Historically, the Temple of Heaven (Tiantan) was where emperors went to worship heaven for good harvests. The temple was listed on the UNESCO World Heritage List in 1998. Built in 1420, the 18th year of the reign of Ming Emperor Yongle, the temple hosts unique architectural features which have inspired numerologists, acoustic engineers, and architects alike.

The northern part of the Temple is circular while the southern part is square, representing heaven and earth respectively. The whole compound is enclosed by two walls – a square wall outside, and a round one inside. The outer area contains most of the temple grounds, while the inner part consists of the Hall of Prayer for Good Harvest and the Circular Mound Altar. The stone in the very middle of the altar, the God’s Heart Stone, was of key importance. It served as the emperor’s plinth while reciting his prayer, and had a peculiar quality of acoustically enhancing the clarity and volume of the emperor’s voice, a phenomenon that you can try out yourself when you visit.