A 13th-century solar machine
Constructed in 1276 under the direction of Kublai Khan's chief astronomer, Guo Shoujing, the Gaocheng Astronomical Observatory is an example of elegant, pre-telescopic engineering. Its main feature is a massive brick and stone tower, a type of gnomon called a gaobiao. This structure stands 9.46 meters tall, or 12.62 meters including the small rooms built atop it. An important innovation by Guo Shoujing was a horizontal bar placed between these two rooms, which narrowed the shadow's edge for more precise measurements.
Extending north from the base of the tower is a 31.19-meter-long stone path called the shigui, or shadow-measuring scale. This scale is built from 36 blocks of stone and has two parallel water channels carved along its length, which were filled to ensure the entire scale was perfectly level. By recording the precise position of the gnomon's shadow on the shigui at noon each day, astronomers could accurately determine the dates of the solstices and equinoxes, and the exact length of a tropical year. This observatory was the primary station in a network of 27 similar facilities across the Yuan empire.
The Season-Granting Calendar
The data gathered at Gaocheng formed the basis of the Shoushi Calendar, or "Season-Granting Calendar," which was put into use in 1281. This document was remarkably accurate. Guo Shoujing and his team calculated the length of a year to be 365 days, 5 hours, 49 minutes, and 20 seconds. This value is only 26 seconds different from the modern measurement of a tropical year.
The Shoushi Calendar's calculation was achieved nearly 300 years before the adoption of the similar Gregorian calendar in Europe. Its precision was so effective that it remained the official calendar of China for 364 years. The site at Gaocheng has an even longer history of astronomical observation, with records showing that a Tang dynasty astronomer named Yi Xing built a standardized gnomon here in 729 AD to help create the Da Yan calendar. The location was considered the "center of heaven and earth." In 2010, the Gaocheng Observatory was included as part of the "Historic Monuments of Dengfeng" UNESCO World Heritage Site.