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.