Emperor Justinan mandated Anthemius of Tralles to build the 'Great Church'. Anthemius was from Izmir in western Asia Minor, and was the last of the great architects of the Roman Empire. More of a mathematician than a builder, Anthemius' mastery in geometry allowed him to build Haghia Sophia's spectacular dome. The secret of balancing a dome over a square was the most important contribution of Byzantium to architecture. The Haghia Sophia was ready for consecration on December 24, 537. It was completed in 5 years, 10 months and 4 days. In comparison, St. Peter's in Rome took 120 years to build; St. Paul's in London took 35 years, while Notre Dame in Paris took 72 years.
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