The Temple of Heaven is a park containing three separate architectural set pieces, and most visitors see only one of them. The Hall of Prayer for Good Harvests, the round blue-roofed building on every postcard, draws the crowds in the northern half. In the southern half, reached through a gate across the park, stand the Circular Mound Altar and the Imperial Vault of Heaven with its Echo Wall - the two structures in which the ritual of the temple was actually performed, and where the mathematics behind the whole design is visible on the ground.
The Circular Mound Altar
The altar is a three-tiered circular platform of white marble, open to the sky, with no building on it. It was here, not in the great hall, that the emperor made the annual sacrifice at the winter solstice, and the design follows the number nine and its multiples, which in Chinese cosmology belong to heaven. Each tier is bounded by balustrades whose panels are counted in nines, and the central slab on the top platform, known as the heart of heaven, is surrounded by rings of paving stones that expand in multiples of nine. Stand on the centre stone and the acoustics of the circular wall around you become noticeably strange: your own voice returns slightly louder than you expect, because the balustrade reflects sound back to the middle.
The Echo Wall
The Imperial Vault of Heaven is a small round hall with a blue tiled roof, set inside a smooth circular wall. The wall is the attraction: it is built with a careful curvature and a hard, even surface, and it transmits sound along its length, so that a word spoken close to the wall on one side can be heard by someone standing against it on the far side, some sixty metres away. The effect is real but fragile - a crowd talking normally will drown it out, and the wall is usually crowded. The three echo stones in front of the vault, where clapping produces multiple returning claps, are the easier demonstration.
Practical notes
The park entrance ticket covers the main grounds, but the Hall of Prayer for Good Harvests, the Circular Mound Altar and the Echo Wall are inside a separate inner charge, and you need to buy the combined ticket if you want to see all three. The park opens early, before the paying halls do, and the southern approach from the east gate is usually quieter than the main north entrance. The altar is entirely exposed, so it is punishing at midday in summer; early morning and late afternoon are far better, and the low sun on the white marble of the circular tiers is the best light you will get.
