Located to the south of the Imperial Vault of Heaven, the Circular Mound Altar is a circular platform to offer sacrifice to heaven. It consists of three circular platforms of white marble, decreasing in diameter.There are 360 pillars in the balustrades, representing the 360 days of the ancient Chinese lunar year. The numbers of various elements of the Altar, including its balusters and steps, are either the sacred number nine or its nonuples.The imperial throne is supposed to be set up in the centre of the uppermost platform, symbolizing the role of the Emperor as the Son of Heaven and hence the mediator between Heaven and Earth. [Photo by Jia Yue for Chinadaily.com.cn]