apsis

A recess or projection, with a dome or vault, at the east end of a church.

Like Henry's collegiate church […] the cathedral is a cruciform basilica of the double bay system with four square bays in the central nave, western transoms, a quadrum for the choir, an apsis, and a transept with auxiliary apses.

Either of the points in the elliptical orbit of a planet, comet, or moon where it is closest or furthest from the body it orbits; examples are perihelion, aphelion, perigee, and apogee.