Byzantine architecture

In 330 the emperor Constantine moves the capital to Byzantion on the Bosporus. The city takes his name, Constantinople, and becomes the centre of the Eastern Roman Empire for more than a thousand years.

The Romans could cast domes on round walls. The Byzantines want something harder: a round dome over a square room. The solution is called the pendentive – four curved triangles in the corners that carry the weight down to four piers.

Constantinople, 532. The emperor Justinian has just crushed a riot, and the church of Hagia Sophia lies in ashes. In five years the mathematician Anthemius of Tralles and Isidore of Miletus build a new one. The dome is about 33 metres across and rests on pendentives. Around its base runs a ring of windows, so that it seems to float.

It doesn't quite hold. In 558 part of it collapses after an earthquake, and the dome is rebuilt higher. But the idea lives on. Inside, mosaics shimmer in gold, and the marble on the walls is chosen for its pattern.

Later the rooms become smaller and tighter – cross-shaped churches with a dome in the middle and four smaller ones around. The style spreads to Ravenna, Venice, Greece and Russia.

And in Hagia Sophia's gallery someone has carved runes into the marble parapet. The only legible part is the name Halfdan. From the North to Constantinople – the Vikings called the city Miklagård.

In 1453 the city falls to the Ottomans. Hagia Sophia becomes a mosque – and the Ottomans take up the dome and build on it.

Novgorod and Pskov. The Russian churches build on Byzantium – but with white lime, plain cubes and bell walls instead of gold and marble.

Romanesque. Same period and the same round arches. But a Romanesque church is long and heavy with vaults; a Byzantine one gathers itself under a dome.