A Critique of ‘Zhang Heng’s Theory of a Spherical Earth’

Abstract

In his annotations to Commentary on the Armillary Sphere (Hun Tian Yi Zhu) Zhang Heng (78–139) explained the Celestial Sphere theory, commenting “heaven is like an egg and the earth is like the yolk of the egg.” On the basis of this remark, many people have argued that Zhang Heng already realized that the earth is spherical. For the convenience of discussion, we call this view ‘Zhang’s theory of a spherical earth.’ After systematically analyzing the relevant literature, Tang Ruchuan pointed out in 1962 that this “theory of a spherical earth” is only a superficial “false appearance” and the Celestial Sphere theory actually refers to a spherical heaven and a flat earth.1 Zheng Wenguang argued that the cosmic model of the Celestial Sphere theory is a hollow heavenly ball, half-filled with water on which the spherical earth floats.2 He regards the theory of Huang Renyu of the Ming dynasty, who compared “the shape of heaven and earth” to a “pig bladder” half of which is “full of water” with “a big dry soil pellet” within it as a “theory of a spherical earth,” It’s worth noting that Zheng Wenguang’s viewpoint has become pervasive among historians of science. The present article disagrees and tries, by taking a broad perspective, to determine whether, in Zhang Heng’s mind, the earth was spherical or flat.