Guo Shoujing, also known as Ruosi, was native of Xingtai, Shunde (now Xingtai, Hebei Province). He was the great astronomer, mathematician, hydraulic expert and the outstanding instrument maker. He devised a number of astronomical instruments, had compiled and published the Shoushi Calender, and conducted large-scale geodetic surveys and celestial observations. According to him, the specific length of a tropical year is 365.2425 days, which figure is absolutely identical to today’s Gregorian calendar.

 

    Major Contributions

Record: “In the reign of Emperor Kaiyuan of Tang Dynasty, Ling Nangong mentioned thirteen places for measuring the shadow cast by the sun in his book. Since today’s territory of China is much larger than that in Tang Dynasty, it is required to increase the points of measurement in the remote areas. At the time of solar and lunar eclipses, as the times, lengths of day and night, and distances between the sun, moon, and stars all differ, the measurer may scale the shadows in a shortcut way and south-to-north direction.”

 

- Biography of Guo Shoujing, Yuan History

    In his statement submitted to Kublai Khan, the Shizu of Yuan, 1279, Guo Shoujing addressed that “actual observation” was the foundation to compilation of calendar. The Dayan Calendar of Tang Dynasty even defines thirteen observation points across China. As by then territory was much larger than that in Tang Dynasty, more accurate data could not be available until the observation was made in remoter areas, thus giving rise to an applicable, nationwide calendar. The Emperor accepted Guo’s suggestions, having twenty-seven points establishing in the country. The territory covered the present Siberia, Korea, Gansu Corridor, and the South China Sea. It was the feat as “Survey across the Four Seas” in the history of astronomy. In this regard Guo Shoujing ever crossed numerous mountains and rivers in carrying out observations at various points.