Byzantine church design depended heavily on natural light which was used for evocative purposes. The orientation of the main axis of the church, the form of the apse and the location and size of its windows are affected by the need for sunlight to shine at the altar on a desired time and for a certain duration. Until now the process of accurately taking account of all the above parameters has been rather difficult and tedious. This paper illustrates the use of digital tools both for the analysis of the geometry of existing apse designs and the parametric generation of new ones. A sophisticated computer program was used to calculate sun angles during the Byzantine period incorporating calendar changes. In addition, Bentley Systemis Generative Components software was used to construct a parametric model that allows the user to define the geometric conditions and parameters of the apse. The software would then iterate through multiple solutions to satisfy the desired conditions as well as allow the user to change the conditions at will. The tools were used to discover the geometry of the apse of the Byzantine church of Hagia Sophia in Constantinople and to analyze the apse of the Post-Byzantine church of the Xeropotamou monastery on Mount Athos in Greece. The paper concludes with a discussion of the role of parametric tools for architectural analysis and the generation of possible design solutions.