A magnificent triple-gabled circular building, built on a three-tiered marble stone base (6 m high), where the Emperor prayed for good harvests. Initially constructed in 1420 in a square design, it was reconstructed in 1545 during Emperor Jiajing's reign as a round building and reconstructed again in 1751 under Qianlong. At that time the roof tiles were redone all in blue. The building is completely wooden, with no nails used. The current building was rebuilt (still in the Ming style) several years after being burned down (caused by lightning) in 1889. The hall has four inner, twelve middle and twelve outer pillars, representing the four seasons, twelve months and twelve traditional Chinese hours (sh�chen) respectively. Combined together, the twelve middle and twelve outer pillars represent the traditional solar term.