In the spring of 1999, the Xi'an Tang City Archaeological Team, IA, CASS, excavated the ruined round altar at Tang Chang' an city. The round altar was the ritual building where the emperor sacrificed to Heaven. Lying in the east of the southern suburbs of Tang Chang' an city, it was constructed in the Sui and abandoned at the end of the Tang period. As known from the ruins, it was an open-air building with four tiers of circular platforms, each measuring about 2m in height and having in the periphery 12 flights of ascending steps to symbolize the 12 two-hour periods of the day. The whole altar is constructed of rammed yellow earth and covered with a layer of white plaster. Its excavation furnishes material evidence to the literary records on ceremonies of sacrificing to Heaven in the Tang period and provides firsthand sources for studying the evolution and historical role of the ancient Chinese ritual system.
