FORM AND FUNCTION OF THE APSE
One of the integral features of the imperial basilica was its projecting apse at the terminal end of the church. It served as a natural focus point for the authoritative figure to issue judgment in the court, and when the Church adopted the model it kept the same aura of political authority.
Apse comes from the Latin word apsis, meaning “arch” or “vault”. It was the most sacred space in the church and served as a niche behind the arched opening in the wall. Most often semicircular, it was roofed with a half-dome and often vaulted. The bema was a raised platform which held the altar and seating for the clergy and was placed underneath the apse when used. The apse was a symbol of “the cave in Bethlehem where Christ was born, and the cave where he was buried” [1].
The longitudinal plan was used in the basilicas in the years of Constantine to Justinian to center the church around the Liturgy of the Eucharist. The longitudinal axis held the forecourt in the western end and the apse at the eastern end, and it served "to pronounce the entrance and exit of the ceremonial liturgy" [2]. The veneration of the relics of martyrs brought about the centrally planned church structure which implemented the apse as a sacred space for the shrine.
With the division of the Roman Empire, execution and design of the apse went distinctly different ways
in the East and the West but kept many similar aspects of style. The capital cities of Rome and Constantinople along with the regional capitols of Thessaloniki and Ravenna espoused ideas from one another and became models for churches built elsewhere. While the design of the ecclesiastical architecture was executed using different techniques, the use of mosaic as the medium unified the designs in their resplendence.
[1] Nicholas N. Patricios, The Sacred Architecture of Byzantium: Art, Liturgy and Symbolism in Early
Christian Churches (N.Y., New York: I.B. Tauris, 2014), 65.
[2] Allan Doig, Liturgy and Architecture: From the Early Church to the Middle Ages (Aldershot,
England: Ashgate, 2008), 80.