Gaocheng Astronomical Observatory, also known as the Dengfeng Observatory, is a World Heritage Site in Duke Zhou Gong's shrine, Gaocheng Town, near Dengfeng in Henan Province, China. This site has a long tradition of astronomical observations, from the time of the Western Zhou up to the early Yuan Dynasty. Actually, one can see a gnome used for the Da Yan calendar in 729 AD and the great observatory of the Yuan Dynasty.
It is believed that the Duke of Zhou had erected at this place a Ceyingtai (observatory measuring the shade or gnomon) to observe the Sun. His interest in mathematics, astronomy/astrology is reported in the Chou Pei Suan Ching.
The astronomer Yi Xing of the Tang Dynasty built 20 standardized gnomons spread out over China to measure the equation of time dependent on the geographical location. Following a proposition of Liu Zhuo from 604 AD, 10 of these were aligned along the meridian 114° east of Greenwich from Central Asia down to Vietnam in order to determine the circumference of the Earth and derivations from a perfect sphere. One of these 10 observatories was situated at Gaocheng. The observations were used to establish the Da Yan calendar.