Zhang Heng

Zhang Heng the Chinese astronomer, mathematician, inventor and man of letters of the Eastern Han dynasty (78-139 AD), best known for his seismoscope, an instrument that detected the direction of distant earthquakes, and for his star catalogue and his account of the motions of the heavens. He stands among the most versatile scientists of the ancient world.

Early Life and Education

Zhang Heng was born in 78 AD in Nanyang, in present-day Henan, the son of an official family of modest means. He was a gifted student and travelled to the capital of Luoyang to complete his education and, later, to Chang'an, the old western capital, where he studied the monuments and the machinery of the former dynasty. He was also an accomplished writer of prose and poetry.

He held a series of official posts, including a place at the observatory and the office of court astronomer, and he was renowned for his learning across many fields. He wrote on astronomy, mathematics, geography and literature, and he was admired both as a scholar and as an administrator of integrity, though he declined some of the highest offices that were offered to him.

His curiosity ranged over the whole of nature, and he combined the speculative interests of the philosopher with the practical skills of the craftsman. He was, in the fullest sense, a polymath, and his work touched nearly every branch of knowledge that his age pursued.

Astronomy and the Armillary Sphere

Zhang Heng made fundamental contributions to Chinese astronomy. He maintained that the heavens are spherical and that the earth is a sphere of smaller size within them, and he gave a reasoned account of the apparent motions of the sun, moon and stars. He estimated the number of visible stars at about two thousand five hundred, and he catalogued and named many of them, giving the Chinese sky a systematic inventory.

He argued that the moon derives its light from the sun, and he gave the earliest clear Chinese explanation of eclipses, holding that a lunar eclipse occurs when the earth comes between the sun and the moon and casts its shadow upon it. This was a decisive step away from the view that eclipses are omens or the actions of spirits, and toward a physical account of celestial phenomena.

He improved and made use of the armillary sphere, the framework of rings that models the celestial sphere and enables observers to measure the positions of the stars. He is credited with constructing a geared sphere, driven by water, that rotated to display the movements of the heavens, an instrument that would have represented the turning of the sky in the observatory and impressed visitors with its ingenuity.

The Seismoscope

Zhang Heng's most famous invention is the seismoscope, described in the dynastic history and built in 132 AD. It was a large bronze vessel, shaped like a jar, with eight dragons arranged around its outer surface and facing the eight directions of the compass. Each dragon held a bronze ball in its mouth, and beneath each dragon sat a bronze toad with its mouth open to receive it.

The device was designed with a mechanism inside, sensitive to the first tremor of an earthquake. When a quake occurred, the mechanism released the ball from the dragon that faced the direction of the disturbance, and the ball fell with a sound into the mouth of the toad below. The instrument thus indicated not merely that an earthquake had happened but the direction from which its shock had come.

The history records that one day the device released a ball although no tremor was felt in the capital, and that sceptics mocked it as useless. Some days later a messenger arrived with the news that an earthquake had struck in the west, and the accuracy of the instrument was vindicated. The account has made Zhang Heng's seismoscope one of the most celebrated instruments of the ancient world, a monument to carefully reasoned mechanical design.

Cartography, Mathematics and Literature

Zhang Heng worked on the mapping of the empire and on the mathematics that underlay measurement and calculation. He is associated with an estimate of the ratio of the circumference of a circle to its diameter, using an approximation that, while not exact, was among the most accurate available in his time. He also studied the problems of calculating distances and volumes.

He was a skilled poet and prose writer, and his compositions on the two capitals are among the notable literary works of the Han period. He wrote on the pleasures and duties of scholarly life, and his essays display a wide and humane culture. That he was both a maker of instruments and a master of literary form marks the breadth of the ideal of learning in his age.

He was concerned with the practical welfare of the people as well as with the heavens. He wrote about the administration of the state, the state of agriculture and the conduct of officials, and he served in provincial posts where he attended to the needs of the population. His many-sided career embodied the classical ideal of the scholar who serves both knowledge and the common good.

Influence and Legacy

Zhang Heng is honoured as one of the greatest scientists of ancient China. His star catalogue, his explanation of eclipses, his armillary and water-driven spheres, and above all his seismoscope advanced the understanding of the natural world and demonstrated the power of careful observation joined to mechanical invention. He brought the study of the heavens into the domain of reasoned, measurable inquiry.

Later Chinese scholars regarded him as a model of the complete man of learning, equally at home in astronomy, mathematics, literature and government. Temples were raised in his honour, and his name was invoked in later centuries as an emblem of scientific achievement. The craters on the moon and other features have been named after him in modern times, a tribute to his standing among the scientists of the ancient world.

He died in 139 AD, leaving a body of work that long outlasted him. His career shows how much a single gifted individual could contribute to knowledge in an age before the modern separation of the disciplines, and it illustrates the vigour of Han science at its height. He remains a central figure in the history of Chinese astronomy and invention.

See also

• Seismoscope
• Armillary sphere
• Chinese astronomy
• Han dynasty