The most important structure at the Temple of Heaven is not the famous one, and its emptiness is the point.
Two structures
Visitors photograph the Hall of Prayer for Good Harvests: circular, triple-eaved, blue-tiled, on a three-tiered marble terrace. It is the image on every guidebook.
The principal sacrifice was not made there. It was made at the Circular Mound Altar to the south: three concentric tiers of open marble, with a round stone at the centre, and no building at all.
The emperor sacrificed to Heaven under the sky, because Heaven is not a thing that can be housed. Putting a roof over it would have been a category error.
The geometry
The whole enclosure is doctrine in stone. Round for Heaven, square for Earth: the altars and halls are circular, the enclosing walls squared, and the northern wall is rounded while the southern is square, so the precinct itself is a diagram of Heaven over Earth.
The numerology is the odd number nine and its multiples throughout (the stones of the altar tiers, the balusters, the paving rings), nine being the greatest single digit and associated with the emperor.
The site covers two hundred and sixty-seven hectares, considerably larger than the Forbidden City, because Heaven outranked the emperor.
The rite
Built in 1420 under the Yongle Emperor, as part of the new capital.
The emperor came twice a year, most importantly at the winter solstice. He fasted beforehand in a dedicated hall inside the precinct. The processional route was closed, commoners were required to shut their doors and windows, and looking at the procession was prohibited.
The rite could be performed by no one else. It was the sharpest practical expression of what the Mandate of Heaven meant: the emperor was the sole intermediary, and the harvest and the legitimacy of the dynasty both depended on his performing it correctly.
Neither Taoist nor Confucian, this is the state cult itself, older as an institution than either, and administered by officials rather than by clergy.
1914
The last performance was in 1914, by Yuan Shikai, president of the new Republic of China.
He had the rite revived, travelled the route in an armoured car, and performed the sacrifice months before declaring himself emperor in December 1915.
It fooled nobody. The declaration collapsed within eighty-three days against near-universal opposition, and he died the following year.
That a republican president attempted the ceremony at all is the useful detail. He understood exactly what it was for, and he was the last person to think it still worked.
Now
The grounds opened as a public park in 1918. UNESCO inscribed the site in 1998.
It is heavily used at dawn: exercise, singing, dancing, instruments, and water calligraphy written on the paving that evaporates as it is written.