It is said in the ancient times, Gong Gong (a half man and half snake monster) and Zhuan Xu (a god) had a hard fight for control of the heaven, with Zhuan Xu emerging as the victor. Exasperated by failure, Gong Gong went to the northwest corner of the earth and knocked down the Buzhou Mountain, which was one of the eight pillars supporting the sky. As a result, the northwest sky collapsed and the southeast earth sank. Subsequently, the sun, moon, and stars in the sky all slid to the northwest, while the water and silt on the earth flowed to the southeast.

Whenever encountered with natural phenomena they could not explain, the ancient Chinese people tended to create various legends with their imagination to show their conjectures. The abovementioned is a legend showing their speculation on the cosmic structure, on which disputes were abundant.

The period between the 3rd and 6th centuries witnessed enormous development in astronomy, with the rise of many theories on the cosmic structure. The most important of these were the theory of canopy-heavens, the theory of sphere heavens, and the theory of expounding appearance in the night sky.

  The Theory of Canopy-Heavens

The oldest theory on cosmic structure, the early school of canopy-heavens claimed that the heaven, shaped like a big pan, covered the earth, shaped like a chessboard.

According to the theory, the sun moves all year round along seven paths, called Qi Heng, and the innermost one was called Nei Heng (Inner Path), along which the sun moved on the Summer Solstice (around June 22-23). The outermost one was called Wai Heng (Outer Path), and the moved along on the Winter Solstice (around December 22-23). During other solar terms, the sun moved along the middle five paths.

Advocators of this school also claimed the sunlight could only reach a maximum distance of 167,000 li (83,500 km), and nothing could be seen beyond. Therefore, they said, the sun moved within the said distance during the day, and beyond during the night.

  The Theory of Sphere-Heavens

In his book Note to the Armillary Sphere, Zhang Heng, a famous astronomer in the Eastern Han Dynasty (25-220)，wrote that the heaven was like an egg, with the earth as the yolk inside, and that the heaven wrapped up the earth just as the eggshell did the yolk. This represents a classic elaboration on the theory of sphere-heavens.

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