Constantinus pictus a S. Pietro

The paper examines the mosaic on the triumphal arch of the Constantinian Basilica of St. Peter in the Vatican, now lost but known through medieval and Renaissance accounts. The scene depicted Emperor Constantine in the act of offering the model of the basilica to Christ and St. Peter, accompanied by a well-known distich (ICUR II, 4092). A recently published source, the Historiae Senenses by Sigismondo Tizio (before 1515), offers further details about the mosaic and describes Constantine kneeling before Christ and, at his feet, what has been interpreted as the imperial diadem—perhaps more plausibly a victory crown. The analysis of a range of historical and hagiographical sources suggests that the mosaic was indeed of Constantinian date and part of a political-religious program linked to the unification of the empire and the celebration of the vicennalia.