Built in 1530 (the ninth year of the Ming-dynasty Jiajing Emperor’s reign), this altar was where the Heaven-worship ceremonies were held on the winter solstice. It was expanded in 1749 (the 14th year of the Qing-dynasty Qianlong Emperor’s reign). The numbers of steps, paved stone slabs on terrace surface, balustrade slabs and tiers, and the sizes of the terraces all follow the principle of using "heavenly number"—— 9. Also, the circular layout symbolizes the traditional belief in the cyclical nature of heavenly order.