Mausoleo di santa Costanza - The 4th-century mosaics on the ambulatory vault @manortiz46az

Mausoleum
The church was originally a part of the imperial funerary complex established here by the family of Emperor Constantine I, which may have been originally intended for the emperor himself. For a wider treatment of this complex, see Sant'Agnese fuori le Mura -Basilica Constantiniana.
After an archaeological excavation in 1992, the status of this church has been under review.
The traditional historical analysis was that it was built between 351 and 357 as a joint mausoleum for Constantina, a daughter of the emperor, and Fausta, who was resident at Rome at the time but who died at Bithynia in Asia Minor in 354. However, her body was brought back and interred here in a sarcophagus of imperial porphyry quarried at Mons Porphyrites in the Eastern Desert of Egypt. She was later joined by her sister Helena, who died in 360 and who had been the wife of Emperor Julian the Apostate. Another, smaller sarcophagus was duly provided for Helena.
The excavation revealed an earlier building on the site, in the form of a small (ten metres wide) triconch (clover-leaf shaped) edifice attached to the Basilica Constantina and entered from it. Hence, the present mausoleum cannot have been built as part of the original funerary complex but the triconch was, as its fabric was integral with that of the basilica.
The revisionist thesis is that Constantina had been buried within this triconch, in the smaller sarcophagus that used to be ascribed to Helena. After the latter empress died, the present building and the larger sarcophagus were then provided for her by Emperor Julian. This would push the date of the structure back to after 360, when Helena's body was brought back from Gaul where she had died. A tentative terminus ad quem for construction is the late 370's, based on stylistic evidence provided by the mosaics.
Middle ages
The building survived the collapse of ancient Roman civilization intact. It first appears in mediaeval history when Pope Nicholas I celebrated Mass here in 865, and this occasion was also the first time that the erroneous name Sanctae Constantiae appeared. So, by this time a legend had grown up identifying Constantina as a saint called Constantia, who was an alleged (non-existent) daughter of Constantine and a hermit at the catacombs of St Agnes. Flavia Julia Constantia was actually the half-sister of the emperor. Although the Roman martyrology never listed this St Constantia, who is the source of the English name Constance, she was to be celebrated at the church with a feast-day on February 25th.
The mausoleum was only formally converted into a church in 1256 by Pope Alexander IV, when he consecrated it. In the process he took the alleged relics of "St Constantia" from the larger sarcophagus and installed them under the central altar; unfortunately, these are very likely to belong to the empress Helena wife of Julian!
https://romanchurches.fandom.com/wiki/Santa_Costanza

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