Zhang Heng: Great Chinese Inventor

By David Wu
David Wu
David Wu
August 6, 2013 Updated: August 6, 2013

“Ding, dong!” a bronze ball fell from the dragon’s mouth into the frog’s mouth. In A.D. 138, during the golden age of the Eastern Han Dynasty (A.D. 25–220), this sound attracted the attention of everyone in the imperial palace.

The dragon spout and the frog receptacle are part of the world’s first earthquake detector, or seismometer, invented by Zhang Heng (張衡)(A.D. 78–139), a renowned scientist, mathematician, painter, and poet of ancient China. Because of the depth and breadth of his talents and contributions, he is considered the Chinese counterpart of Leonardo da Vinci.

Zhang’s device is in the shape of a jug and made of copper. On the surface of the jug, there are eight dragons hanging upside down. There is a bronze ball in each dragon’s mouth. Under each dragon, there is an open-mouthed frog facing up toward the dragons. The heads of the eight dragons point to the directions of east, west, north, south, northeast, northwest, southeast, and southwest.

As the Earth Shakes
When an earthquake happens in a certain area, the wave of the earthquake triggers a pendulum-like device inside the seismometer, and the pendulum sways toward the direction where the quake wave originates. Then, a lever that connects to the dragon of that direction makes the dragon’s mouth open, and a bronze ball drops out.

On that day in A.D. 138, the sound of the bronze ball dropping caused a stir among all the imperial officials in the palace. “It’s Zhang Heng’s seismometer! Is it true that something has happened?”

According to the direction in which the dragon that dropped the ball was oriented, it was determined that the quake had occurred to the west of Luoyang, the capital city. Since no one had sensed anything in Luoyang proper, people were skeptical.

However, a few days later, a messenger from the western Long region (today, southwest Gansu province), which was west of Luoyang, reported that there had been an earthquake there. As it happened exactly the same time that the seismometer was triggered, people were greatly impressed by Zhang Heng’s instrument.

It was the first earthquake detector in human history and Zhang Heng is regarded worldwide as a pioneer of seismic research.

From then on, Eastern Han imperial historians recorded earthquakes happening all over the country. Today, from an advanced modern science and technology point of view, the seismometer Zhang Heng invented is still considered amazingly refined and remarkable.

Zhang Heng was also a mathematician and he eventually calculated pi (π) as being between 3.1466 and 3.1622. Though this is slightly different from the pi that we recognize today, considering it was 1,800 years ago, his level of accuracy is an astonishing achievement.

Understanding the Universe
There are two ancient Chinese theories regarding the shape of the Earth and its position in the universe. One, Kai Tian, is that the Earth is believed to be flat and the sky to be round, like a cover above the Earth. Another is the theory of Hun Tian, which states that the Earth is inside the sky, just like a yolk is inside an egg.

Zhang Heng was a proponent of the Hun Tian theory, He wrote: “The sky is like a hen’s egg, and is as round as a crossbow pellet; the Earth is like the yolk of the egg, lying alone at the center. The sky is large and the Earth small.”

He thought that Heaven and Earth were supported by air and floated in water. Although he believed that Heaven had a solid external shell, he did not think that this shell was the boundary of the cosmos. He believed that the cosmos outside of the shell was infinite in terms of space and time.

In the beginning of the book Ling Xian (靈憲), an exposition of astronomical theories, Zhang attempted to explain the origin of Heaven and Earth and issues of evolution. He believed that before Heaven and Earth were separated, everything was chaotic. Once they were separated, the light substances rose to form the heavens and the heavy substances coagulated to form the Earth.

Heaven contained the qi of yang and the Earth contained the qi of yin. When the two kinds of qi interacted with each other, all things in the universe were created. The qi that was ejected from the Earth formed the stars.

Zhang believed that stars moved slowly when they were close to Heaven, and those farther from Heaven moved faster. In other words, he tried to explain the speed of the planets in terms of the distance between them and their suns.

A Spiritual Foundation
In ancient China, quite distinct from modern Western empirical science, the traditional Chinese approach to the study of the Earth, life, and the universe was from a unified perspective, which integrated spirituality.

Zhang Heng’s great scientific achievements were inspired by his spiritual experiences, which were also expressed in his literary works.

“Contemplating the Mystery,” one of Zhang Heng’s famous poems, was inspired by his spiritual wonderings and dreams and helped to shape his view of the universe. Some researchers believe that the egg yolk metaphor of the Earth’s place in the universe is what Zhang Heng actually saw in a dream or during his spiritual travels into space.

The history recorded during the Eastern Han Dynasty contains an extensive biography of Zhang Heng. According to this biography, he wrote 32 articles in the disciplines of science, mathematics, philosophy, and literature. The biography contains full quotations of two of his poems: “Leisure” and “Contemplating the Mystery.”

The two poems indeed reflect Zhang Heng’s frame of mind. The former indicates his attitude toward academic research, whereas the latter is a rare piece about astral travel.

Zhang Heng’s scientific achievements were honored by later generations. A golden yellow mineral discovered in 1986 was named after him. In 1970, the United Nations named a lunar crater, the Chang [Zhang] Heng, after him. And in 1977, asteroid 1802 was also named after him, a well-deserved recognition for this great Chinese astronomer.

David Wu
David Wu