The present build dating to around 1540, this is the business end of the Temple of Heaven. Here on this open-air, raised platform (a circle within a square arrangement) is where the esoteric ceremonies to heaven actually took place, performed according to solemn protocol every winter solstice by the emperor himself.
The number nine here is significant, as your eyes will tell you. Odd numbers were considered sacrosanct in imperial China – nine (jiǔ) is the highest single-digit odd number, as well as being a homonym for long life. The altar is arranged in three tiers, with the top tier containing nine rings of stones, arranged in multiples of nine. The stairs and balustrades are also multiples of nine.