One of several historical Chinese astronomical instruments at the Beijing Ancient Observatory in Beijing, China, an astronomical observatory that dates from the 1400s. The gnomon was designed primarily to determine the length of the year. Chinese astronomer Guo Shoujing improved the design of the basic gnomon by adding a scale to the base and a bar across the top of the gnomon to measure the shadow position more accurately. The water trough on the base is for accurate levelling.