PureInsight | July 15, 2002
Biography of Zhang Heng
Zhang Heng, alias Pingzhi, was born at Shiqiao town, Xie County, Nanyang Shire, in A.D. 78 (25 km north of the current Nanyang city, Henan province). At the age of sixteen, he left his hometown to travel around China and study. He met many famous scholars. Once he went to Chang’an, the old capital of the Han Dynasty. There he visited local scenic and historical sites, and investigated the terrain, products, customs and human relationships in the surrounding mountainous area. Later, he went to Luoyang, the capital city of the East Han Dynasty, and studied at Taixue, its the highest institution.
Zhang had a particular interest in literature. He wrote many literary works using different styles, notably including Going Back to the Field, Two Capitals, Four kinds of Melancholy, and A Song of Simultaneous Sounds. In A.D. 111, Zhang Heng was called to serve in the government. He successively served in junior and senior officer positions, including with Longzhong, Taishiling and Gongche Shimaling. Among these, his longest period of service, fourteen years, was with Taishiling. Taishiling was an officer in charge of observing astronomical phenomena, compiling calendars, forecasting weather, and generally arranging for meteorological and temperature studies. When he was assigned to this duty, Zhang carried out a detailed study of the astronomical calendar and made major contributions.
According to his knowledge and actual observations of the moving patterns of celestial bodies, Zhang Heng created an apparatus called the armillary sphere that accurately demonstrated the main points of the Hun Theory (theory that says the sky was like the shell of an egg and the earth, the yolk). He was an expert in astronomy and calendar calculation. He wrote a lot of astronomical books, including Lingxian, The Drawing of Lingxian, and The Chart and Interpretation of Armillary Sphere. He was one of the representatives of the Hun Theory in the mid-term of the East Han Dynasty.
Astronomy Book --- Lingxian
Lingxian was the most famous of the books written by Zhang Heng. It is an astronomy book that describes the development and movements of Heaven, Earth, Sun, Moon and stars. In the book Lingxian, Zhang proposed that the dimensions we can observe are limited and the dimensions we can not observe are unlimited and ceaseless. His work clearly proposed the theory that the universe is infinite in both space and time.
In Lingxian, Zhang Heng indicated that the moon itself does not emit light but rather reflects sunlight. He thought the sun and the moon were just like fire and water. Fire can emit light and water can reflect light. He pointed out that the generation of moonlight was due to the reflection of sunlight and that moonlight cannot be observed in the daytime because it is overpowered by sunlight. At the same time, Zhang also explained the occurrence of the lunar eclipse. He believed that when the moon was full, we should be able to see the full moon; but there would be times when we could not. This was because the earth then covered sunlight. He called the shadow of the earth “Anxu” and suggested that when the moon passes the place of “Anxu”, a lunar eclipse would occur. His explanation of the principle of the lunar eclipse was very insightful.
Additionally, in Lingxian, Zhang Heng also calculated the angular diameter of the sun and the moon, and recorded 2,500 stars that he observed while in Luoyang, calculations that are very close to the results modern astronomers have observed. In another astronomy book, The Chart and Interpretation of Armillary Sphere, he measured one solar year as “365 and a quarter degrees”, which is very similar to the figure that modern astronomers calculated with 365 days, 5 hours and 48 minutes and 46 seconds.
In Lingxian, Zhang Heng used some modern terminologies including equator, elliptic, South Pole and North Pole. He also drew the first complete star chart in China, including 2,500 stars. According to Lingxian, “There are 124 stars which are always bright and 320 stars which have names. The total number of stars is 2500. There are still some stars which are not included”. The star chart Zhang Heng made not only greatly surpassed its predecessors, but also was the leading chart for a long time after. During the last period of the Han dynasty, China was in disorder and the star chart Zhang Heng made was lost. In the beginning of the Jin dynasty, the star chart Chen Zhuo made included 1,464 stars, which was only half of the stars charted by Zhang. It wasn’t until the Kangxi Emperor of the Qin Dynasty that a more complicated star chart was created using a telescope, and this one included more than 3,000 stars.
Armillary Sphere and Houfeng Seismograph
Armillary Spheres (Ming Dynasty)
In A.D. 117, Zhang Heng created the world’s first armillary sphere and it was driven by copper kettle gears. The armillary sphere had an outside sphere and an inside sphere, both of which rotated. On the surface was carved the South Pole, the North Pole, the equator, elliptics, the 24 solar terms, the sun, the moon, and the stars. The positions of the sun, the moon and the stars, as well as the state of their orbits were consistent with their actual positions in the universe.
Zhang Heng Seismograph
In A.D. 132, Zhang Heng invented the Houfeng Seismograph that was made of fine copper in the shape of a wine pot. Eight dragons were mounted on the surface. The heads of these dragons pointed out the eight directions of east, south, west, north, northeast, southeast, northwest and southwest. Each dragon had a copper ball inside its mouth and a frog squatting under its head. When an earthquake occurred, the mouth of the dragon pointing in the direction of the earthquake would open automatically, and the copper ball would fall into the mouth of the corresponding frog. The instrument staff could then record the time and direction of an earthquake immediately. In A.D. 138, this seismograph accurately detected an earthquake that occurred in the Longxi Shire. The seismograph Zhang Heng invented was the first apparatus in the world that could measure the direction of an earthquake, and this was 1700 years earlier than the European seismograph. Zhang Heng also invented an odometer that had a figure that struck a drum as each li (0.5 km) went by to measure distance, a compass vehicle in which a figure always indicated the southerly direction, an ancient Chinese sundial to measure the position of the sun, and a wooden flying bird, among other things. He also estimated pi to be the square root of 10, wrote more than thirty books in astronomy and literature and made great contributions to the calendar, arithmetic, literature and art.