Though Yu Yu was as thin as firewood, his spirit was vigorous. Wearing a tall cap and broad sash, he knelt behind his desk and watched Chen Caozhi and Ran Sheng enter the thatched cottage one after the other. Chen Caozhi was handsome and tall, refined and scholarly, while Ran Sheng was built like an iron tower, powerfully imposing. With Ran Sheng inside, the thatched study immediately seemed cramped, as though he could overturn the whole cottage with one sweep of his hand.
Once Yu Xiaofu had made the introductions, Yu Yu said, "I hear that Left Supervisor Chen also studies astronomy and calendrical science. Do you know the similarities and differences between the Canopy Heaven Theory, the Spherical Heaven Theory, and the Xuan Ye Theory?"
The Canopy Heaven Theory, the Spherical Heaven Theory, and the Xuan Ye Theory were the three most representative cosmological theories of ancient China. The Canopy Heaven Theory was the oldest. Its premise was that "heaven resembles a conical hat, and earth takes the form of an overturned platter." It held that the universe was hemispherical, with the earth stretching boundlessly and heaven covering it like a conical hat-
The Spherical Heaven Theory, represented by Zhang Heng of the Eastern Han, held that "the spherical heavens resemble an egg: the celestial sphere is round as a pellet, and earth is like the yolk within the egg, dwelling alone inside heaven. Heaven is large and earth small, and water lies both within and beyond heaven. Heaven envelops earth as the shell wraps the yolk. Heaven and earth each stand upon qi and float upon water." The Spherical Heaven Theory was a great advance over the Canopy Heaven Theory, broadly reflecting the celestial configurations within the solar system. With the armillary sphere providing corroboration, it was the