Zhang Heng’s Cosmology

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Among the most famous astronomers in ancient China, Zhang Heng of the East Han Dynasty was obviously the most brilliant.

Zhang Heng was a proponent of the Hun theory, which stated that the earth is inside the sky, just like the yolk is inside an egg. 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 hard external shell, he did not think that this shell was the boundary of the cosmos. He believed that the cosmos outside the shell was infinite in terms of space and time.

In the beginning of the book Ling Xian, Zhang tried to explain the origin of heaven and earth and the issues of evolution. He thought that before heaven and earth were separated, it 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. The two kinds of qi interacted with each other, and that’s how things in the universe were created. The qi that was ejected from the earth formed stars. Zhang believed that stars moved slowly when they were close to heaven, and those far from heaven moved faster. In other words, he tried to explain speed of planets in terms of the distance between them and their suns.

Zhang Heng paid attention not only to theoretical studies but also to practice. He himself designed and made an armillary sphere driven by water, which is similar to today’s celestial globe. He also invented the Houfeng seismograph. Geng Shouzhang in the Western Han Dynasty invented a prototype of the armillary sphere, and Zhang Heng improved on it. He used the instrument to demonstrate his theory of Hun. With a gear system, he connected the celestial globe to a timing clepsydra. The water dropped from the clepsydra to make the globe turn on a regular basis. It made a circuit in one day, so people watching the globe indoors would be able to locate celestial bodies at different times. His seismograph, made in 132 A.D., was the first instrument in the world to detect earthquakes. Zhang’s seismograph is recognized by the world as an instrument that was well ahead of his time. To this day, in fact, no one has been able to reproduce it.

Zhang Heng also observed and analyzed many specific celestial phenomena. He calculated that in central China, people could see about 2,500 stars. His studies in regard to lunar eclipses were almost completely accurate. His measurement of the angular diameter of the sun and the moon was 29’24”, which is 1/736 of the celestial globe. This result is very close to the true average angular diameter of the sun and the moon, 31’59”26. We can see that Zhang Heng’s measurement was quite close. Zhang Heng also realized that the sun people saw in the morning, at noon, and in the evening was the same size even though it looked bigger in the morning and evening but smaller at noon. He understood that this was just an optical effect due to the fact that in the morning and evening, the observer is in a rather dark environment. A light object appears to be bigger when it is seen from a dark environment. However, at noon, because it is full daylight and is so bright, the sun in the sky seems to be smaller. It is just like a fire, which seems much larger at night than it appears when seen during the day.

The History of the Eastern Han Dynasty contains a biography of Zhang Heng. According to this biography, he wrote thirty-two articles in the areas of science, philosophy, and literature. The biography fully quoted two of his poems: “Leisure” and “Thinking About Mysteries.” These two poems indeed reflect Zhang Heng’s frame of mind. The former indicates his attitude toward academic research, while the latter is a rare piece about astral travel. In this second poem, he said:

I walked out of the quiet and beautiful “Ziwei Palace” and reached the bright and spacious “Taiwei”;
I let “Wang Liang” drive the “Junma,” striding across the “Gedao”!
I wove a tight “Liewang” and patrolled the forest of “Tianyuan.”
I opened up the “Jugong” and aimed at the “Fenglang”!
I observed the strongly fortified “Bilei” at the “Beiluo” and then heavily beat the “Hegu.”
I boarded the boat of “Tianhuang” and wandered in the vast “Yinhe.”
Standing at the tip of the “Beidou,” I looked back and saw the sun and the moon revolving continuously.

(Note: The words in quotation marks are the ancient Chinese names of the constellations.)

This poem of Zhang Heng’s, “Thinking About Mysteries,” describes how his main consciousness left his body and traveled between stars. This indicates why he was able to know that the earth was a sphere instead of an infinite plane. This also explains why he could propose the theory of Hun, which conformed to the cosmic structure. The method that Zhang Heng used to study the cosmos was totally different from that of today’s scientists. Furthermore, it is clear that Zhang Heng’s achievements were closely related to his conduct and attitude toward research.


China Encyclopedia (Astronomy Section)
A Chronicle of Zhang Heng

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