A ruler for the sky
The Gaocheng Observatory is a precision astronomical instrument made of brick and stone. Built in 1276, its main components are a high platform and a long, stone measuring scale. The platform is a truncated pyramid, 9.46 meters high, with two small rooms on top that bring the total structure height to 12.62 meters. A horizontal bar is mounted between these two rooms, 9.75 meters above the base. This bar, precisely 40 chi (a traditional Chinese unit of length), acts as the gnomon, casting a shadow to the north.
Extending north from the platform's base is the shigui, or "sky-measuring ruler." This 31.19-meter-long scale is constructed from 36 square stone blocks set perfectly level along the meridian. Two parallel troughs carved into its surface could be filled with water to verify its horizontality. To take a reading, astronomers would use a device called a jingfu, an instrument with a small aperture, to project a sharp image of the gnomon's shadow onto the ruler. This method allowed for a measurement precision within two millimeters, a detail for tracking the exact moment of the winter and summer solstices.
The 26-second calendar
The observatory was constructed under an order from Kublai Khan of the Yuan Dynasty. His chief astronomer and engineer, Guo Shoujing, was tasked with creating a new, highly accurate calendar for the empire. The Gaocheng Observatory was the central instrument in a network of 27 observing stations established across China to gather the necessary data. The location itself was significant, as ancient Chinese cosmology considered the area the "Center of Heaven and Earth."
The observations produced the Shoushi Calendar, or "Season-Granting Calendar," which was put into use in 1281. Based on the meticulous shadow measurements, Guo Shoujing's team calculated the length of the tropical year to be 365.2425 days. This figure is only 26 seconds different from the modern scientific value. The calculation achieved the same accuracy as the Gregorian calendar, but predated its introduction in Europe by nearly 300 years. The Shoushi Calendar was so accurate that it remained the official calendar of China for 364 years.