A monumental astronomic work, Ling Xian, translated as Mystical Laws, is a distillation of the years of practice and theoretical research by Zhang Heng, a famous astronomer of the Eastern Han Dynasty (25-220). Dealing with such topics as the formation and structure of the heaven and earth and the cosmic evolvement as well as the nature and movement of celestial bodies, the book promoted the level of ancient China's astronomical level to a new high and cast a profound impact on the following generations.

In terms of the formation of the heaven and earth, Ling Xian represents the theory of sphere heavens, believing the heaven, the earth, and all the other things have their origins in primitive and chaotic yuanqi (vitality).

Based on the theory of sphere heavens, the book elaborates on the lunar eclipse. As a explanation of the phenomenon, Zhang Heng wrote in the book, "The moon itself is not luminescent, but rather reflecting the sunlight. The part of the moon where the sunlight fails to reach will not give out light. If the moon moves into the shadow of the earth, a lunar eclipse occurs." His explanation was, and still is, indeed quite scientific.

In relation of the finity and infinity of the cosmos, Zhang Heng compared the heaven to an eggshell, and the earth to the egg yolk. He viewed the cosmos that people could see with the naked eye as limited while the part that could not be seen as infinite.

Zhang recorded his measurement of both the sun and the moon's angular diameter, which is 1/736 of the celestial sphere, or rather 29°21″(or 29'21"), an absolute difference of 2°from the average value of 31°5″(or 31'5"). Considering the scientific level and observatory conditions, the number is rather precise.

Based on his careful observation of the celestial bodies, Zhang Heng sorted out the various star charts made by his predecessors and re-established a new chart, recording the position of over 3,000 stars.

Concerning the movement of the five planets, Zhang wrote: "The sun, the moon and the five planets all move in-between the heaven and earth, rather than on the wall of the celestial sphere. Moreover, the seven celestial bodies move at different speeds."

Ling Xian, as one of the most outstanding astronomic works in the history of ancient Chinese astronomy, represents a milestone in the development of Chinese astronomy. Its importance is not lessened at all by its mistakes and shortcomings.

Author: Jessie