Guo Shoujing
The Gaocheng Astronomical Observatory (also known as the Dengfeng Observatory or the Tower of Chou Kong) stands south-east of Luoyang. It was constructed in 1276 during the Yuan Dynasty by Guo Shoujing as a giant gnomon for "the measurement of the sun's shadow". Prior to the Jesuit China Missions, it was used for establishing the summer and winter solstices in traditional Chinese astronomy.