Yuan Dynasty Astronomer and Hydraulic Engineer, Compiler of the Shoushi Calendar
Guo Shoujing (1231–1316) was a Yuan dynasty astronomer and hydraulic engineering expert, commissioned by Kublai Khan to lead nationwide astronomical observation and calendar reform, who designed and improved a series of astronomical instruments such as the simplified armilla and the tall gnomon, and organized twenty-seven observation stations across the country for large-scale astronomical measurement; the length of the tropical year computed by the Shoushi Calendar he thereby compiled differs from the precise value of modern astronomy by only about 26 seconds, a precision ahead of the great majority of calendar systems in the world of the time, and this calendar was used for nearly four hundred years thereafter, the peak of the development of the ancient Chinese calendar system. He was also an outstanding hydraulic engineering expert, directing the building of important hydraulic works such as the Tonghui Canal from Yuan Dadu to Tongzhou, greatly improving the efficiency of grain transport on the northern section of the Grand Canal and making an important contribution to the grain supply of the Yuan capital and the exchange of goods between north and south.
Guo Shoujing advised Kublai Khan to establish a systematic astronomical observation network across the country, at last setting up observation stations at twenty-seven locations including present-day Hainan Island and southern Siberia for simultaneous astronomical measurement, a breadth of measurement exceedingly rare in the world of the time, providing a solid first-hand observational basis for compiling a precise calendar.
The length of the tropical year computed by the Shoushi Calendar Guo Shoujing directed was 365.2425 days, differing from the precise value of modern astronomy by only about 26 seconds, a precision not matched in the West until the Gregorian calendar reform more than three hundred years later; the Shoushi Calendar was thereafter used by the Yuan and Ming for more than three hundred and sixty years, the longest-used and most precise calendar system in the history of the ancient Chinese calendar.
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