constitution of the Universe, written around 120, he described the cosmos as shaped like an egg, with the Earth at the centre like a yolk. He recorded about 2,500 bright stars and 124 constellations, explained that the Moon shines by reflected sunlight, and discussed solar and lunar eclipses.

His most famous device, presented to the court in 132, was a bronze vessel containing a pendulum. When the ground shook, a ball dropped from the mouth of one of eight dragons into a toad below, showing the direction of the earthquake. The official history of the Later Han records that it once registered a tremor no one in the capital had felt, and that a messenger soon arrived reporting an earthquake in Longxi, in modern Gansu, in the direction indicated.

Zhang was equally celebrated as a writer. His Western Metropolis and Eastern Metropolis rhapsodies, modelled on works by the historian Ban Gu, described the splendour of the capitals but warned that the Eastern Han could decline into decadence like its predecessor. His Four Sorrows is among the earliest Chinese poems with seven characters to a line. Several of his works were later preserved in the Selections of Refined Literature, compiled by a Liang crown prince in the 6th century.

Under Emperor Shun he rose to Palace Attendant and urged the emperor to curb the power of the court eunuchs, who in turn slandered him. He eventually left the central court to administer the kingdom of Hejian, in present-day Hebei, and then returned briefly to Nanyang. Recalled to the capital in 138, he died there the following year.

Later Chinese inventors built on his work; the 11th-century official Su Song cited Zhang's armillary sphere as the inspiration for his astronomical clock tower, and the scholar Shen Kuo developed the theory of the spherical Sun and Moon. Some modern scholars compare his astronomy to that of his Greek contemporary Ptolemy. A lunar crater, an asteroid and a Chinese earthquake-research satellite launched in 2018 bear his name.

Zhang Heng lived under the Eastern Han dynasty in China. Our 100 map shows the Han empire at the time of his career, and the 200 map shows it in decline after his death.